Publications of GEG Members

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Venstrom, L, J Yager, T Vervynckt, J Ogland-Hand, and S Nudehi Measurement of the Natural Convection Heat Transfer in a Magnesium Oxide Electrolytic Cell Concept Thermal Science and Engineering Applications, (in press). [Download PDF]

Ezekiel, J., A. Ebigbo, B. M. Adams, and M. O. Saar Combining natural gas recovery and CO2-based geothermal energy extraction for electric power generation Applied Energy, (in press).

Horacek, J., et al., M. Ezzat, et al., EUROfusion MST1 Team, JET Contributors, and MAST-U Team Scaling of L-mode heat flux for ITER and COMPASS-U divertors, based on five tokamaks Nuclear Fusion, (in press). [Download PDF] [View Abstract]This contribution aims to improve existing scalings of the L-mode power decay length, especially for plasma configurations with strike points at the ITER-relevant location - closed vertical divertor targets. We propose 13 new scalings based on data from the tokamaks JET, EAST, MAST, Alcator C-mod and COMPASS, and validate them against the output of the 2D turbulence code HESEL. The analysis covers 500 divertor heat flux profiles (obtained by probes or IR cameras), measured in L-mode discharges with varying 12 global plasma parameters (all well predictable). We find that two previously published scalings [Eich, J.Nucl.Mat. 438 (2013) S72; Scarabosio, J.Nucl.Mat. 438 (2013) S426] (based on outer targets of AUG and JET) describe well all the JET, C-mod and COMPASS profiles, not only at outer horizontal and vertical targets, but surprisingly also at the inner vertical targets. In contrast, EAST, HESEL and especially MAST data are poorly described by these scalings. We therefore derive 13 new scalings describing 85-92 % of the measured decay lengths variability. The reader is suggested to use as many as possible scalings from here, depending on which parameters have available. Despite the fact that the scaling candidates are based on different parameters, predictions for the highest current L-modes in ITER are all very similar. Just prior to the L-H transition, in ITER baseline scenario, all the scalings predict values 2.5-3.5 mm (mapped to outer midplane), shorter for a single scaling based on predicted stored plasma energy. 1.6-2.6 mm is predicted for the COMPASS-Upgrade tokamak. In attached L-mode plasma, our results imply (using significant assumptions) steady-state surface-perpendicular heat flux around 10 MW/m2 for ITER, and 20 MW/m2 for COMPASS-Upgrade.

Vogler, D., S.D.C. Walsh, Ph. Rudolf von Rohr, and M.O. Saar Simulation of rock failure modes in thermal spallation drilling Acta Geotechnica, 2020. [Download PDF] [View Abstract]Thermal spallation drilling is a contact-less means of borehole excavation that works by exposing a rock surface to a high-temperature jet flame. In this study, we investigate crucial factors for the success of such thermal drilling operations using numerical simulations of the thermomechanical processes leading to rock failure at the borehole surface. To that end, we integrate a model developed for spalling failure with our thermomechanical simulations. In particular, we consider the role of material heterogeneities, maximum jet-flame temperature and maximum jet-flame temperature rise time on the onset of inelastic deformation and subsequent damage. We further investigate differences in energy consumption for the studied system configurations. The simulations highlight the importance of material composition, as thermal spallation is favored in fine-grained material with strong material heterogeneity. The model is used to test the relationship between the jet-flame temperature and the onset of thermal spallation.

Rossi, E., S. Jamali, V. Wittig, M.O. Saar, and Ph. Rudolf von Rohr A combined thermo-mechanical drilling technology for deep geothermal and hard rock reservoirs Geothermics, 85/101771, 2020. [Download PDF] [View Abstract]Combined thermo-mechanical drilling is a novel technology to enhance drilling performance in deep hard rock formations. In this work, we demonstrate this technology in the field by implementing the concept on a full-scale drilling rig, and we show its feasibility under realistic process conditions. We provide evidence that the novel drilling method can increase the removal performance in hard rocks by up to a factor of three, compared to conventional drilling methods. From the findings of this work, we conclude that integration of thermal assistance to conventional rotary drilling constitutes an interesting approach to facilitate the drilling process, and therefore increase the access viability to deep georesources in hard rocks.

Rossi, E., S. Jamali, M.O. Saar, and Ph. Rudolf von Rohr Field test of a Combined Thermo-Mechanical Drilling technology. Mode I: Thermal spallation drilling Journal of Petroleum Science and Engineering, 190/107005, 2020. [Download PDF] [View Abstract]Accessing hydrocarbons, geothermal energy and mineral resources requires more and more drilling to great depths and into hard rocks, as many shallow resources in soft rocks have been mined already. Drilling into hard rock to great depths, however, requires reducing the effort (i.e., energy), time (i.e., increasing the rate of penetration) and cost associated with such operations. Thus, a Combined Thermo-Mechanical Drilling (CTMD) technology is proposed, which employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) Thermal spallation drilling, investigated here as a field test and (II) Flame-assisted rotary drilling, investigated as a field test in the companion paper. The CTMD technology is expected to reduce drilling costs, especially in hard rocks, by enhancing the rock penetration rate and increasing the bit lifetime. Mode I of the CTMD technology (thermal spallation drilling) is investigated here by implementing the concept on a full-scale drilling rig to investigate its feasibility and performance under realistic field conditions. During the test, the successful thermal spallation process is monitored, employing a novel acoustic emission system. The effects of thermal spallation in the granite rock are analyzed to provide conclusions regarding the rock removal performance and the application potential of the technology. The field test shows that thermal spallation of the granitic rock can be successfully achieved even when a liquid (water) is used as the drilling fluid, as long as the heat source is appropriately shielded by compressed-air jets. Thermal damage of the surrounding rock is investigated after the spallation test, employing micro-computer tomography imaging and modeling the stability of the cracks, generated by the spallation field test. This study shows that thermally induced damage is mainly confined within a narrow region close to the rock surface, suggesting that thermal spallation only marginally affects the overall mechanical stability of the borehole. Thus, this confirms that, as part of the Combined Thermo- Mechanical Drilling (CTMD) technology, thermal spallation drilling is a promising mode that has a high potential of facilitating the drilling of deep boreholes in hard rocks.

Rossi, E., S. Jamali, D. Schwarz, M.O. Saar, and Ph. Rudolf von Rohr Field test of a Combined Thermo-Mechanical Drilling technology. Mode II: Flame-assisted rotary drilling Journal of Petroleum Science and Engineering, 190/106880, 2020. [Download PDF] [View Abstract]To enhance the drilling performance in deep hard rocks and reduce overall drilling efforts, this work proposes a Combined Thermo-Mechanical Drilling (CTMD) technology. This technology employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) thermal spallation drilling, investigated in the companion paper and (II) flame-assisted rotary drilling, investigated here as a field test. The CTMD technology is expected to reduce drilling efforts, especially in hard rocks, enhancing the rock penetration rate and increasing the bit lifetime, all of which reduces the drilling costs. The present work investigates Mode II (flame-assisted rotary drilling) of the CTMD technology by implementing the concept in an existing drilling rig and testing the technology under relevant process conditions. This contribution studies the underlying rock removal mechanism of CTMD and demonstrates its drilling performance, compared to conventional rotary drilling methods. Acoustic emission monitoring, and analysis of the collected drill cuttings provide multiple evidences for thermal-cracking-enhanced rock removal during the flame-assisted rotary drilling. This removal mechanism appears to represent an optimal compromise to minimize rock fragmentation and cutting transport efforts during drilling, compared to a less efficient mechanical scraping of the hard granite rock, observed during the standalone-mechanical drill test. The drilling performance, in terms of removal and wear rates, are evaluated for the flame-assisted rotary drilling. This shows that the proposed drilling approach is capable of enhancing the removal process in hard granite rock, by a factor of 2.5, compared to standalone-mechanical drilling. The implementation of this drilling approach into a conventional drilling system shows that integration of thermal assistance to conventional rotary drilling requires marginal technical efforts. Additionally, this technology can profit from established knowledge in conventional mechanical drilling, facilitating its implementation to improve drilling performance in hard rocks. Hence, this study demonstrates that the Combined Thermo- Mechanical Drilling method is feasible and concludes that this technology constitutes a promising approach to improve the drilling process, thereby increasing the viability of accessing deep geo-resources in hard rocks.

Kittilä, A., M.R. Jalali, M. Somogyvári, K.F. Evans, M.O. Saar, and X.-Z. Kong Characterization of the effects of hydraulic stimulation with tracer-based temporal moment analysis and tomographic inversion Geothermics, 86/101820, 2020. [Download PDF] [View Abstract]Tracer tests were conducted as part of decameter-scale in-situ hydraulic stimulation experiments at the Grimsel Test Site to investigate the hydraulic properties of a stimulated crystalline rock volume and to study the stimulation-induced hydrodynamic changes. Temporal moment analysis yielded an increase in tracer swept pore volume with prominent flow channeling. Post-stimulation tomographic inversion of the hydraulic conductivity, K, distribution indicated an increase in the geometric mean of logK and a decrease in the Dykstra-Parsons heterogeneity index. These results indicate that new flow path connections were created by the stimulation programs, enabling the tracers to sweep larger volumes, while accessing flow paths with larger hydraulic conductivities.

Nejati, M., A. Aminzadeh, T. Driesner, and M.O. Saar On the directional dependency of Mode I fracture toughness in anisotropic rocks Theoretical and Applied Fracture Mechanics, 107/102494, 2020. [Download PDF] [View Abstract]This paper presents a theoretical and experimental analysis of the directional variations of different measures of Mode fracture toughness in anisotropic rocks and possibly other types of solids. We report the theoretical basis for the directional dependence of three measures of fracture toughness: the critical stress intensity factor, the critical energy release rate and the critical strain energy density. The equivalency of these three measures in anisotropic materials is discussed. We then provide a full set of experimental results on the fracture toughness variation in an anisotropic rock that exhibits transverse isotropy. The results give supporting evidence that the critical Mode stress intensity factor in fact varies with direction based on a sinusoidal function. This indicates that there exist two principal values of the fracture toughness along with the principal material directions within the plane. Once these two principal values are determined, all three measures of the fracture toughness can be predicted in any direction, provided that the elastic constants of the material are known, and that the symmetry condition employed in this analysis is fulfilled.

Gischig, V.S., D. Giardini, F. Amann, "et al.", Keith F. Evans, M. Jalali, "et al.", A. Kittilä, X. Ma, "et al.", and M.O. Saar Hydraulic stimulation and fluid circulation experiments in underground laboratories: Stepping up the scale towards engineered geothermal systems Geomechanics for Energy and the Environment, 100175, 2020. [Download PDF] [View Abstract]The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS.

Krietsch, H., L. Villiger, J. Doetsch, V. Gischig, K.F. Evans, B. Brixel, M.R. Jalali, S. Loew, D. Giardini, and F. Amann Changing Flow Paths Caused by Simultaneous Shearing and Fracturing Observed During Hydraulic Stimulation Geophysical Research Letters, 47, 2020. [Download PDF]

von Planta, C., D. Vogler, X. Chen, M.G.C. Nestola, M.O. Saar, and R. Krause Modelling of hydro-mechanical processes in heterogeneous fracture intersections using a fictitious domain method with variational transfer operators Computational Geosciences, 2020. [Download PDF] [View Abstract]Fluid flow in rough fractures and the coupling with the mechanical behavior of the fractures pose great difficulties for numerical modeling approaches, due to complex fracture surface topographies, the non-linearity of hydromechanical processes and their tightly coupled nature. To this end, we have adapted a fictitious domain method to enable the simulation of hydromechanical processes in fracture-intersections. The main characteristic of the method is the immersion of the fracture domain, modelled as a linear elastic solid, in the surrounding fluid, modelled with the incompressible Navier Stokes equations. The fluid and the solid problems are coupled with variational transfer operators. Variational transfer operators are also used to solve contact within the fracture using a mortar approach and to generate problem specific fluid grids. With respect to our applications, the key features of the method are the usage of different finite element discretizations for the solid and the fluid problem and the automatically generated representation of the fluid-solid boundary. We demonstrate that the presented methodology resolves small-scale roughness on the fracture surface, while capturing fluid flow field changes during mechanical loading. Starting with 2D/3D benchmark simulations of intersected fractures, we end with an intersected fracture composed of complex fracture surface topographies, which are in contact under increasing loads. The contributions of this article are: (1) the application of the fictitious domain method to study flow in fractures with intersections, (2) a mortar based contact solver for the solid problem, (3) generation of problem specific grids using the geometry information from the variational transfer operators.

Ziegler, M., and K.F. Evans Comparative study of Basel EGS reservoir faults inferred from analysis of microseismic cluster datasets with fracture zones obtained from well log analysis Journal of Structural Geology, 130, 2020.

Ahkami, M., A. Parmigiani, P.R. Di Palma, M.O. Saar, and X.-Z. Kong A lattice-Boltzmann study of permeability-porosity relationships and mineral precipitation patterns in fractured porous media Computational Geosciences, 2020. [Download PDF] [View Abstract]Mineral precipitation can drastically alter a reservoir’s ability to transmit mass and energy during various engineering/natural subsurface processes, such as geothermal energy extraction and geological carbon dioxide sequestration. However, it is still challenging to explain the relationships among permeability, porosity, and precipitation patterns in reservoirs, particularly in fracture-dominated reservoirs. Here, we investigate the pore-scale behavior of single-species mineral precipitation reactions in a fractured porous medium, using a phase field lattice-Boltzmann method. Parallel to the main flow direction, the medium is divided into two halves, one with a low-permeability matrix and one with a high-permeability matrix. Each matrix contains one flow-through and one dead-end fracture. A wide range of species diffusivity and reaction rates is explored to cover regimes from advection- to diffusion-dominated, and from transport- to reaction-limited. By employing the ratio of the Damköhler (Da) and the Peclet (Pe) number, four distinct precipitation patterns can be identified, namely (1) no precipitation (Da/Pe < 1), (2) near-inlet clogging (Da/Pe > 100), (3) fracture isolation (1 < Da/Pe < 100 and Pe > 1), and (4) diffusive precipitation (1 < Da/Pe < 100 and Pe < 0.1). Using moment analyses, we discuss in detail the development of the species (i.e., reactant) concentration and mineral precipitation fields for various species transport regimes. Finally, we establish a general relationship among mineral precipitation pattern, porosity, and permeability. Our study provides insights into the feedback loop of fluid flow, species transport, mineral precipitation, pore space geometry changes, and permeability in fractured porous media.

Walsh, S.D.C., and D. Vogler Simulating Electropulse Fracture of Granitic Rock International Journal of Rock Mechanics and Mining Sciences, 128, pp. 104238, 2020. [Download PDF] [View Abstract]Electropulse treatments employ a series of high-voltage discharges to break rock into small fragments. As these methods are particularly suited to fracturing hard brittle rocks, electropulse treatments can serve to enhance or substitute for more traditional mechanical approaches to drilling and processing of these materials. Nevertheless, while these treatments have the potential to improve hard-rock operations, the coupled electro-mechanical processes responsible for damaging the rock are poorly described. The lack of accurate models for these processes increases the difficulty of designing, controlling and optimizing tools that employ electropulse treatments and limits their range of application. This paper describes a new modeling method for studying electropulse treatments in geotechnical operations. The multiphysics model simulates the passage of the pulse, electrical breakdown in the rock, and the mechanical response at the grain-scale. It also accounts for the contributions from different minerals and porosities, allowing the effect of material composition to be considered. In so doing, it provides a means to investigate the different physical and operational factors influencing electropulse treatments.

Vogler, D., S.D.C. Walsh, and M.O. Saar A Numerical Investigation into Key Factors Controlling Hard Rock Excavation via Electropulse Stimulation Journal of Rock Mechanics and Geotechnical Engineering, (in press). [View Abstract]Electropulse stimulation provides an energy-efficient means of excavating hard rocks through repeated application of high voltage pulses to the rock surface. As such, it has the potential to confer significant advantages to mining and drilling operations for mineral and energy resources. Nevertheless, before these benefits can be realized, a better understanding of these processes is required to improve their deployment in the field. In this paper, we employ a recently developed model of the grain-scale processes involved in electropulse stimulation to examine excavation of hard rock under realistic operating conditions. To that end, we investigate the maximum applied voltage within ranges of 120~kV to 600~kV, to observe the onset of rock fragmentation. We further study the effect of grain size on rock breakage, by comparing fine and coarse grained rocks modeled after granodiorite and granite, respectively. Lastly, the pore fluid salinity is investigated, since the electric conductivity of the pore fluid is shown to be a governing factor for the electrical conductivity of the system. This study demonstrates that all investigated factors are crucial to the efficiency of rock fragmentation by electropulsing.

Ascasíbar, E., et al., M. Ezzat, et al., J. M. García-Regaña, et al., and the TJ-II team Overview of recent TJ-II stellarator results Nuclear Fusion, 59/11, pp. 1-13, 2019. [Download PDF] [View Abstract]The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components. As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented.

Morschhauser, A., A.V. Grayver, A. Kuvshinov, F. Samrock, and J. Matzka Tippers at island geomagnetic observatories constrain electrical conductivity of oceanic lithosphere and upper mantle Earth, Planets and Space, 71/17, pp. 1-9, 2019. [Download PDF] [View Abstract]Geomagnetic field variations as recorded at geomagnetic observatories are important for global electromagnetic studies. However, this data set is rarely used for studying the local electrical conductivity at depths $<200$ km. The main reasoning being that given a single geomagnetic observatory, one can at most constrain the one-dimensional (1-D) conductivity structure beneath it. At the same time, tippers, magnetic transfer functions resolving these depths, are zero for any 1-D conductivity distribution. We show that the ocean induction effect alleviates these limitations for observatories on islands and develop a method to invert tippers for a 1-D conductivity profile in the presence of three-dimensional conductivity structure due to bathymetry. This allows to recover 1-D upper mantle conductivity profiles at remote oceanic locations where little or no knowledge is available and that would otherwise be difficult to access. We apply the method to Gan in the Indian Ocean and to Tristan da Cunha in the South Atlantic, and the obtained conductivity profiles indicate a normal oceanic mantle and elevated conductivities, respectively, which fits well with their geological settings.

Valley, B., and K.F. Evans Stress magnitudes in the Basel enhanced geothermal system International Journal of Rock Mechanics and Mining Sciences, 118, pp. 1-20, 2019. [Download PDF] [View Abstract]This paper presents the results of an evaluation of stress magnitudes in the granitic EGS reservoir in Basel, Switzerland. The profile of minimum principal horizontal stress, Shmin, is constrained by hydraulic tests, but the magnitude of the maximum horizontal principal stress, SHmax is uncertain. Here we derive estimates for SHmax by analysing breakout width data from an acoustic televiewer logs run in the granitic basement section of the BS-1 borehole. Some 81% of the borehole in the granite is affected by breakouts. The approach employed to derive SHmax magnitude from the estimated breakout widths is taking into account all stress components at the borehole wall including the remnant thermal stress arising from the cooling of the borehole wall by the drilling. In BS-1, breakouts width tends to decrease with depth. Assuming there is no significant systematic change in the strength characteristics of the rock along the length of the hole, for which there is no evidence, the large-scale trend has the consequence of implying a small gradient of the SHmax profile. A low Shmin gradient was also implied by a stress analysis that additionally considered the occasionally coincident presence of drilling induced tension fractures. The absolute values of SHmax depend upon the failure criterion used. Criteria that consider the strengthening effect of the intermediate stress (Mogi-Coulomb and Hoek-Brown 3D) yield profiles that violate frictional limits on the strength of the crust above 4 km, whereas the profiles of the Mohr-Coulomb and Rankine criteria do not. The Mohr-Coulomb criteria profiles indicate a trend in SHmax from favoring strike-slip faulting above 4200m to strike-slip/normal faulting below. This is consistent with focal mechanisms recorded during the reservoir stimulation which show a mix of strike-slip and normal faulting throughout the depth range considered.

Cunningham, A. B., H. Class, A. Ebigbo, R. Gerlach, A. J. Phillips, and J. Hommel Field-scale modeling of microbially induced calcite precipitation Computational Geosciences, 23/2, pp. 399-414, 2019. [Download PDF] [View Abstract]The biogeochemical process known as microbially induced calcite precipitation (MICP) is being investigated for engineering and material science applications. To model MICP process behavior in porous media, computational simulators must couple flow, transport, and relevant biogeochemical reactions. Changes in media porosity and permeability due to biomass growth and calcite precipitation, as well as their effects on one another must be considered. A comprehensive Darcy-scale model has been developed by Ebigbo et al. (Water Resour. Res. 48(7), W07519, 2012) and Hommel et al. (Water Resour. Res. 51, 3695–3715, 2015) and validated at different scales of observation using laboratory experimental systems at the Center for Biofilm Engineering (CBE), Montana State University (MSU). This investigation clearly demonstrates that a close synergy between laboratory experimentation at different scales and corresponding simulation model development is necessary to advance MICP application to the field scale. Ultimately, model predictions of MICP sealing of a fractured sandstone formation, located 340.8 m below ground surface, were made and compared with corresponding field observations. Modeling MICP at the field scale poses special challenges, including choosing a reasonable model-domain size, initial and boundary conditions, and determining the initial distribution of porosity and permeability. In the presented study, model predictions of deposited calcite volume agree favorably with corresponding field observations of increased injection pressure during the MICP fracture sealing test in the field. Results indicate that the current status of our MICP model now allows its use for further subsurface engineering applications, including well-bore cement sealing and certain fracture-related applications in unconventional oil and gas production.

Parmigiani, A., P.R. Di Palma, S. Leclaire, F. Habib, and X.-Z. Kong Characterization of transport-enhanced phase separation in porous media using a lattice-Boltzmann method Geofluids, 2019, 2019. [Download PDF] [View Abstract]Phase separation of formation fluids in subsurface introduces hydrodynamic perturbations which are critical for mass and energy transport of geofluids. Here, we present pore-scale lattice-Boltzmann simulations to investigate the hydrodynamical response of a porous-system to the emergence of non-wetting droplets under background hydraulic gradients. A wide parameter space of capillary number and fluid saturation is explored to characterize the droplet evolution, the droplet size and shape distribution, and the capillary-clogging patterns. We find that clogging is favored by high capillary stress; nonetheless clogging occurs at high non-wetting saturation (larger than 0.3), denoting the importance of convective transport on droplet-growth and permeability. Moreover, droplets are more sheared at low capillary number, however solid matrix plays a key role on droplet's volume-to-surface ratio.

Romano, E., J. Jimenez-Martinez, A. Parmigiani, X.-Z. Kong, and I. Battiato Contribution of Pore-Scale Approach to Macroscale Geofluids Modelling in Porous Media Geofluids, 2019, 2019. [Download PDF] [View Abstract]Understanding the fundamental mechanisms of fluid flows and reactive transport in natural systems is a major challenge for several fields of Earth sciences (e.g. hydrology, soil science, volcanology) and Geo/Environmental-engineering (CO2 sequestration, NAPLS contamination, geothermal energy, oil&gas reservoir exploitation). The hierarchical structures of natural system (e.g. heterogeneity of geological formations) as well as the different behaviour of single and multi-phase fluids at the pore-scale coupled with the nonlinearity of underlying reactive processes necessitate investigating these aspects at the scale at which they physically occur, the scale of pore and fractures. Recent improvements in pore-scale computational modelling, together with the development of non- invasive microscopic imaging technology and the latest microfluidics technics are allowing the vast field of porous & fractured media research to benefit of major advances due to: 1) an improved understanding and description of pore-scale mechanisms and 2) the ability of thinking in terms of coupled processes. The contributions collected in this Special Issue, although far from constituting a comprehensive picture of the “pore-scale world”, however offer a good example of the potentialities of such an approach to investigate a wide range of processes usually observed at macro-scale, but whose underlying physical and chemical processes take place at micro-scale.

Perras, M.A., and D. Vogler Compressive and Tensile Behavior of 3D-Printed and Natural Sandstones Transport in Porous Media, 129/2, pp. 559-581, 2019. [Download PDF] [View Abstract]The presented work compares the mechanical behavior from standard unconfined compressive strength and indirect tensile strength tests of natural sandstone and artificial sand-based specimens created by 3D additive manufacturing. Three natural sandstones of varying strength and stiffness were tested to capture a wide range of behavior for comparison with the 3D-printed specimens. Sand grains with furan and silicate binders, as well as, ceramic beads with silicate binder were 3D-printed by commercial suppliers. The tensile and compressive strength, the stiffness, the crack initiation and the crack damage thresholds and the strain behavior were examined to determine if the mechanical behavior of the 3D-printed specimens is similar to natural sandstones. The Sand-Furan 3D-prints behaved the closest to the weak natural sandstone. The compressive strength to stiffness ratio, also known as the modulus ratio, and the compressive to tensile strength ratio of the 3D-printed Sand-Furan specimens were found to be similar to the natural sandstones tested in this study and literature values. The failed specimens composed of ceramic beads with silicate binder, both in compression and tension, showed fracture growth not commonly observed in natural specimens. The other 3D-printed specimens generally fractured in a similar manner to natural specimens, although several of the quartz sand with furan binder specimens showed fracturing behavior similar to high porosity natural specimens. Over all, using the commercially available quartz sand with furan binder 3D-print materials showed promise to be able to replicate natural rock specimen behavior.

Elison, P., J. Niederau, C. Vogt, and C. Clauser Quantification of thermal conductivity uncertainty for basin modeling AAPG Bulletin, 103/8, pp. 1787-1809, 2019. [Download PDF]

Niederau, J., F. Wellmann, and N. Börsing Analyzing the influence of correlation length in permeability on convective systems in heterogeneous aquifers using entropy production Geothermal Energy Science – Society – Technology, 7/35, 2019. [Download PDF] [View Abstract]Hydrothermal convection in porous geothermal reservoir systems can be seen as a double-edged sword. On the one hand, regions of upflow in convective systems can increase the geothermal energy potential of the reservoir; on the other hand, convection introduces uncertainty, because it can be difficult to locate these regions of upflow. Several predictive criteria, such as the Rayleigh number, exist to estimate whether convection might occur under certain conditions. As such, it is of interest which factors influence locations of upwelling regions and how these factors can be determined. We use the thermodynamic measure entropy production to describe the influence of spatially heterogeneous permeability on a hydrothermal convection pattern in a 2D model of a hot sedimentary aquifer system in the Perth Basin, Western Australia. To this end, we set up a Monte Carlo study with multiple ensembles. Each ensemble contains several hundred realizations of spatially heterogeneous permeability. The ensembles only differ in the horizontal spatial continuity (i.e., correlation length) of permeability. The entropy production of the simulated ensembles shows that the convection patterns in our models drastically change with the introduction and increase of a finite, lateral correlation length in permeability. An initial decrease of the average entropy production number with increasing lateral correlation length shows that fewer ensemble members show convection. When neglecting the purely conductive ensembles in our analysis, no significant change in the number of convection cells is seen for lateral correlation lengths larger than 2000 m. The result suggests that the strength of convective heat transfer is not sensitive to changes in lateral correlation length beyond a specific factor. It does, however, change strongly compared to simulations with a homogeneous permeability field. As such, while the uncertainty in spatial continuity of permeability may not strongly influence the convective heat transfer, our findings show that it is important to consider spatial heterogeneity and continuity of permeability when simulating convective heat transfer in an aquifer.

Kyas, S., U. Langer, and S. Repin Guaranteed error bounds and local indicators for adaptive solvers using stabilized space-time IgA approximations to parabolic problems Computers and Mathematics with Applications, 78/8, pp. 2641-2671, 2019. [Download PDF] [View Abstract]The paper is concerned with space-time IgA approximations to parabolic initial-boundary value problems. We deduce guaranteed and fully computable error bounds adapted to special features of such type of approximations and investigate their efficiency. The derivation is based on the analysis of the corresponding integral identity and exploits purely functional arguments in the maximal parabolic regularity setting. The estimates are valid for any approximation from the admissible (energy) class and do not contain mesh-dependent constants. They provide computable and fully guaranteed error bounds for the norms arising in stabilised space-time approximations. Furthermore, a posterior error estimates yield efficient error indicators enhancing the performance of adaptive solvers. Theoretical results are verified on a series of numerical examples, in which approximate solutions and auxiliary fluxes are recovered by IgA techniques. The mesh refinement algorithm is governed by local error indicators that naturally follow from the global error majorants. The numerical results confirm high efficiency of the method in the context of the two main goals of a posteriori error analysis: estimation of global errors and mesh adaptation.

Afshari Moein, M.J., B. Valley, and K.F. Evans Scaling of Fracture Patterns in Three Deep Boreholes and Implications for Constraining Fractal Discrete Fracture Network Models Rock Mechanics and Rock Engineering, 52/6, pp. 1723-1743, 2019. [Download PDF]

Javanmard, H., M. Seyyedi, S.A. Jones, and S.M. Nielsen Dimethyl Ether Enhanced Oil Recovery in Fractured Reservoirs and Aspects of Phase Behavior Energy & Fuels, 2019. [Download PDF] [View Abstract]The efficiency of the dimethyl ether (DME) enhanced oil recovery (EOR) technique in a fractured chalk reservoir core plug was investigated. The coreflood experiment showed that DME EOR could lead to 44.2% additional oil recovery, amounting to 80.6% of the ultimate oil recovery. A comprehensive set of laboratory experiments, including density measurements of miscible fluids, DME-induced oil swelling factor, and partition coefficient of DME between the aqueous and oleic phase, were performed. The experimental results show that the partition coefficient of DME for the mixture of DME–brine–oil can reach up to 18.3. The oil swelling factor for such a system can reach up to 2.7 under realistic reservoir conditions. Comparing this data set to the available data for other mutually soluble solvent-based EOR techniques shows that the oil swelling caused by DME is far stronger than for other common solvents. Due to the strong partitioning of DME between the phases, the DME from the DME–brine solution rapidly partitions into the bypassed oil in the low permeability matrix, which leads to strong oil swelling and production.

Miron, G. D., A.M.M. Leal, and A. Yapparova Thermodynamic Properties of Aqueous Species Calculated Using the HKF Model: How Do Different Thermodynamic and Electrostatic Models for Solvent Water Affect Calculated Aqueous Properties? Geofluids, a43, pp. 1-24, 2019. [Download PDF] [View Abstract]Thermodynamic properties of aqueous species are essential for modeling of fluid-rock interaction processes. The Helgeson-Kirkham-Flowers (HKF) model is widely used for calculating standard state thermodynamic properties of ions and complexes over a wide range of temperatures and pressures. To do this, the HKF model requires thermodynamic and electrostatic models of water solvent. In this study, we investigate and quantify the impact of choosing different models for calculating water solvent volumetric and dielectric properties, on the properties of aqueous species calculated using the HKF model. We identify temperature and pressure conditions at which the choice of different models can have a considerable effect on the properties of aqueous species and on fluid mineral equilibrium calculations. The investigated temperature and pressure intervals are 25–1000°C and 1–5 kbar, representative of upper to middle crustal levels, and of interest for modeling ore-forming processes. The thermodynamic and electrostatic models for water solvent considered are: Haar, Gallagher and Kell (1984), Wagner and Pruß (2002), and Zhang and Duan (2005), to calculate water volumetric properties, and Johnson and Norton (1991), Fernandez and others (1997), and Sverjensky and others (2014), to calculate water dielectric properties. We observe only small discrepancies in the calculated standard partial molal properties of aqueous species resulting from using different water thermodynamic models. However, large differences in the properties of charged species can be observed at higher temperatures (above 500°C) as a result of using different electrostatic models. Depending on the aqueous speciation and the reactions that control the chemical composition, the observed differences can vary. The discrepancy between various electrostatic models is attributed to the scarcity of experimental data at high temperatures. These discrepancies restrict the reliability of the geochemical modeling of hydrothermal and ore formation processes, and the retrieval of thermodynamic parameters from experimental data at elevated temperatures and pressures.

von Planta, C., D. Vogler, X. Chen, M.G.C. Nestola, M.O. Saar, and R. Krause Simulation of hydro-mechanically coupled processes in rough rock fractures using an immersed boundary method and variational transfer operators Computational Geosciences, 23/5, pp. 1125-1140, 2019. [Download PDF] [View Abstract]Hydro-mechanical processes in rough fractures are highly non-linear and govern productivity and associated risks in a wide range of reservoir engineering problems. To enable high-resolution simulations of hydro-mechanical processes in fractures, we present an adaptation of an immersed boundary method to compute fluid flow between rough fracture surfaces. The solid domain is immersed into the fluid domain and both domains are coupled by means of variational volumetric transfer operators. The transfer operators implicitly resolve the boundary between the solid and the fluid, which simplifies the setup of fracture simulations with complex surfaces. It is possible to choose different formulations and discretization schemes for each subproblem and it is not necessary to remesh the fluid grid. We use benchmark problems and real fracture geometries to demonstrate the following capabilities of the presented approach: (1) resolving the boundary of the rough fracture surface in the fluid; (2) capturing fluid flow field changes in a fracture which closes under increasing normal load; and (3) simulating the opening of a fracture due to increased fluid pressure.

Ma, J., L. Querci, B. Hattendorf, M.O. Saar, and X.-Z. Kong Toward a Spatiotemporal Understanding of Dolomite Dissolution in Sandstone by CO2‑Enriched Brine Circulation Environmental Science & Technology, 2019. [Download PDF] [View Abstract]In this study, we introduce a stochastic method to delineate the mineral effective surface area (ESA) evolution during a re-cycling reactive flow-through transport experiment on a sandstone under geologic reservoir conditions, with a focus on the dissolution of its dolomite cement, Ca$_{1.05}$Mg$_{0.75}$Fe$_{0.2}$(CO$_3$)$_2$. CO$_2$-enriched brine was circulated through this sandstone specimen for 137 cycles ($\sim$270 hours) to examine the evolution of in-situ hydraulic properties and CO$_2$-enriched brine-dolomite geochemical reactions. The bulk permeability of the sandstone specimen decreased from 356 mD before the reaction to 139 mD after the reaction, while porosity increased from 21.9\% to 23.2\% due to a solid volume loss of 0.25 ml. Chemical analyses on experimental effluents during the first cycle yielded a dolomite reactivity of $\sim$2.45 mmol~m$^{-3}$~s$^{-1}$, a corresponding sample-averaged ESA of $\sim$8.86$\times 10^{-4}$~m$^2$/g, and an ESA coefficient of 1.36$\times 10^{-2}$, indicating limited participation of the physically exposed mineral surface area. As the dissolution reaction progressed, the ESA is observed to first increase, then decrease. This change in ESA can be qualitatively reproduced employing SEM-image-based stochastic analyses on dolomite dissolution. These results provide a new approach to analyze and upscale the ESA during geochemical reactions, which are involved in a wide range of geo-engineering operations.

Lima, M.M., D. Vogler, L. Querci, C. Madonna, B. Hattendorf, M.O. Saar, and X.-Z. Kong Thermally driven fracture aperture variation in naturally fractured granites Geothermal Energy Journal, 7/1, pp. 1-23, 2019. [Download PDF] [View Abstract]Temperature variations often trigger coupled thermal, hydrological, mechanical, and chemical (THMC) processes that can significantly alter the permeability/impedance of fracture-dominated deep geological reservoirs. It is thus necessary to quantitatively explore the associated phenomena during fracture opening and closure as a result of temperature change. In this work, we report near-field experimental results of the effect of temperature on the hydraulic properties of natural fractures under stressed conditions (effective normal stresses of 5-25 MPa). Two specimens of naturally fractured granodiorite cores from the Grimsel Test Site in Switzerland were subjected to flow-through experiments with a temperature variation of 25-140 °C to characterize the evolution of fracture aperture/permeability. The fracture surfaces of the studied specimens were morphologically characterized using photogrammetry scanning. Periodic measurements of the efflux of dissolved minerals yield the net removal mass, which is correlated to the observed rates of fracture closure. Changes measured in hydraulic aperture are significant, exhibiting reductions of 20-75 % over the heating/cooling cycles. Under higher confining stresses, the effects in fracture permeability are irreversible and notably time-dependent. Thermally driven fracture aperture variation was more pronounced in the specimen with the largest mean aperture width and spatial correlation length. Gradual fracture compaction is likely controlled by thermal dilation, mechanical grinding, and pressure dissolution due to increased thermal stresses exerted over the contacting asperities, as confirmed by the analyses of hydraulic properties and efflux mass.

Myre, J.M., I. Lascu, E.A. Lima, J.M. Feinberg, M.O. Saar, and B.P. Weiss Using TNT-NN to Unlock the Fast Full Spatial Inversion of Large Magnetic Microscopy Datasets Earth Planets and Space, 71/14, 2019. [Download PDF] [View Abstract]Modern magnetic microscopy (MM) provides high-resolution, ultra-high sensitivity moment magnetometry, with the ability to measure at spatial resolutions better than 10^−4 m and to detect magnetic moments weaker than 10^−15 Am^2 . These characteristics make modern MM devices capable of particularly high resolution analysis of the magnetic properties of materials, but generate extremely large data sets. Many studies utilizing MM attempt to solve an inverse problem to determine the magnitude of the magnetic moments that produce the measured component of the magnetic field. Fast Fourier techniques in the frequency domain and non-negative least-squares (NNLS) methods in the spatial domain are the two most frequently used methods to solve this inverse problem. Although extremely fast, Fourier techniques can produce solutions that violate the non-negativity of moments constraint. Inversions in the spatial domain do not violate non-negativity constraints, but the execution times of standard NNLS solvers (the Lawson and Hanson method and Matlab’s lsqlin) prohibit spatial domain inversions from operating at the full spatial resolution of an MM. In this paper we present the applicability of the TNT-NN algorithm, a newly developed NNLS active set method, as a means to directly address the NNLS routine hindering existing spatial domain inversion methods. The TNT-NN algorithm enhances the performance of spatial domain inversions by accelerating the core NNLS routine. Using a conventional computing system, we show that the TNT-NN algorithm produces solutions with residuals comparable to conventional methods while reducing execution time of spatial domain inversions from months to hours or less. Using isothermal remanent magnetization measurements of multiple synthetic and natural samples, we show that the capabilities of the TNT-NN algorithm allow scans with sizes that made them previously inaccesible to NNLS techniques to be inverted. Ultimately, the TNT- NN algorithm enables spatial domain inversions of MM data on an accelerated timescale that renders spatial domain analyses for modern MM studies practical. In particular, this new technique enables MM experiments that would have required an impractical amount of inversion time such as high-resolution stepwise magnetization and demagnetization and 3-dimensional inversions.

Schädle, P., P. Zulian, D. Vogler, S. Bhopalam R., M.G.C. Nestola, A. Ebigbo, R. Krause, and M.O. Saar 3D non-conforming mesh model for flow in fractured porous media using Lagrange multipliers Computers & Geosciences, 132, pp. 42-55, 2019. [Download PDF] [View Abstract]This work presents a modeling approach for single-phase flow in 3D fractured porous media with non-conforming meshes. To this end, a Lagrange multiplier method is combined with a parallel variational transfer approach. This Lagrange multiplier method enables the use of non-conforming meshes and depicts the variable coupling between fracture and matrix domain. The variational transfer allows general, accurate, and parallel projection of variables between non-conforming meshes (i.e. between fracture and matrix domain). Comparisons of simulations with 2D benchmarks show good agreement, and the applied finite element Lagrange multiplier spaces show good performance. The method is further evaluated on 3D fracture networks by comparing it to results from conforming mesh simulations which were used as a reference. Application to realistic fracture networks with hundreds of fractures is demonstrated. Mesh size and mesh convergence are investigated for benchmark cases and 3D fracture network applications. Results demonstrate that the Lagrange multiplier method, in combination with the variational transfer approach, is capable of modeling single-phase flow through realistic 3D fracture networks.

Dambly, M.L.T., M. Nejati, D. Vogler, and M.O. Saar On the direct measurement of shear moduli in transversely isotropic rocks using the uniaxial compression test International Journal of Rock Mechanics and Mining Sciences (IJRMMS), 113, pp. 220-240, 2019. [Download PDF] [View Abstract]This paper introduces a methodology for the direct determination of the shear moduli in transversely isotropic rocks, using a single test, where a cylindrical specimen is subjected to uniaxial compression. A method is also developed to determine the orientation of the isotropy plane as well as the dynamic elastic constants using ultrasonic measurements on a single cylindrical specimen. Explicit formulae are developed to calculate the shear moduli from strain gauge measurements at different polar angles. The calculation of shear moduli from these formulae requires no knowledge about Young's moduli or Poisson's ratios and depends only on the orientation of the isotropy plane. Several strain gauge setups are designed to obtain the shear moduli from different numbers and arrangements of strain gauges. We demonstrate, that the shear moduli can be determined accurately and efficiently with only three strain gauge measurements. The orientation of the isotropy plane is measured with different methods, including ultrasonic measurements. The results show, that the isotropy plane of the tested granitic samples slightly deviates from the foliation plane. However, the foliation plane can still determine the orientation of the isotropy plane with a good approximation.

Ogland-Hand, J.D., J.M. Bielicki, Y. Wang, B.M. Adams, T.A. Buscheck, and M.O. Saar The value of bulk energy storage for reducing CO2 emissions and water requirements from regional electricity systems. Energy Conversion and Management, 181, pp. 674-685, 2019. [Download PDF] [View Abstract]The implementation of bulk energy storage (BES) technologies can help to achieve higher penetration and utilization of variable renewable energy technologies (e.g., wind and solar), but it can also alter the dispatch order in regional electricity systems in other ways. These changes to the dispatch order affect the total amount of carbon dioxide (CO2) that is emitted to the atmosphere and the amount of total water that is required by the electricity generating facilities. In a case study of the Electricity Reliability Council of Texas system, we separately investigated the value that three BES technologies (CO2- Geothermal Bulk Energy Storage, Compressed Air Energy Storage, Pumped Hydro Energy Storage) could have for reducing system-wide CO2 emissions and water requirements. In addition to increasing the utilization of wind power capacity, the dispatch of BES also led to an increase in the utilization of natural gas power capacity and of coal power capacity, and a decrease in the utilization of nuclear power capacity, depending on the character of the net load, the CO2 price, the water price, and the BES technology. These changes to the dispatch order provided positive value (e.g., increase in natural gas generally reduced CO2 emissions; decrease in nuclear utilization always decreased water requirements) or negative value (e.g., increase in coal generally increased CO2 emissions; increase in natural gas sometimes increased water requirements) to the regional electricity system. We also found that these values to the system can be greater than the cost of operating the BES facility. At present, there are mechanisms to compensate BES facilities for ancillary grid services, and our results suggest that similar mechanisms could be enacted to compensate BES facilities for their contribution to the environmental sustainability of the system.

Kittilä, A., M.R. Jalali, K.F. Evans, M. Willmann, M.O. Saar, and X.-Z. Kong Field Comparison of DNA-Labeled Nanoparticle and Solute Tracer Transport in a Fractured Crystalline Rock Water Resources Research, 2019. [Download PDF]

Ahkami, M., T. Roesgen, M.O. Saar, and X.-Z. Kong High-resolution temporo-ensemble PIV to resolve pore-scale flow in 3D-printed fractured porous media Transport in Porous Media, 129/2, pp. 467-483, 2019. [Download PDF] [View Abstract]Fractures are conduits that can enable fast advective transfer of (fluid, solute, reactant, particle, etc.) mass and energy. Such fast transfer can significantly affect pore-scale physico-chemical processes, which can in turn affect macroscopic mass and energy transport characteristics. Here, flooding experiments are conducted in a well-characterized fractured porous medium, manufactured by 3D printing. Given steady-state flow conditions, the micro-structure of the two-dimensional (2D) pore fluid flow field is delineated to resolve fluid velocities on the order of a sub-millimeter per second. We demonstrate the capabilities of a new temporo-ensemble Particle Image Velocimetry (PIV) method by maximizing its spatial resolution, employing in-line illumination. This method is advantageous as it is capable of minimizing the number of pixels, required for velocity determinations, down to one pixel, thereby enabling resolving high spatial resolutions of velocity vectors in a large field of view (FOV). While the main goal of this study is to introduce a novel experimental and velocimetry framework, this new method is then applied to specifically improve the understanding of fluid flow through fractured porous media. Histograms of measured velocities indicate log-normal and Gaussian-type distributions of longitudinal and lateral velocities in fractures, respectively. The magnitudes of fluid velocities in fractures and the flow interactions between fractures and matrices are shown to be influenced by the permeability of the background matrix and the orientation of the fractures.

Amann, F., V. Gischig, K.F. Evans, et al., A. Kittilä, S. Wiemer, M.O. Saar, S. Löw, Th. Driesner, H. Maurer, and D. Giardini The seismo-hydro-mechanical behaviour during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in-situ stimulation experiment Solid Earth, 9, pp. 115-137, 2018. [Download PDF] [View Abstract]In this contribution, we present a review of scientific research results that address seismo-hydromechanically coupled processes relevant for the development of a sustainable heat exchanger in low-permeability crystalline rock and introduce the design of the In situ Stimulation and Circulation (ISC) experiment at the Grimsel Test Site dedicated to studying such processes under controlled conditions. The review shows that research on reservoir stimulation for deep geothermal energy exploitation has been largely based on laboratory observations, large-scale projects and numerical models. Observations of full-scale reservoir stimulations have yielded important results. However, the limited access to the reservoir and limitations in the control on the experimental conditions during deep reservoir stimulations is insufficient to resolve the details of the hydromechanical processes that would enhance process understanding in a way that aids future stimulation design. Small-scale laboratory experiments provide fundamental insights into various processes relevant for enhanced geothermal energy, but suffer from (1) difficulties and uncertainties in upscaling the results to the field scale and (2) relatively homogeneous material and stress conditions that lead to an oversimplistic fracture flow and/or hydraulic fracture propagation behavior that is not representative of a heterogeneous reservoir. Thus, there is a need for intermediate-scale hydraulic stimulation experiments with high experimental control that bridge the various scales and for which access to the target rock mass with a comprehensive monitoring system is possible. The ISC experiment is designed to address open research questions in a naturally fractured and faulted crystalline rock mass at the Grimsel Test Site (Switzerland). Two hydraulic injection phases were executed to enhance the permeability of the rock mass. During the injection phases the rock mass deformation across fractures and within intact rock, the pore pressure distribution and propagation, and the microseismic response were monitored at a high spatial and temporal resolution.

Gischig, V.S., J. Doetsch, H. Maurer, H. Krietsch, F. Amman, K.F. Evans, M. Nejati, M. Jalali, B. Valley, A.C. Obermann, and S. Wiemer On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity Solid Earth, 9, pp. 39-61, 2018. [Download PDF] [View Abstract]To characterize the stress field at the Grimsel Test Site (GTS) underground rock laboratory a series of hydrofracturing test and overcoring test were performed. Hydrofracturing was accompanied by seismic monitoring using a network of highly sensitive piezosensors and accelerometers that were able to record small seismic events associated with decimeter-sized fractures. Due to potential discrepancies between the hydro-fracture orientation and stress field estimates from overcoring, it was essential to obtain high-precision hypocenter locations that reliably illuminate fracture growth. Absolute locations were improved using a transverse isotropic P-wave velocity model and by applying joint hypocenter determination that allowed computation of station corrections. We further exploited the high degree of waveform similarity of events by applying cluster analysis and relative relocation. Resulting clouds of absolute and relative located seismicity showed a consistent east-west strike and 70° dip for all hydrofractures. The fracture growth direction from microseismicity is consistent with the principal stress orientations from the overcoring stress tests provided an anisotropic elastic model for the rock mass is used in the data inversions. σ1 is significantly larger than the other two principal stresses, and has a reasonably well-defined orientation that is subparallel to the fracture plane. σ2 and σ3 are almost equal in magnitude, and thus lie on a circle defined by the standard errors of the solutions. The poles of the microseismicity planes also lie on this circle towards the north. The trace of the hydrofracture imaged at the borehole wall show that they initiated within the foliation plane, which differs in orientation from the microseismicity planes. Thus, fracture initiation was most likely influenced by a foliation-related strength anisotropy. Analysis of P-wave polarizations suggested double-couple focal mechanisms with both thrust and normal faulting mechanisms present, whereas strike-slip and thrust mechanisms would be expected from the overcoring-derived stress solution. The reasons for these discrepancies can be explained by pressure leak-off, but possibly may also involve stress field rotation around the propagating hydrofracture. Our study demonstrates that microseismicity monitoring along with high-resolution event locations provides valuable information for interpreting stress characterization measurements.

Vogler, D., S. Ostvar, R. Paustian, and B.D. Wood A Hierarchy of Models for Simulating Experimental Results from a 3D Heterogeneous Porous Medium Advances in Water Resources, 114, pp. 149-163, 2018. [Download PDF] [View Abstract]In this work we examine the dispersion of conservative tracers (bromide and fluorescein) in an experimentally-constructed three-dimensional dual-porosity porous medium. The medium is highly heterogeneous ($\sigma_Y^2=5.7$), and consists of spherical, low-hydraulic-conductivity inclusions embedded in a high-hydraulic-conductivity matrix. The bi-modal medium was saturated with tracers, and then flushed with tracer-free fluid while the effluent breakthrough curves were measured. The focus for this work is to examine a hierarchy of four models (in the absence of adjustable parameters) with decreasing complexity to assess their ability to accurately represent the measured breakthrough curves. The most information-rich model was (1) a direct numerical simulation of the system in which the geometry, boundary and initial conditions, and medium properties were fully independently characterized experimentally with high fidelity. The reduced models included; (2) a simplified numerical model identical to the fully-resolved direct numerical simulation (DNS) model, but using a domain that was one-tenth the size; (3) an upscaled mobile-immobile model that allowed for a time-dependent mass-transfer coefficient; and, (4) an upscaled mobile-immobile model that assumed a space-time constant mass-transfer coefficient. The results illustrated that all four models provided accurate representations of the experimental breakthrough curves as measured by global RMS error. The primary component of error induced in the upscaled models appeared to arise from the neglect of convection within the inclusions. We discuss the necessity to assign value (via a utility function or other similar method) to outcomes if one is to further select from among model options. Interestingly, these results suggested that the conventional convection-dispersion equation, when applied in a way that resolves the heterogeneities, yields models with high fidelity without requiring the imposition of a more complex non-Fickian model.

Vogler, D., R. R. Settgast, C. Annavarapu, C. Madonna, P. Bayer, and F. Amann Experiments and Simulations of Fully Hydro-Mechanically coupled Response of Rough Fractures exposed to High Pressure Fluid Injection Journal of Geophysical Research: Solid Earth, 123, pp. 1186-1200, 2018. [Download PDF] [View Abstract]In this work, we present the application of a fully-coupled hydro-mechanical method to investigate the effect of fracture heterogeneity on fluid flow through fractures at the laboratory scale. Experimental and numerical studies of fracture closure behavior in the presence of heterogeneous mechanical and hydraulic properties are presented. We compare the results of two sets of laboratory experiments on granodiorite specimens against numerical simulations in order to investigate the mechanical fracture closure and the hydro-mechanical effects, respectively. The model captures fracture closure behavior and predicts a non-linear increase in fluid injection pressure with loading. Results from this study indicate that the heterogeneous aperture distributions measured for experiment specimens can be used as model input for a local cubic law model in a heterogeneous fracture to capture fracture closure behavior and corresponding fluid pressure response.

Xia, T., E. Dontsov, Z. Chen, F. Zhang, and X.-Z. Kong Fluid Flow in Unconventional Gas Reservoirs Geofluids, 2018, 2018. [Download PDF] [View Abstract]Unconventional gas (including tight, shale, and coal seam gas) production has led to a drastic change of global energy landscape. The fundamental understanding of gas flow behaviours in unconventional gas reservoirs is essential to elevate the potential gas resource recovery. The behaviours of gas flow follow a chain of physicochemical processes in unconventional gas reservoirs, which can be labeled as “coupled processes” implying that one process affects the initiation and progress of another. This process chain is linked together through different disciplines, including geoscience, rock mechanics, multiphase flow, engineering chemistry, and thermodynamics, among others. Although progress on evaluation of migration, control, and recovery of unconventional gas has been achieved using mathematical models and physical experiments, the role of different fluids (eg, CH4 and other hydrocarbons, CO2, and water) in unconventional gas flow is not well understood. Filling this knowledge gap is likely to play a critical impact on raising the potential of unconventional gas resource recovery and on reducing the environmental risks.

Kling, T., D. Vogler, L. Pastewka, F. Amann, and P. Blum Numerical simulations and validation of contact mechanics in a granodiorite fracture Rock Mechanics and Rock Engineering, 51/9, pp. 2805-2824, 2018. [Download PDF] [View Abstract]Numerous rock engineering applications require a reliable estimation of fracture permeabilities to predict fluid flow and transport processes. Since measurements of fracture properties at great depth are extremely elaborate, representative fracture geometries typically are obtained from outcrops or core drillings. Thus, physically valid numerical approaches are required to compute the actual fracture geometries under in situ stress conditions. Hence, the objective of this study is the validation of a fast Fourier transform (FFT)-based numerical approach for a circular granodiorite fracture considering stress-dependent normal closure. The numerical approach employs both purely elastic and elastic--plastic contact deformation models, which are based on high-resolution fracture scans and representative mechanical properties, which were measured in laboratory experiments. The numerical approaches are validated by comparing the simulated results with uniaxial laboratory tests. The normal stresses applied in the axial direction of the cylindrical specimen vary between 0.25 and 10 MPa. The simulations indicate the best performance for the elastic--plastic model, which fits well with experimentally derived normal closure data (root-mean-squared error $=9 \mu m$). The validity of the elastic--plastic model is emphasized by a more realistic reproduction of aperture distributions, local stresses and contact areas along the fracture. Although there are differences in simulated closure for the elastic and elastic--plastic models, only slight differences in the resulting aperture distributions are observed. In contrast to alternative interpenetration models or analytical models such as the Barton--Bandis models and the ''exponential repulsion model'', the numerical simulations reproduce heterogeneous local closure as well as low-contact areas (<2\%) even at high normal stresses (10 MPa), which coincides with findings of former experimental studies. Additionally, a relative hardness value of 0.14 for granitic rocks, which defines the general resistance to non-elastic deformation of the contacts, is introduced and successfully applied for the elastic--plastic model.

Birdsell, D., S. Karra, and H. Rajaram On the Representation of the Porosity‐Pressure Relationship in General Subsurface Flow Codes Water Resources Research, 54/2, pp. 1382-1388, 2018. [Download PDF] [View Abstract]The governing equations for subsurface flow codes in a deformable porous media are derived from the balance of fluid mass and Darcy's equation. One class of these codes, which we call general subsurface flow codes (GSFs), allow for more general constitutive relations for material properties such as porosity, permeability and density. Examples of GSFs include PFLOTRAN, FEHM, TOUGH2, STOMP, and some reservoir simulators such as BOAST. Depending on the constitutive relations used in GSFs, an inconsistency arises between the standard groundwater flow equation and the governing equation of GSFs, and we clarify that the reason for this inconsistency is because the Darcy's equation used in the GSFs should account for the velocity of fluid with respect to solid. Due to lack of awareness of this inconsistency, users of the GSFs tend to use a porosity‐pressure relationship that comes from the standard groundwater flow equation and assumes that the relative velocity is already accounted for. For the Theis problem, we show that using this traditional relationship in the GSFs leads to significantly large errors. We propose an alternate porosity‐pressure relationship that is consistent with the derivation of the governing equations in the GSFs where the solid velocity is not tracked, and show that, with this relationship, the results are more accurate for the Theis problem. The purpose of this note is to make the users and developers of these GSFs aware of this inconsistency and to advocate that the alternate porosity model derived here should be incorporated in GSFs.

Matculevich, S., and M. Wolfmayr On the a posteriori error analysis for linear Fokker-Planck models in convection-dominated diffusion problems Applied Mathematics and Computation, 339, pp. 779-804, 2018. [Download PDF] [View Abstract]This work is aimed at the derivation of reliable and efficient a posteriori error estimates for convection-dominated diffusion problems motivated by a linear Fokker–Planck problem appearing in computational neuroscience. We obtain computable error bounds of functional type for the static and time-dependent case and for different boundary conditions (mixed and pure Neumann boundary conditions). Finally, we present a set of various numerical examples including discussions on mesh adaptivity and space-time discretisation. The numerical results confirm the reliability and efficiency of the error estimates derived.

Holm, B., and S. Matculevich Fully reliable error control for evolutionary problems Comput. Math. Appl. (CAMWA), 75/4, pp. 1302-1329, 2018. [Download PDF] [View Abstract]This work is focused on the application of functional-type a posteriori error estimates and corresponding indicators to a class of time-dependent problems. We consider the algorithmic part of their derivation and implementation and also discuss the numerical properties of these bounds that comply with obtained numerical results. This paper examines two different methods of solution approximation for evolutionary models, i.e., a time-marching technique and a space–time approach. The first part of the study presents an algorithm for global minimisation of the majorant on each of discretisation time-cylinders (time-slabs), the effectiveness of this approach to error estimation is confirmed by extensive numerical tests. In the second part of the publication, the application of functional error estimates is discussed with respect to a space–time approach. It is followed by a set of extensive numerical tests that demonstrates the efficiency of proposed error control method. The numerical results obtained in this paper rely on the implementation carried out using open source software, which allows formulating the problem in a weak setting. To work in a variational framework appears to be very natural for functional error estimates due to their derivation method. The search for the optimal parameters for the majorant is done by a global functional minimisation, which to the authors’ knowledge is the first work using this technique in an evolutionary framework.

Javanmard, H., M. Seyyedi, and S.M. Nielsen On Oil Recovery Mechanisms and Potential of DME–Brine Injection in the North Sea Chalk Oil Reservoirs Industrial & Engineering Chemistry Research, 57/46, pp. 15898-15908, 2018. [Download PDF] [View Abstract]North Sea tight chalk oil reservoirs are well-known for their submicron pore throat sizes and heterogeneous porosity pattern that includes fractures and microfractures. The host rock of these reservoirs is extremely sensitive and can easily react with the injected fluid, which in turn adversely affects the permeability and thus injectivity. The combined effect of these parameters makes oil production in chalk reservoirs extremely difficult. A novel solvent-based enhanced oil recovery (EOR) method that can address these issues is investigated for the first time in the chalk reservoirs. We thouroughly investigate the oil recovery potential and dominant oil recovery mechanisms by dimethyl ether (DME)–brine injection under conditions pertinent to the North Sea tight chalk oil reservoirs. A series of systematically designed high-pressure and high-temperature flooding experiments were carried out using reservoir core and crude oil. The experimental results revealed the strong oil recovery potential of tertiary DME–brine injection with two different DME contents. Furthermore, both secondary and tertiary DME–brine injection scenarios significantly improved the oil recovery with the better performance in the secondary scenario. The results show that the dominant oil recovery mechanism is rapid and strong oil swelling is caused by the preferential partitioning of DME into the oil phase. During DME–brine injection, no indications of rock mineral dissolution and adverse effects on rock permeability were observed. This is one of the advantages of this method over CO2, CO2–water alternating gas (WAG), and alkaline injections in which the EOR agent causes calcite dissolution, wormhole formation, and scaling issues in fragile chalk reservoirs.

Krietsch, H., V. Gischig, K.F. Evans, J. Doetsch, N.O. Dutler, B. Valley, and F. Amman Stress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic Methods Rock Mechanics and Rock Engineering, 2018. [Download PDF] [View Abstract]An extensive campaign to characterize rock stresses on the decameter scale was carried out in three 18–24 m long boreholes drilled from a tunnel in foliated granite at the Grimsel Test Site, Switzerland. The survey combined stress relief methods with hydrofracturing (HF) tests and concomitant monitoring of induced seismicity. Hydrofracture traces at the borehole wall were visualized with impression packer tests. The microseismic clouds indicate sub-vertical south-dipping HFs. Initial inversion of the overcoring strains with an isotropic rock model yielded stress tensors that disagreed with the HF and microseismic results. The discrepancy was eliminated using a transversely isotropic rock model, parametrized by a novel method that used numerical modelling of the in situ biaxial cell data to determine the requisite five independent elastic parameters. The results show that stress is reasonably uniform in the rock volume that lies to the south of a shear zone that cuts the NNW of the study volume. Stress in this volume is considered to be unperturbed by structures, and has principal stress magnitudes of 13.1–14.4 MPa for σ1, 9.2–10.2 MPa for σ2, and 8.6–9.7 MPa for σ3 with σ1 plunging to the east at 30–40°. To the NNW of the uniform stress regime, the minimum principal stress declines and the principal axes rotate as the shear zone is approached. The stress perturbation is clearly associated with the shear zone, and may reflect the presence of more fragmented rock acting as a compliant inclusion, or remnant stresses arising from slip on the shear zone in the past.

Matculevich, S. Functional approach to the error control in adaptive IgA schemes for elliptic boundary value problems Journal of Computational and Applied Mathematics, 344, pp. 394-423, 2018. [Download PDF] [View Abstract]This work presents a numerical study of functional type a posteriori error estimates for IgA approximation schemes in the context of elliptic boundary-value problems. Along with the detailed discussion of the most crucial properties of such estimates, we present the algorithm of a reliable solution approximation together with the scheme of an efficient a posteriori error bound generation. In this approach, we take advantage of B-(THB-) spline’s high smoothness for the auxiliary vector function reconstruction, which, at the same time, allows to use much coarser meshes and decrease the number of unknowns substantially. The most representative numerical results, obtained during a systematic testing of error estimates, are presented in the second part of the paper. The efficiency of the obtained error bounds is analysed from both the error estimation (indication) and the computational expenses points of view. Several examples illustrate that functional error estimates (alternatively referred to as the majorants and minorants of deviation from an exact solution) perform a much sharper error control than, for instance, residual-based error estimates. Simultaneously, assembling and solving routines for an auxiliary variables reconstruction, which generate the majorant (or minorant) of an error, can be executed several times faster than the routines for a primal unknown.

Comeau, M.J., J.S. Kaufl, M. Becken, A. Kuvshinov, A.V. Grayver, J. Kamm, B. Erdenechimeg, and S. Demberel Evidence for fluid and melt generation in response to an asthenospheric upwelling beneath the Hangai Dome, Mongolia Earth and Planetary Science Letters, 487, pp. 201-209, 2018. [Download PDF] [View Abstract]The Hangai Dome, Mongolia, is an unusual high-elevation, intra-continental plateau characterized by dispersed, low-volume, intraplate volcanism. Its subsurface structure and its origin remains unexplained, due in part to a lack of high-resolution geophysical data. Magnetotelluric data along a ∼610 km profile crossing the Hangai Dome were used to generate electrical resistivity models of the crust and upper mantle. The crust is found to be unexpectedly heterogeneous. The upper crust is highly resistive but contains several features interpreted as ancient fluid pathways and fault zones, including the South Hangai fault system and ophiolite belt that is revealed to be a major crustal boundary. South of the Hangai Dome a clear transition in crustal properties is observed which reflects the rheological differences across accreted terranes. The lower crust contains discrete zones of low-resistivity material that indicate the presence of fluids and a weakened lower crust. The upper mantle contains a large low-resistivity zone that is consistent with the presence of partial melt within an asthenospheric upwelling, believed to be driving intraplate volcanism and supporting uplift.

Mikutis, G., C.A. Deuber, L. Schmid, A. Kittilä, N. Lobsiger, M. Puddu, D.O. Asgeirsson, R.N. Grass, M.O. Saar, and W.J. Stark Silica-encapsulated DNA-based tracers for aquifer characterization Environmental Science & Technology, 52, pp. 12142-12152, 2018. [Download PDF] [View Abstract]Environmental tracing is a direct way to characterize aquifers, evaluate the solute transfer parameter in underground reservoirs, and track contamination. By performing multitracer tests, and translating the tracer breakthrough times into tomographic maps, key parameters such as a reservoir’s effective porosity and permeability field may be obtained. DNA, with its modular design, allows the generation of a virtually unlimited number of distinguishable tracers. To overcome the insufficient DNA stability due to microbial activity, heat, and chemical stress, we present a method to encapsulated DNA into silica with control over the particle size. The reliability of DNA quantification is improved by the sample preservation with NaN3 and particle redispersion strategies. In both sand column and unconsolidated aquifer experiments, DNA-based particle tracers exhibited slightly earlier and sharper breakthrough than the traditional solute tracer uranine. The reason behind this observation is the size exclusion effect, whereby larger tracer particles are excluded from small pores, and are therefore transported with higher average velocity, which is pore size-dependent. Identical surface properties, and thus flow behavior, makes the new material an attractive tracer to characterize sandy groundwater reservoirs or to track multiple sources of contaminants with high spatial resolution.

Hobé, A., D. Vogler, M.P. Seybold, A. Ebigbo, R.R. Settgast, and M.O. Saar Estimating Fluid Flow Rates through Fracture Networks using Combinatorial Optimization Advances in Water Resources, 122, pp. 85-97, 2018. [Download PDF] [View Abstract]To enable fast uncertainty quantification of fluid flow in a discrete fracture network (DFN), we present two approaches to quickly compute fluid flow in DFNs using combinatorial optimization algorithms. Specifically, the presented Hanan Shortest Path Maxflow (HSPM) and Intersection Shortest Path Maxflow (ISPM) methods translate DFN geometries and properties to a graph on which a max flow algorithm computes a combinatorial flow, from which an overall fluid flow rate is estimated using a shortest path decomposition of this flow. The two approaches are assessed by comparing their predictions with results from explicit numerical simulations of simple test cases as well as stochastic DFN realizations covering a range of fracture densities. Both methods have a high accuracy and very low computational cost, which can facilitate much-needed in-depth analyses of the propagation of uncertainty in fracture and fracture-network properties to fluid flow rates.

Rossi, E., M.A. Kant, C. Madonna, M.O. Saar, and Ph. Rudolf von Rohr The Effects of High Heating Rate and High Temperature on the Rock Strength: Feasibility Study of a Thermally Assisted Drilling Method Rock Mechanics and Rock Engineering, 51/9, pp. 2957-2964 , 2018. [Download PDF] [View Abstract]In this paper, the feasibility of a thermally assisted drilling method is investigated. The working principle of this method is based on the weakening effect of a flame-jet to enhance the drilling performance of conventional, mechanical drilling. To investigate its effectiveness, we study rock weakening after rapid, localized flame-jet heating of Rorschach sandstone and Central Aare granite. We perform experiments on rock strength after flame treatments in comparison to oven heating, for temperatures up to 650 \(^{\circ} \)C and heating rates from 0.17 to 20 \(^{\circ} \)C/s. The material hardening, commonly observed at moderate temperatures after oven treatments, can be suppressed by flame heating the material at high heating rates. Our study highlights the influence of the heating rate on the mechanism of thermal microcracking. High heating rate, flame treatments appear to mostly induce cracks at the grain boundaries, opposed to slow oven treatments, where also a considerable number of intragranular cracks are found. Herewith, we postulate that at low heating rates, thermal expansion stresses cause the observed thermal cracking. In contrast, at higher heating rates, thermal cracking is dominated by the stress concentrations caused by high thermal gradients.

Samrock, F., A.V. Grayver, H. Eysteinsson, and M.O. Saar Magnetotelluric image of transcrustal magmatic system beneath the Tulu Moye geothermal prospect in the Ethiopian Rift Geophysical Research Letters, 2018. [Download PDF] [View Abstract]Continental rifting is initiated by a dynamic interplay between tectonic stretching and mantle upwelling. Decompression melting assists continental break-up through lithospheric weakening and enforces upflow of melt to the Earth’s surface. However, the details about melt transport through the brittle crust and storage under narrow rift-aligned magmatic segments remain largely unclear. Here we present a crustal scale electrical conductivity model for a magmatic segment in the Ethiopian Rift, derived from 3-D phase tensor inversion of magnetotelluric data. Our subsurface model shows that melt migrates along pre-existing weak structures and is stored in different concentrations on two major interconnected levels, facilitating the formation of a convective hydrothermal system. The obtained model of a transcrustal magmatic system offers new insights into rifting mechanisms, evolution of magma ascent, and prospective geothermal reservoirs.

Kong, X.-Z., C. Deuber, A. Kittilä, M. Somogyvari, G. Mikutis, P. Bayer, W.J. Stark, and M.O. Saar Tomographic reservoir imaging with DNA-labeled silica nanotracers: The first field validation Environmental Science &Technology, 52/23, pp. 13681-13689, 2018. [Download PDF] [View Abstract]This study presents the first field validation of using DNA-labeled silica nanoparticles as tracers to image subsurface reservoirs by travel time based tomography. During a field campaign in Switzerland, we performed short-pulse tracer tests under a forced hydraulic head gradient to conduct a multisource−multireceiver tracer test and tomographic inversion, determining the two-dimensional hydraulic conductivity field between two vertical wells. Together with three traditional solute dye tracers, we injected spherical silica nanotracers, encoded with synthetic DNA molecules, which are protected by a silica layer against damage due to chemicals, microorganisms, and enzymes. Temporal moment analyses of the recorded tracer concentration breakthrough curves (BTCs) indicate higher mass recovery, less mean residence time, and smaller dispersion of the DNA-labeled nanotracers, compared to solute dye tracers. Importantly, travel time based tomography, using nanotracer BTCs, yields a satisfactory hydraulic conductivity tomogram, validated by the dye tracer results and previous field investigations. These advantages of DNA-labeled nanotracers, in comparison to traditional solute dye tracers, make them well-suited for tomographic reservoir characterizations in fields such as hydrogeology, petroleum engineering, and geothermal energy, particularly with respect to resolving preferential flow paths or the heterogeneity of contact surfaces or by enabling source zone characterizations of dense nonaqueous phase liquids.

Kant, M.A., E. Rossi, J. Duss, F. Amman, M.O. Saar, and P. Rudolf von Rohr Demonstration of thermal borehole enlargement to facilitate controlled reservoir engineering for deep geothermal, oil or gas systems Applied Energy, 212, pp. 1501-1509, 2018. [Download PDF] [View Abstract]The creation of deep reservoirs for geothermal energy or oil and gas extraction is impeded by insu cient stimulation. Direction and extension of the created fractures are complex to control and, therefore, large stimulated and interconnected fracture networks are di cult to create. This poses an inherent risk of un- economic reservoirs, due to insu cient heat-sweep surfaces or hydraulic shortcuts. Therefore, we present a new technique, which locally increases the cross section of a borehole by utilizing a thermal spallation process on the sidewalls of the borehole. By controlled and local enlargement of the well bore diameter, initial fracture sources are created, potentially reducing the injection pressure during stimulation, initiating fracture growth, optimizing fracture propagation and increasing the number of accessible preexisting frac- tures. Consequently, local thermal borehole enlargement reduces project failure risks by providing better control on subsequent stimulation processes. In order to show the applicability of the suggested technique, we conducted a shallow field test in an underground rock laboratory. Two types of borehole enlargements were created in a 14.5 m deep borehole, confirming that the technology is applicable, with implications for improving the productivity of geothermal, oil and gas reservoirs.

Cui, G., S. Ren, L. Zhang, J. Ezekiel, C. Enechukwu, Y. Wang, and R. Zhang Geothermal exploitation from hot dry rocks via recycling heat transmission fluid in a horizontal well Energy, 128, pp. 366-377, 2017. [Download PDF] [View Abstract]A new method for geothermal exploitation from hot dry rocks by recycling heat transmission fluid in a horizontal well via a closed loop is proposed, in which the costly and complex hydro-fracturing can be avoided. In this paper, numerical simulation models were established to calculate the heat mining rate for the new technology to assess its technical and economic feasibility. Sensitivity studies were performed to analyze the effects of various parameters on heat mining rate, including the injection rate, the horizontal segment length and the thermal conductivity of the tubing. The results show that a high heat mining rate over 1.7 MW can be obtained using a 3000 m long horizontal well to extract geothermal energy from a typical hot dry rock of 235 °C with a water circulation rate of 432 m3/d. For low-temperature geothermal reservoirs, higher injection rate, longer horizontal wells and better thermal insulation of tubing can be applied to increase the heat mining rate. The cost of geothermal power generation using a single horizontal well is estimated as 0.122 $/kWh, and this could be further reduced to 0.084 $/kWh when the multi-branch horizontal well pattern was adopted, slightly lower than a fractured vertical well case.

Niederau, J., A. Ebigbo, G. Marquart, J. Arnold, and C. Clauser On the impact of spatially heterogenous permeability on free convection in the Perth Basin, Australia Geothermics, 66, pp. 119-133, 2017. [Download PDF] [View Abstract]We study the impact of spatially heterogeneous permeability on the formation and shape of hydrothermal porous flow convection in the Yarragadee Aquifer by modelling three simulation scenarios, each with differing permeability distributions. In all scenarios, the southern part of the model is characterised by convection rolls, while the north is dominated by a stable region of decreased temperatures at depth due to hydraulic interaction with shallower aquifers. This suggests that reservoir structure is a first-order controlling factor for the formation of the free con- vective system. The convective system adjusts to the spatially heterogeneous permeability distribution, yielding locally different convection patterns.

Büsing, H., C. Vogt, A. Ebigbo, and N. Kitzsch Numerical study on CO2 leakage detection using electrical streaming potential data Water Resour. Res, 53, pp. 1-15, 2017. [Download PDF] [View Abstract]We study the feasibility of detecting carbon dioxide (CO2) movement in the overburden of a storage reservoir due to CO2 leakage through an abandoned well by self-potential (SP) measurements at the surface. This is achieved with three-dimensional numerical (SP) modeling of two-phase fluid flow and electrokinetic coupling between flow and streaming potential. We find that, in typical leakage scenarios, for leaky and/or injection wells with conductive metal casing, self-potential signals originating from injection can be identified at the surface. As the injection signal is also observed at the leaky well with metal casing, SP monitoring can be applied for detecting abandoned wells. However, leakage signals are much smaller than the injection signal and thus masked by the latter. We present three alternatives to overcome this problem: (i) simulate the streaming potential of the nonleaky scenario and subtract the result from the measured streaming potential data; (ii) exploit the symmetry of the injection signal by analyzing the potential difference of dipoles with the dipole center at the injection well; or (iii) measure SP during periods where the injection is interrupted. In our judgement, the most promising approach for detecting a real-world CO2 leakage is by combining methods (i) and (ii), because this would give the highest signal from the leakage and omit signals originating from the injection well. Consequently, we recommend SP as monitoring method for subsurface CO2 storage, especially because a leakage can be detected shortly after the injection started even before CO2 arrives at the leaky well.

Ezekiel, J., R. Shaoran, Z. Liang, and W. Yuting Displacement Mechanisms of Air Injection for IOR in Low Permeability Light Oil Reservoirs International Journal of Oil, Gas and Coal Technology, 16/1, pp. 1-26, 2017. [Download PDF] [View Abstract]Air injection into light oil reservoirs has been proven to be a valuable improved oil recovery (IOR) process and is being successfully implemented worldwide in many oilfields. It specially offers unique technical and economic opportunities for tertiary or secondary oil recovery in light oil reservoirs with low permeability in which conventional water injection techniques have been unsuccessful and/or uneconomical. This paper provides a comprehensive overview on the oxidation and IOR process of air injection into low permeability light oil reservoir based on detailed analysis of some field projects and the results of laboratory testing and reservoir simulation of a typical light oil reservoir, the Q131 Block. The reaction mechanisms of low temperature oxidation (LTO) and high temperature oxidation (HTO or in-situ combustion) are particularly addressed in this study. Air flooding displacement efficiency experiment was carried out without water injection, and an oil recovery of more than 40% of hydrocarbon pore volume (HCPV) was observed. A series of high-pressure oxidation experiments using the typical light oil were conducted in the temperature range of 98°C to 180°C. The results showed high oxidation and carbon dioxide (CO2) conversion rates, which are both favourable in terms of oxygen consumption. A conceptual full field compositional reservoir simulation model of the targeted low permeability block was also used to examine the reaction schemes, thermal effect of LTO reactions and IOR mechanisms.

Vogler, D., S.D.C. Walsh, E. Dombrovski, and M.A. Perras A Comparison of Tensile Failure in 3D-Printed and Natural Sandstone Engineering Geology, 226, pp. 221-235, 2017. [Download PDF] [View Abstract]This work investigates the possibility of replication of natural rock specimens, which can be used to analyze rock mechanical behavior by subjecting a number of identical specimens to tensile tests and a variety of analysis methods. We compare the properties of fractures generated in artificial sandstone specimens to those generated in natural sandstone specimens. Artificial sandstone specimens, created using 3D additive manufacturing printing processes, were subject to tensile failure using the Brazilian test method and the results from these tests were compared to results from Brazilian tests conducted on natural sandstones. The specimens included two distinct types of synthetic rock, unaltered from the manufacturers typical process, and three natural sandstones. For each test, the loading history to failure of the specimens were recorded and the failure mode was confirmed using digital imaging techniques. In addition, three dimensional images were taken of the fracture surfaces, which were then used to compare the geometric characteristics of all materials tested. The indirect tensile strength of the artificial sandstone specimens ranged between 1.0 and 2.8 MPa. Natural sandstone specimens with a wide range of indirect tensile strengths were tested for comparison. These included a strong sandstone, an intermediate sandstone, and a weak sandstone; which were found to have indirect tensile strength ranges of 10.5-25.5 MPa, 4.4-6.4 MPa, and 0.9-1.1 MPa, respectively. Digital image correlation confirmed that the artificial specimens generally failed in a tensile (mode I) fracture, similar to the natural specimens. Likewise, fracture surface roughness measures showed no clear distinction between weak natural and artificial sandstones. This indicates that there are distinct similarities between the fractures generated in the natural and artificial sandstones of comparable indirect tensile strengths. The three dimensionally printed sandstone specimens are shown to exhibit indirect tensile strength, surface roughness and crack propagation behavior which resembles a weak natural sandstone.

Xu, R.N., R. Li, J. Ma, D. He, and P.X. Jiang Effect of Mineral Dissolution/Precipitation and CO2 Exsolution on CO2 transport in Geological Carbon Storage ACCOUNTS OF CHEMICAL RESEARCH, 50/9, pp. 2056-2066, 2017. [Download PDF] [View Abstract]Geological carbon sequestration (GCS) in deep saline aquifers is an effective means for storing carbon dioxide to address global climate change. As the time after injection increases, the safety of storage increases as the CO2 transforms from a separate phase to CO2(aq) and HCO3- by dissolution and then to carbonates by mineral dissolution. However, subsequent depressurization could lead to dissolved CO2(aq) escaping from the formation water and creating a new separate phase which may reduce the GCS system safety. The mineral dissolution and the CO2 exsolution and mineral precipitation during depressurization change the morphology, porosity, and permeability of the porous rock medium, which then affects the two-phase flow of the CO2 and formation water. A better understanding of these effects on the CO2 water two-phase flow will improve predictions of the long-term CO2 storage reliability, especially the impact of depressurization on the long-term stability. In this Account, we summarize our recent work on the effect of CO2 exsolution and mineral dissolution/precipitation on CO2 transport in GCS reservoirs. We place emphasis on understanding the behavior and transformation of the carbon components in the reservoir, including CO2(sc/g), CO2(aq), HCO3-, and carbonate minerals (calcite and dolomite), highlight their transport and mobility by coupled geochemical and two-phase flow processes, and consider the implications of these transport mechanisms on estimates of the long-term safety of GCS. We describe experimental and numerical pore- and core-scale methods used in our lab in conjunction with industrial and international partners to investigate these effects. Experimental results show how mineral dissolution affects permeability, capillary pressure, and relative permeability, which are important phenomena affecting the input parameters for reservoir flow modeling. The porosity and the absolute permeability increase when CO2 dissolved water is continuously injected through the core. The MRI results indicate dissolution of the carbonates during the experiments since the porosity has been increased after the core-flooding experiments. The mineral dissolution changes the pore structure by enlarging the throat diameters and decreasing the pore specific surface areas, resulting in lower CO2/water capillary pressures and changes in the relative permeability. When the reservoir pressure decreases, the CO2 exsolution occurs due to the reduction of solubility. The CO2 bubbles preferentially grow toward the larger pores instead of toward the throats or the finer pores during the depressurization. After exsolution, the exsolved CO2 phase shows low mobility due to the highly dispersed pore-scale morphology, and the well dispersed small bubbles tend to merge without interface contact driven by the Ostwald ripening mechanism. During depressurization, the dissolved carbonate could also precipitate as a result of increasing pH. There is increasing formation water flow resistance and low mobility of the CO2 in the presence of CO2 exsolution and carbonate precipitation. These effects produce a self-sealing mechanism that may reduce unfavorable CO2 migration even in the presence of sudden reservoir depressurization.

Vogler, D., S.D.C. Walsh, P. Bayer, and F. Amann Comparison of Surface Properties in Natural and Artificially Generated Fractures in a Crystalline Rock Rock Mechanics and Rock Engineering, 50/11, pp. 2891-2909, 2017. [Download PDF] [View Abstract]This work studies the roughness characteristics of fracture surfaces from a crystalline rock by analyzing differences in surface roughness between fractures of various types and sizes. We compare the surface properties of natural fractures sampled in situ and artificial (i.e., man-made) fractures created in the same source rock under laboratory conditions. The topography of the various fracture types is compared and characterized using a range of different measures of surface roughness. Both natural and artificial, and tensile and shear fractures are considered, along with the effects of specimen size on both the geometry of the fracture and its surface characterization. The analysis shows that fracture characteristics are substantially different between natural shear and artificial tensile fractures, while natural tensile fracture often spans the whole result domain of the two other fracture types. Specimen size effects are also evident, not only as scale sensitivity in the roughness metrics, but also as a by-product of the physical processes used to generate the fractures. Results from fractures generated with Brazilian tests show that fracture roughness at small scales differentiates fractures from different specimen sizes and stresses at failure.

Schaedle, P., T. Kaempfer, G. Pepin, J. Wendling, and J. Bommundt Combining high-resolution two-phase with simplified single-phase simulations in order to optimize the performance of PA/SA simulations for a deep geological repository for radioactive waste Geological Society, London, Special Publications, 433/443/SP443.4, 2017. [Download PDF] [View Abstract]The transport of a radioactive solute during the transient thermo-hydraulic regime with gas generation in and around a disposal cell depends on complex multi-phase processes. Numerical simulations can improve the understanding of the system by providing detailed information on the temporal and spatial distribution of the radionuclides. In particular, their fluxes can be computed under the given transient conditions considering radionuclide, heat and gas release from the waste. However, such detailed multi-phase simulations are very demanding with respect to com- putational resources and time. Based on the knowledge gained from such complex simulations, we have developed a robust simplified single-phase approach for performance and safety assessment, the improved efficiency of which enables extensive parameter studies. The simplified approach comprises, on the one hand, homogenization of features of high detail and, on the other hand, the employment of two-phase simulation results that are used to deduce equivalent single-phase parameterizations. The results have been validated with various benchmark criteria at well-defined interfaces in the modelled disposal cells based on the simulated radionuclides fluxes.

Rudolf von Rohr, Ph., M. Kant, and E. Rossi An apparatus for thermal spallation of a borehole Patent EP3450675A1, 2017.

Walsh, S.D.C., N. Garapati, A.M.M. Leal, and M.O. Saar Calculating thermophysical fluid properties during geothermal energy production with NESS and Reaktoro Geothermics, 70, pp. 146-154, 2017. [Download PDF] [View Abstract]We investigate how subsurface fluids of different compositions affect the electricity generation of geothermal power plants. First, we outline a numerical model capable of accounting for the thermophysical properties of geothermal fluids of arbitrary composition within simulations of geothermal power production. The behavior of brines with varying compositions from geothermal sites around the globe are then examined using the model. The effect of each brine on an idealized binary geothermal power plant is simulated, and their performances compared by calculating the amount of heat exchanged from the fluid to the plant's secondary cycle. Our simulations combine (1) a newly developed Non-linear Equation System Solver (NESS), for simulating individual geothermal power plant components, (2) the advanced geochemical speciation solver, Reaktoro, used for calculation of thermodynamic fluid properties, and (3) compositional models for the calculation of fluid-dynamical properties (e.g., viscosity as a function of temperature and brine composition). The accuracy of the model is verified by comparing its predictions with experimental data from single-salt, binary-salt, and multiple-salt solutions. The geothermal power plant simulations show that the brines considered in this study can be divided into three main categories: (1) those of largely meteoric origin with low salinity for which the effect of salt concentration is negligible; (2) moderate-depth brines with high concentrations of Na+ and K+ ions, whose performance is well approximated by pure NaCl solutions of equivalent salinity; and (3) deeper, high-salinity brines that require a more detailed consideration of their composition for accurate simulation of plant operations.

Leal, A.M.M., D.A. Kulik, W.R. Smith, and M.O. Saar An overview of computational methods for chemical equilibrium and kinetic calculations for geochemical and reactive transport modeling Pure and Applied Chemistry, 89/5, pp. 597-643, 2017. [Download PDF] [View Abstract]We present an overview of novel numerical methods for chemical equilibrium and kinetic calculations for complex non-ideal multiphase systems. The methods we present for equilibrium calculations are based either on Gibbs energy minimization (GEM) calculations or on solving the system of extended law of mass-action (xLMA) equations. In both methods, no a posteriori phase stability tests, and thus no tentative addition or removal of phases during or at the end of the calculations, are necessary. All potentially stable phases are considered from the beginning of the calculation, and stability indices are immediately available at the end of the computation to determine which phases are actually stable at equilibrium. Both GEM and xLMA equilibrium methods are tailored for computationally demanding applications that require many rapid local equilibrium calculations, such as reactive transport modeling. The numerical method for chemical kinetic calculations we present supports both closed and open systems, and it considers a partial equilibrium simplification for fast reactions. The method employs an implicit integration scheme that improves stability and speed when solving the often stiff differential equations in kinetic calculations. As such, it requires compositional derivatives of the reaction rates to assemble the Jacobian matrix of the resultant implicit algebraic equations that are solved at every time step. We present a detailed procedure to calculate these derivatives, and we show how the partial equilibrium assumption affects their computation. These numerical methods have been implemented in Reaktoro (, an open-source software for modeling chemically reactive systems. We finish with a discussion on the comparison of these methods with others in the literature.

Myre, J.M., E. Frahm, D.J. Lilja, and M.O. Saar TNT-NN: A Fast Active Set Method for Solving Large Non-Negative Least Squares Problems Procedia Computer Science, 108C, pp. 755-764, 2017. [Download PDF] [View Abstract]In 1974 Lawson and Hanson produced a seminal active set strategy to solve least-squares prob- lems with non-negativity constraints that remains popular today. In this paper we present TNT-NN, a new active set method for solving non-negative least squares (NNLS) problems. TNT-NN uses a different strategy not only for the construction of the active set but also for the solution of the unconstrained least squares sub-problem. This results in dramatically improved performance over traditional active set NNLS solvers, including the Lawson and Hanson NNLS algorithm and the Fast NNLS (FNNLS) algorithm, allowing for computational investigations of new types of scientific and engineering problems. For the small systems tested (5000 × 5000 or smaller), it is shown that TNT-NN is up to 95× faster than FNNLS. Recent studies in rock magnetism have revealed a need for fast NNLS algorithms to address large problems (on the order of 105 × 105 or larger). We apply the TNT- NN algorithm to a representative rock magnetism inversion problem where it is 60× faster than FNNLS. We also show that TNT-NN is capable of solving large (45000 × 45000) problems more than 150× faster than FNNLS. These large test problems were previously considered to be unsolvable, due to the excessive execution time required by traditional methods.

Luhmann, A.J., B.M. Tutolo, C. Tan, B.M. Moskowitz, M.O. Saar, and W.E. Seyfried, Jr. Whole rock basalt alteration from CO2-rich brine during flow-through experiments at 150°C and 150 bar Chemical Geology, 453, pp. 92-110, 2017. [Download PDF] [View Abstract]Four flow-through experiments at 150 °C were conducted on intact cores of basalt to assess alteration and mass transfer during reaction with CO2-rich fluid. Two experiments used a flow rate of 0.1 ml/min, and two used a flow rate of 0.01 ml/min. Permeability increased for both experiments at the higher flow rate, but decreased for the lower flow rate experiments. The experimental fluid (initial pH of 3.3) became enriched in Si, Mg, and Fe upon passing through the cores, primarily from olivine and titanomagnetite dissolution and possibly pyroxene dissolution. Secondary minerals enriched in Al and Si were present on post-experimental cores, and an Fe2O3-rich phase was identified on the downstream ends of the cores from the experiments at the lower flow rate. While we could not specifically identify if siderite (FeCO3) was present in the post-experimental basalt cores, siderite was generally saturated or supersaturated in outlet fluid samples, suggesting a thermodynamic drive for Fe carbonation from basalt-H2O-CO2 reaction. Reaction path models that employ dissolution kinetics of olivine, labradorite, and enstatite also suggest siderite formation at low pH. Furthermore, fluid-rock interaction caused a relatively high mobility of the alkali metals; up to 29% and 99% of the K and Cs present in the core, respectively, were preferentially dissolved from the cores, likely due to fractional crystallization effects that made alkali metals highly accessible. Together, these datasets illustrate changes in chemical parameters that arise due to fluid-basalt interaction in relatively low pH environments with elevated CO2.

Luhmann, A.J., B.M. Tutolo, B.C. Bagley, D.F.R. Mildner, W.E. Seyfried Jr., and M.O. Saar Permeability, porosity, and mineral surface area changes in basalt cores induced by reactive transport of CO2-rich brine Water Resources Research, 53, pp. 1-20, 2017. [Download PDF] [View Abstract]Four reactive flow-through laboratory experiments (two each at 0.1 mL/min and 0.01 mL/min flow rates) at 150°C and 150 bar (15 MPa) are conducted on intact basalt cores to assess changes in porosity, permeability, and surface area caused by CO2-rich fluid-rock interaction. Permeability decreases slightly during the lower flow rate experiments and increases during the higher flow rate experiments. At the higher flow rate, core permeability increases by more than one order of magnitude in one experiment and less than a factor of two in the other due to differences in preexisting flow path structure. X-ray computed tomography (XRCT) scans of pre- and post-experiment cores identify both mineral dissolution and secondary mineralization, with a net decrease in XRCT porosity of ∼0.7%–0.8% for the larger pores in all four cores. (Ultra) small-angle neutron scattering ((U)SANS) data sets indicate an increase in both (U)SANS porosity and specific surface area (SSA) over the ∼1 nm to 10 µm scale range in post-experiment basalt samples, with differences due to flow rate and reaction time. Net porosity increases from summing porosity changes from XRCT and (U)SANS analyses are consistent with core mass decreases. (U)SANS data suggest an overall preservation of the pore structure with no change in mineral surface roughness from reaction, and the pore structure is unique in comparison to previously published basalt analyses. Together, these data sets illustrate changes in physical parameters that arise due to fluid-basalt interaction in relatively low pH environments with elevated CO2 concentration, with significant implications for flow, transport, and reaction through geologic formations.

Kuvshinov, A., J. Matzka, B. Poedjono, F. Samrock, N. Olsen, and S. Pai Probing Earth’s conductivity structure beneath oceans by scalar geomagnetic data: autonomous surface vehicle solution Earth, Planets and Space, 68 (1)/189, 2016. [Download PDF]

Vogler, D., F. Amann, P. Bayer, and D. Elsworth Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation Rock Mechanics and Rock Engineering, 49/9, pp. 3463-3479, 2016. [Download PDF] [View Abstract]Increasing fracture aperture by lowering effective normal stress and by inducing dilatant shearing and thermo-elastic effects is essential for transmissivity increase in enhanced geothermal systems. This study investigates transmissivity evolution for fluid flow through natural fractures in granodiorite at the laboratory scale. Processes that influence transmissivity are changing normal loads, surface deformation, the formation of gouge and fracture offset. Normal loads were varied in cycles between 1 and 68 MPa and cause transmissivity changes of up to three orders of magnitude. Similarly, small offsets of fracture surfaces of the order of millimeters induced changes in transmissivity of up to three orders of magnitude. During normal load cycling, the fractures experienced significant surface deformation, which did not lead to increased matedness for most experiments, especially for offset fractures. The resulting gouge material production may have caused clogging of the main fluid flow channels with progressing loading cycles, resulting in reductions of transmissivity by up to one order of magnitude. During one load cycle, from low to high normal loads, the majority of tests show hysteretic behavior of the transmissivity. This effect is stronger for early load cycles, most likely when surface deformation occurs, and becomes less pronounced in later cycles when asperities with low asperity strength failed. The influence of repeated load cycling on surface deformation is investigated by scanning the specimen surfaces before and after testing. This allows one to study asperity height distribution and surface deformation by evaluating the changes of the standard deviation of the height, distribution of asperities and matedness of the fractures. Surface roughness, as expressed by the standard deviation of the asperity height distribution, increased during testing. Specimen surfaces that were tested in a mated configuration were better mated after testing, than specimens tested in shear offset configuration. The fracture surface deformation of specimen surfaces that were tested in an offset configuration was dominated by the breaking of individual asperities and grains, which did not result in better mated surfaces.

Ebigbo, A., J. Niederau, G. Marquart, I. Dini, M. Thorwart, W. Rabbel, R. Pechnig, R. Bertani, and C. Clauser Influence of depth, temperature, and structure of a crustal heat source on the geothermal reservoirs of Tuscany: numerical modelling and sensitivity study Geothermal Energy, 4/5, 2016. [Download PDF] [View Abstract]Granitoid intrusions are the primary heat source of many deep geothermal reservoirs in Tuscany. The depth and shape of these plutons, characterised in this study by a prominent seismic reflector (the K horizon), may vary significantly within the spatial scale of interest. In an exploration field, simulations reveal the mechanisms by which such a heat source influences temperature distribution. A simple analysis quantifies the sensitivity of potentially measurable indicators (i.e. vertical temperature profiles and surface heat flow) to variations in depth, temperature, and shape of the heat source within given ranges of uncertainty.

Seidler, R., K. Padalkina, H.M. Buecker, A. Ebigbo, M. Herty, G. Marquart, and J. Niederau Optimal experimental design for reservoir property estimates in geothermal exploration Computational Geomechanics, 20/375, 2016. [Download PDF] [View Abstract]During geothermal reservoir development, drilling deep boreholes turns out to be extremely expensive and risky. Thus, it is of great importance to work out the details of suitable borehole locations in advance. Here, given a set of existing boreholes, we demonstrate how a sophisticated numerical technique called optimal experimental design helps to find a location of an additional exploratory borehole that reduces risk and, ultimately, saves cost. More precisely, the approach minimizes the uncertainty when deducing the effective permeability of a buried reservoir layer from a temperature profile measured in this exploratory borehole. In this paper, we (1) outline the mathematical formulation in terms of an optimization problem, (2) describe the numerical implementation involving various software components, and (3) apply the method to a 3D numerical simulation model representing a real geothermal reservoir in northern Italy. Our results show that optimal experimental design is conceptually and computationally feasible for industrial-scale applications. For the particular reservoir and the estimation of permeability from temperature, the optimal location of the additional borehole coincides with regions of high flow rates and large deviations from the mean temperature of the reservoir layer in question. Finally, the presentation shows that, methodologically, the optimization method can be generalized from estimating permeability to finding any other reservoir properties.

Ebigbo, A., P.A. Lang, A. Paluszny, and R.W. Zimmerman Inclusion-based effective medium models for the permeability of a 3D fractured rock mass Transport in Porous Media, 113/1, pp. 137-158, 2016. [Download PDF] [View Abstract]Effective permeability is an essential parameter for describing fluid flow through fractured rock masses. This study investigates the ability of classical inclusion-based effective medium models (following the work of Sævik et al. in Transp Porous Media 100(1):115–142, 2013. doi:10.1007/s11242-013-0208-0) to predict this permeability, which depends on several geometric properties of the fractures/networks. This is achieved by comparison of various effective medium models, such as the symmetric and asymmetric self-consistent schemes, the differential scheme, and Maxwell’s method, with the results of explicit numerical simulations of mono- and poly-disperse isotropic fracture networks embedded in a permeable rock matrix. Comparisons are also made with the Hashin–Shtrikman bounds, Snow’s model, and Mourzenko’s heuristic model (Mourzenko et al. in Phys Rev E 84:036–307, 2011. doi:10.1103/PhysRevE.84.036307). This problem is characterised by two small parameters, the aspect ratio of the spheroidal fractures, α, and the ratio between matrix and fracture permeability, κ. Two different regimes can be identified, corresponding to α/κ<1 and α/κ>1. The lower the value of α/κ, the more significant is flow through the matrix. Due to differing flow patterns, the dependence of effective permeability on fracture density differs in the two regimes. When α/κ≫1, a distinct percolation threshold is observed, whereas for α/κ≪1, the matrix is sufficiently transmissive that such a transition is not observed. The self-consistent effective medium methods show good accuracy for both mono- and polydisperse isotropic fracture networks. Mourzenko’s equation is very accurate, particularly for monodisperse networks. Finally, it is shown that Snow’s model essentially coincides with the Hashin–Shtrikman upper bound.

Leal, A.M.M., D.A. Kulik, and G. Koskowski Computational methods for reactive transport modeling: A Gibbs energy minimization approach for multiphase equilibrium calculations Advances in Water Resources, 88, pp. 231-240, 2016. [Download PDF] [View Abstract]We present a numerical method for multiphase chemical equilibrium calculations based on a Gibbs energy minimization approach. The method can accurately and efficiently determine the stable phase assemblage at equilibrium independently of the type of phases and species that constitute the chemical system. We have successfully applied our chemical equilibrium algorithm in reactive transport simulations to demonstrate its effective use in computationally intensive applications. We used FEniCS to solve the governing partial differential equations of mass transport in porous media using finite element methods in unstructured meshes. Our equilibrium calculations were benchmarked with GEMS3K, the numerical kernel of the geochemical package GEMS. This allowed us to compare our results with a well-established Gibbs energy minimization algorithm, as well as their performance on every mesh node, at every time step of the transport simulation. The benchmark shows that our novel chemical equilibrium algorithm is accurate, robust, and efficient for reactive transport applications, and it is an improvement over the Gibbs energy minimization algorithm used in GEMS3K. The proposed chemical equilibrium method has been implemented in Reaktoro, a unified framework for modeling chemically reactive systems, which is now used as an alternative numerical kernel of GEMS.

Katika, K., M. Ahkami, P.L. Fosbol, A.Y. Halim, A. Shapiro, K. Thomsen, I. Xiarchos, and I.L. Fabricius Comparative analysis of experimental methods for quantification of small amounts of oil in water Journal of Petroleum Science and Engineering, 147, pp. 459-467, 2016. [Download PDF] [View Abstract]During core flooding experiments where water is injected into oil bearing core plugs, the produced fluids can be sampled in a fraction collector. When the core approaches residual oil saturation, the produced amount of oil is typically small (can be less than a few microliters) and the quantification of oil is then difficult. In this study, we compare four approaches to determine the volume of the collected oil fraction in core flooding effluents. The four methods are: Image analysis, UV/visible spectroscopy, liquid scintillation counting, and low-field nuclear magnetic resonance (NMR) spectrometry. The procedure followed to determine the oil fraction and a summary of advantages and disadvantages of each method are given. Our results show that all four methods are reproducible with high accuracy. The NMR method was capable of direct quantification of both oil and water fractions, without comparison to a pre-made standard curve. Image analysis, UV/visible spectroscopy, and liquid scintillation counting quantify only the oil fraction by comparing with a pre-made standard curve. The image analysis technique is reliable when more than 0.1 ml oil is present, whereas liquid scintillation counting performs well when less than 0.6 ml oil is present. Both UV/visible spectroscopy and NMR spectrometry produced high accuracy results in the entire studied range (0.006–1.1 ml). In terms of laboratory time, the liquid scintillation counting is the fastest and least user dependent, whereas the NMR spectrometry is the most time consuming.

Qin, C.-Z., S.M. Hassanizadeh, and A. Ebigbo Pore-scale network modeling of microbially induced calcium carbonate precipitation: Insight into scale dependence of biogeochemical reaction rates Water Resources Research/52, 2016. [Download PDF] [View Abstract]The engineering of microbially induced calcium carbonate precipitation (MICP) has attracted much attention in a number of applications, such as sealing of CO2 leakage pathways, soil stabilization, and subsurface remediation of radionuclides and toxic metals. The goal of this work is to gain insight into pore-scale processes of MICP and scale dependence of biogeochemical reaction rates. This will help us develop efficient field-scale MICP models. In this work, we have developed a comprehensive pore-network model for MICP, with geochemical speciation calculated by the open-source PHREEQC module. A numerical pseudo-3-D micromodel as the computational domain was generated by a novel pore-network generation method. We modeled a three-stage process in the engineering of MICP including the growth of biofilm, the injection of calcium-rich medium, and the precipitation of calcium carbonate. A number of test cases were conducted to illustrate how calcite precipitation was influenced by different operating conditions. In addition, we studied the possibility of reducing the computational effort by simplifying geochemical calculations. Finally, the effect of mass transfer limitation of possible carbonate ions in a pore element on calcite precipitation was explored.

Buscheck, T.A., J.M. Bielicki, T.A. Edmunds, Y. Hao, Y. Sun, J.B. Randolph, and M.O. Saar Multifluid geo-energy systems: Using geologic CO2 storage for geothermal energy production and grid-scale energy storage in sedimentary basins Geosphere, 12/3, pp. 1-19, 2016. [Download PDF] [View Abstract]We present an approach that uses the huge fluid and thermal storage capac ity of the subsurface, together with geologic carbon dioxide (CO 2 ) storage, to harvest, store, and dispatch energy from subsurface (geothermal) and surface (solar, nuclear, fossil) thermal resources, as well as excess energy on electric grids. Captured CO 2 is injected into saline aquifers to store pres - sure, generate artesian flow of brine, and provide a supplemental working fluid for efficient heat extraction and power conversion. Concentric rings of injection and production wells create a hydraulic mound to store pressure, CO 2 , and thermal energy. This energy storage can take excess power from the grid and excess and/or waste thermal energy and dispatch that energy when it is demanded, and thus enable higher penetration of variable renewable en - ergy technologies (e.g., wind and solar). CO 2 stored in the subsurface func - tions as a cushion gas to provide enormous pressure storage capacity and displace large quantities of brine, some of which can be treated for a variety of beneficial uses. Geo thermal power and energy-storage applications may generate enough revenues to compensate for CO 2 capture costs. While our ap - proach can use nitrogen (N 2 ), in addition to CO 2 , as a supplemental fluid, and store thermal energy, this study focuses on using CO 2 for geothermal energy production and grid-scale energy storage. We conduct a techno-economic assess ment to determine the levelized cost of electricity using this approach to generate geothermal power. We present a reservoir pressure management strategy that diverts a small portion of the produced brine for beneficial con - sumptive use to reduce the pumping cost of fluid recirculation, while reducing the risk of seismicity, caprock fracture, and CO 2 leakage.

Leal, A.M.M., D.A. Kulik, and M.O. Saar Enabling Gibbs energy minimization algorithms to use equilibrium constants of reactions in multiphase equilibrium calculations Chemical Geology, 437, pp. 170-181, 2016. [Download PDF] [View Abstract]The geochemical literature provides numerous thermodynamic databases compiled from equilibrium constants of reactions. These databases are typically used in speciation calculations based on the law of mass action (LMA) approach. Unfortunately, such LMA databases cannot be directly used in equilibrium speciation methods based on the Gibbs energy minimization (GEM) approach because of their lack of standard chemical potentials of species. Therefore, we present in this work a simple conversion approach that calculates apparent standard chemical potentials of species from equilibrium constants of reactions. We assess the consistency and accuracy of the use of apparent standard chemical potentials in GEM algorithms by benchmarking equilibrium speciation calculations using GEM and LMA methods with the same LMA database. In all cases, we use PHREEQC to perform the LMA calculations, and we use its LMA databases to calculate the equilibrium constants of reactions. GEM calculations are performed using a Gibbs energy minimization method of Reaktoro — a unified open-source framework for numerical modeling of chemically reactive systems. By comparing the GEM and LMA results, we show that the use of apparent standard chemical potentials in GEM methods produces consistent and accurate equilibrium speciation results, thus validating our new, practical conversion technique that enables GEM algorithms to take advantage of many existing LMA databases, consequently extending and diversifying their range of applicability.

Tutolo, B.M., D.F. Mildner, C.V. Gagnon, M.O. Saar, and W.E. Seyfried Nanoscale constraints on porosity generation and fluid flow during serpentinization Geology, 44/2, pp. 103-106, 2016. [Download PDF] [View Abstract]Field samples of olivine-rich rocks are nearly always serpentinized—commonly to completion—but, paradoxically, their intrinsic porosity and permeability are diminishingly low. Serpentinization reactions occur through a coupled process of fluid infiltration, volumetric expansion, and reaction-driven fracturing. Pores and reactive surface area generated during this process are the primary pathways for fluid infiltration into and reaction with serpentinizing rocks, but the size and distribution of these pores and surface area have not yet been described. Here, we utilize neutron scattering techniques to present the first measurements of the evolution of pore size and specific surface area distribution in partially serpentinized rocks. Samples were obtained from the ca. 2 Ma Atlantis Massif oceanic core complex located off-axis of the Mid-Atlantic Ridge and an olivine-rich outcrop of the ca. 1.1 Ga Duluth Complex of the North American Mid-Continent Rift. Our measurements and analyses demonstrate that serpentine and accessory phases form with their own, inherent porosity, which accommodates the bulk of diffusive fluid flow during serpentinization and thereby permits continued serpentinization after voluminous serpentine minerals fill reaction-generated porosity.

Leal, A.M.M., D. Kulik, G. Kosakowski, and M.O. Saar Computational methods for reactive transport modeling: An extended law of mass-action, xLMA, method for multiphase equilibrium calculations Advances in Water Resources, 96, pp. 405-422, 2016. [Download PDF] [View Abstract]We present a numerical method for multiphase chemical equilibrium calculations based on a Gibbs energy minimization approach. The method can accurately and efficiently determine the stable phase assemblage at equilibrium independently of the type of phases and species that constitute the chemical system. We have successfully applied our chemical equilibrium algorithm in reactive transport simulations to demonstrate its effective use in computationally intensive applications. We used FEniCS to solve the governing partial differential equations of mass transport in porous media using finite element methods in unstructured meshes. Our equilibrium calculations were benchmarked with GEMS3K, the numerical kernel of the geochemical package GEMS. This allowed us to compare our results with a well-established Gibbs energy minimization algorithm, as well as their performance on every mesh node, at every time step of the transport simulation. The benchmark shows that our novel chemical equilibrium algorithm is accurate, robust, and efficient for reactive transport applications, and it is an improvement over the Gibbs energy minimization algorithm used in GEMS3K. The proposed chemical equilibrium method has been implemented in Reaktoro, a unified framework for modeling chemically reactive systems, which is now used as an alternative numerical kernel of GEMS.

Matculevich, S., and S. Repin Explicit constants in Poincaré-type inequalities for simplicial domains and application to a posteriori estimates Comput. Methods Appl. Math. (CMAM), 16/2, pp. 277-298, 2016. [Download PDF] [View Abstract]The paper is concerned with sharp estimates of constants in the classical Poincaré inequalities and Poincaré-type inequalities for functions with zero mean values in a simplicial domain or on a part of the boundary. These estimates are important for quantitative analysis of problems generated by differential equations where numerical approximations are typically constructed with the help of simplicial meshes. We suggest easily computable relations that provide sharp bounds of the respective constants and compare these results with analytical estimates (if such estimates are known). In the last section, we discuss possible applications and derive a computable majorant of the difference between the exact solution of a boundary value problem and an arbitrary finite dimensional approximation defined on a simplicial mesh.

Zhang, L., D. Li, and J. Ezekiel CO2 Geological Storage into a Lateral Aquifer of an Offshore Gas Field in the South China Sea: Storage Safety and Project Design Frontiers of Earth Science, 2015. [Download PDF] [View Abstract]The DF1-1 gas field, located in the western South China Sea, contains a high concentration of CO2, thus there is great concern about the need to reduce the CO2 emissions. Many options have been considered in recent years to dispose of the CO2 separated from the natural gas stream on the Hainan Island. In this study, the feasibility of CO2 storage in the lateral saline aquifer of the DF1-1 gas field is assessed, including aquifer selection and geological assessment, CO2 migration and storage safety, project design, and economic analysis. Six offshore aquifers have been investigated for CO2 geological storage. The lateral aquifer of the DF1-1 gas field has been selected as the best target for CO2 injection and storage because of its proven sealing ability, and the large storage capacity of the combined aquifer and hydrocarbon reservoir geological structure. The separated CO2 will be dehydrated on the Hainan Island and transported by a long-distance subsea pipeline in supercritical or liquid state to the central platform of the DF1-1 gas field for pressure adjustment. The CO2 will then be injected into the lateral aquifer via a subsea well-head through a horizontal well. Reservoir simulations suggest that the injected CO2 will migrate slowly upwards in the aquifer without disturbing the natural gas production. The scoping economic analysis shows that the unit storage cost of the project is approximately US$26-31/ton CO2 with the subsea pipeline as the main contributor to capital expenditure (CAPEX), and the dehydration system as the main factor of operating expenditure (OPEX).

Li, D., B. Ren, L. Zhang, J. Ezekiel, S. Ren, and Y. Feng CO2-sensitive foams for mobility control and channeling blocking in enhanced WAG process Chemical Engineering Research and Design, 102, pp. 234-243, 2015. [Download PDF] [View Abstract]Mobility control is a key issue in gas and CO2 flooding process, and water-alternating-gas (CO2) injection (WAG) has been used in various field applications. The WAG process can be CO2 foam assisted in order to further improve the sweeping efficiency of the injectants. In this study, a novel foaming method for reducing CO2 mobility and blocking gas channeling is proposed, which is based on a CO2-sensitive chemical (compound with amine group) to generate foams or thicken the injected water in situ. The CO2-sensitive chemical is referred to that, in a reaction triggered by CO2, it can be converted to an effective surfactant or foam agent. The chemical can be dissolved in water and injected as a slug. In this study, the foaming behavior and the CO2-sensitivity of the chemical were tested using a visualization apparatus and a viscometer in the presence of CO2 and N2. The capability of the foams generated using the CO2-sisentive chemical for mobility control was evaluated via gas flooding experiments of sand-packs. Stable CO2 foams have been obtained at high temperature and pressure conditions (up to 140 °C and 16 MPa), and high viscosity was observed in the chemical solution when CO2 was present in comparison with that of N2, indicating the chemical's sensitivity with dissolved CO2 in water. In the sand-pack flooding experiments, a high resistance factor was achieved in a simulated WAG process using the CO2-sensitive chemical, which is attributed to the CO2 foams and viscous micelles generated in situ during CO2 injection.

Li, M., S. Fan, Y. Su, J. Ezekiel, M. Lu, and L. Zhang Mathematical models of the heat-water dissociation of natural gas hydrates considering a moving Stefan boundary Energy, 90/1, pp. 202-207, 2015. [Download PDF] [View Abstract]This paper presents mathematical models for radial, quasi-steady state heat transfer in a semi-infinite hydrate reservoir with a moving boundary that is related to the dissociation of natural gas hydrates. The exact solutions of the temperature in the dissociation zone and hydrate zone, using the Paterson exponential integral function, are obtained, and the dissociation frontal brim location of the hydrates is determined by combining the Deaton method with the Clausius–Claperyron equation. A sample calculation shows that the reservoir temperature falls sharply to the dissociation temperature and then drops gradually with increasing distance to the reservoir temperature. With respect to time, the temperature increases slowly to the dissociation temperature, after which, the dissociation temperature falls sharply to the temperature close to that of the injected hot-water. By increasing the temperature of injected hot-water, more hydrates participate in dissociation; with an increase in time, the radius quickly increases, but the radius of hydrate dissociation increases slowly.

Hommel, J., E. Lauchnor, A. Phillips, R. Gerlach, A.B. Cunningham, R. Helmig, A. Ebigbo, and H. Class A revised model for microbially induced calcite precipitation: Improvements and new insights based on recent experiments Water Resources Research, 51/5, pp. 3695-3715, 2015. [Download PDF] [View Abstract]The model for microbially induced calcite precipitation (MICP) published by Ebigbo et al. (2012) has been improved based on new insights obtained from experiments and model calibration. The challenge in constructing a predictive model for permeability reduction in the underground with MICP is the quantification of the complex interaction between flow, transport, biofilm growth, and reaction kinetics. New data from Lauchnor et al. (2015) on whole-cell ureolysis kinetics from batch experiments were incorporated into the model, which has allowed for a more precise quantification of the relevant parameters as well as a simplification of the reaction kinetics in the equations of the model. Further, the model has been calibrated objectively by inverse modeling using quasi-1D column experiments and a radial flow experiment. From the postprocessing of the inverse modeling, a comprehensive sensitivity analysis has been performed with focus on the model input parameters that were fitted in the course of the model calibration. It reveals that calcite precipitation and concentrations of math formula and math formula are particularly sensitive to parameters associated with the ureolysis rate and the attachment behavior of biomass. Based on the determined sensitivities and the ranges of values for the estimated parameters in the inversion, it is possible to identify focal areas where further research can have a high impact toward improving the understanding and engineering of MICP.

Hommel, J., E. Lauchnor, R. Gerlach, A.B. Cunningham, A. Ebigbo, R. Helmig, and H. Class Investigating the influence of the initial biomass distribution and injection strategies on biofilm- mediated calcite precipitation in porous media Transport in Porous Media, 2015. [Download PDF] [View Abstract]Attachment of bacteria in porous media is a complex mixture of processes resulting in the transfer and immobilization of suspended cells onto a solid surface within the porous medium. Quantifying the rate of attachment is difficult due to the many simultaneous processes possibly involved in attachment, including straining, sorption, and sedimentation, and the difficulties in measuring metabolically active cells attached to porous media. Preliminary experiments confirmed the difficulty associated with measuring active Sporosarcina pasteurii cells attached to porous media. However, attachment is a key process in applications of biofilm-mediated reactions in the subsurface such as microbially induced calcite precipitation. Independent of the exact processes involved, attachment determines both the distribution and the initial amount of attached biomass and as such the initial reaction rate. As direct experimental investigations are difficult, this study is limited to a numerical investigation of the effect of various initial biomass distributions and initial amounts of attached biomass. This is performed for various injection strategies, changing the injection rate as well as alternating between continuous and pulsed injections. The results of this study indicate that, for the selected scenarios, both the initial amount and the distribution of attached biomass have minor influence on the Ca2+ precipitation efficiency as well as the distribution of the precipitates compared to the influence of the injection strategy. The influence of the initial biomass distribution on the resulting final distribution of the precipitated calcite is limited, except for the continuous injection at intermediate injection rate. But even for this injection strategy, the Ca2+ precipitation efficiency shows no significant dependence on the initial biomass distribution.

Manceau, J.C., J. Ma, and R. Li Two-phase flow properties of a sandstone rock for the CO2/water system: Core-flooding experiments, and focus on impacts of mineralogical changes Water Resources Research, 51, pp. 2885-2900, 2015. [Download PDF] [View Abstract]The two-phase flow characterization (CO2/water) of a Triassic sandstone core from the Paris Basin, France, is reported in this paper. Absolute properties (porosity and water permeability), capillary pressure, relative permeability with hysteresis between drainage and imbibition, and residual trapping capacities have been assessed at 9 MPa pore pressure and 28°C (CO2 in liquid state) using a single core-flooding apparatus associated with magnetic resonance imaging. Different methodologies have been followed to obtain a data set of flow properties to be upscaled and used in large-scale CO2 geological storage evolution modeling tools. The measurements are consistent with the properties of well-sorted water-wet porous systems. As the mineralogical investigations showed a nonnegligible proportion of carbonates in the core, the experimental protocol was designed to observe potential impacts on flow properties of mineralogical changes. The magnetic resonance scanning and mineralogical observations indicate mineral dissolution during the experimental campaign, and the core-flooding results show an increase in porosity and water absolute permeability. The changes in two-phase flow properties appear coherent with the pore structure modifications induced by the carbonates dissolution but the changes in relative permeability could also be explained by a potential increase of the water-wet character of the core. Further investigations on the impacts of mineral changes are required with other reactive formation rocks, especially carbonate-rich ones, because the implications can be significant both for the validity of laboratory measurements and for the outcomes of in situ operations modeling.

Leal, A.M.M., M.J. Blunt, and T.C. LaForce A chemical kinetics algorithm for geochemical modelling Applied Geochemistry, 55, pp. 46-61, 2015. [Download PDF] [View Abstract]A chemical kinetics algorithm is presented for geochemical applications. The algorithm is capable of handling both equilibrium- and kinetically-controlled reactions in multiphase systems. The ordinary differential equations (ODEs) are solved using an implicit multistep backward differentiation formula (BDF) algorithm to ensure efficiency and stability when integrating stiff ODEs. An adaptive control scheme of the time step is adopted to guarantee small steps in steeper regions and large steps in smoother regions of the integration. Analytical derivatives of the reaction rates and species activities are used to permit the use of larger time steps, and to increase the robustness of the calculations. The chemical equilibrium calculations are performed using a Gibbs free energy minimisation algorithm, which is based on a trust-region interior-point method adapted with a watchdog strategy that yields quadratic rates of convergence near the solution. The chemical kinetics algorithm is applied to geochemical problems relevant to carbon storage in saline aquifers. The calculations assume aqueous, gaseous and mineral phases, where the kinetics of the water–gas–rock interactions are investigated. The results allow us to estimate the time frames at which brine of different salinities and supercritical CO2 attains equilibrium with a carbonate rock, as well as the amount of carbon dioxide trapped by solubility and mineralisation mechanisms.

Samrock, F., A. Kuvshinov, J. Bakker, A. Jackson, and F. Shimeles 3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia Geophysical Journal International, 202/3, pp. 1923-1948, 2015. [Download PDF] [View Abstract]The Main Ethiopian Rift Valley encompasses a number of volcanoes, which are known to be actively deforming with reoccurring periods of uplift and setting. One of the regions where temporal changes take place is the Aluto volcanic complex. It hosts a productive geothermal field and the only currently operating geothermal power plant of Ethiopia. We carried out magnetotelluric (MT) measurements in early 2012 in order to identify the source of unrest. Broad-band MT data (0.001-1000 s) have been acquired at 46 sites covering the expanse of the Aluto volcanic complex with an average site spacing of 1 km. Based on this MT data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres. Resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones, which can be easily sensed by MT. Thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature, extent and potential for production of energy. 3-D inversions of the observed MT data from Aluto reveal the typical electrical conductivity distribution of a high-enthalpy geothermal system, which is mainly governed by the hydrothermal alteration mineralogy. The recovered 3-D conductivity models provide no evidence for an active deep magmatic system under Aluto. Forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off-axis fault zone a few tens of kilometres west of the central rift axis. The absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the Aluto-Langano geothermal field.

Bakker, J., A. Kuvshinov, F. Samrock, A. Geraskin, and O. Pankratov Introducing inter-site phase tensors to suppress galvanic distortion in the telluric method Earth, Planets and Space: EPS, 67/1, pp. 160, 2015. [Download PDF] [View Abstract]A common problem when interpreting magnetotelluric (MT) data is that they often are distorted by shallow unresolvable local structures, an effect known as galvanic distortion. We present two transfer functions that are (almost) resistant to galvanic distortion. First, we introduce the electric phase tensor, which is derived from the electric tensor, where the electric tensor relates the horizontal electric fields at a field and base site. The electric phase tensor is only affected by galvanic distortion, if present, at the base site. Second, we introduce the quasi-electric phase tensor, which is derived from the quasi-electric tensor, where the quasi-electric tensor relates the electric field at a field site with the magnetic field at a base site. The quasi-electric tensor is not affected by galvanic distortion. Using a synthetic data-set, we show that the sensitivity of the MT phase tensor, the quasi-electric phase tensor, and the electric phase tensor is comparable for our model under consideration. Furthermore, we demonstrate that stable (quasi-) electric phase tensors can be recovered from a real data-set with the use of existing processing software. In addition, we provide a formalism to propagate the uncertainties from the estimated (quasi-) electric and impedance tensors to their respective phase tensors. The uncertainties of the (quasi-) electric phase tensors are of the same order of magnitude as the uncertainties of the MT phase tensor. From our study, we conclude that the (quasi-) electric phase tensors are an attractive complement to the standard MT responses.

Galindo-Torres, S.A., T. Molebatsi, X.Z. Kong, A. Scheuermann, D. Bringemeier, and L. Li Scaling solutions for connectivity and conductivity of continuous random networks Physical Review E, Statistical, Nonlinear, and Soft Matter Physics, 92/4, pp. 041001, 2015. [Download PDF] [View Abstract]Connectivity and conductivity of two-dimensional fracture networks (FNs), as an important type of continuous random networks, are examined systematically through Monte Carlo simulations under a variety of conditions, including different power law distributions of the fracture lengths and domain sizes. The simulation results are analyzed using analogies of the percolation theory for discrete random networks. With a characteristic length scale and conductivity scale introduced, we show that the connectivity and conductivity of FNs can be well described by universal scaling solutions. These solutions shed light on previous observations of scale-dependent FN behavior and provide a powerful method for quantifying effective bulk properties of continuous random networks.

Ma, Y., X. -Z. Kong, A. Scheuermann, S. A. Galindo-Torres, D. Bringemeier, and L. Li Microbubble transport in water-saturated porous media Water Resources Research, 51/6, pp. 4359-4373, 2015. [Download PDF] [View Abstract]Laboratory experiments were conducted to investigate flow of discrete microbubbles through a water-saturated porous medium. During the experiments, bubbles, released from a diffuser, moved upward through a quasi-2-D flume filled with transparent water-based gelbeads and formed a distinct plume that could be well registered by a calibrated camera. Outflowing bubbles were collected on the top of the flume using volumetric burettes for flux measurements. We quantified the scaling behaviors between the gas (bubble) release rates and various characteristic parameters of the bubble plume, including plume tip velocity, plume width, and breakthrough time of the plume front. The experiments also revealed circulations of ambient pore water induced by the bubble flow. Based on a simple momentum exchange model, we showed that the relationship between the mean pore water velocity and gas release rate is consistent with the scaling solution for the bubble plume. These findings have important implications for studies of natural gas emission and air sparging, as well as fundamental research on bubble transport in porous media.

Birdsell, D., H. Rajaram, and G. Lackey Imbibition of hydraulic fracturing fluids into partially saturated shale Water Resources Research, 51/8, pp. 6787-6796, 2015. [Download PDF] [View Abstract]Recent studies suggest that imbibition of hydraulic fracturing fluids into partially saturated shale is an important mechanism that restricts their migration, thus reducing the risk of groundwater contamination. We present computations of imbibition based on an exact semianalytical solution for spontaneous imbibition. These computations lead to quantitative estimates of an imbibition rate parameter A with units of LT−1/2 for shale, which is related to porous medium and fluid properties, and the initial water saturation. Our calculations suggest that significant fractions of injected fluid volumes (15–95%) can be imbibed in shale gas systems, whereas imbibition volumes in shale oil systems is much lower (3–27%). We present a nondimensionalization of A, which provides insights into the critical factors controlling imbibition, and facilitates the estimation of A based on readily measured porous medium and fluid properties. For a given set of medium and fluid properties, A varies by less than factors of ~1.8 (gas nonwetting phase) and ~3.4 (oil nonwetting phase) over the range of initial water saturations reported for the Marcellus shale (0.05–0.6). However, for higher initial water saturations, A decreases significantly. The intrinsic permeability of the shale and the viscosity of the fluids are the most important properties controlling the imbibition rate.

Birdsell, D., H. Rajaram, D. Dempsey, and H. Viswanathan Hydraulic fracturing fluid migration in the subsurface: A review and expanded modeling results Water Resources Research, 51/9, pp. 7159-7188, 2015. [Download PDF] [View Abstract]Understanding the transport of hydraulic fracturing (HF) fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated. Topographically driven flow, overpressured shale reservoirs, permeable pathways such as faults or leaky wellbores, the increased formation pressure due to HF fluid injection, and the density contrast of the HF fluid to the surrounding brine can encourage upward HF fluid migration. In contrast, the very low shale permeability and capillary imbibition of water into partially saturated shale may sequester much of the HF fluid, and well production will remove HF fluid from the subsurface. We review the literature on important aspects of HF fluid migration. Single‐phase flow and transport simulations are performed to quantify how much HF fluid is removed via the wellbore with flowback and produced water, how much reaches overlying aquifers, and how much is permanently sequestered by capillary imbibition, which is treated as a sink term based on a semianalytical, one‐dimensional solution for two‐phase flow. These simulations include all of the important aspects of HF fluid migration identified in the literature review and are performed in five stages to faithfully represent the typical operation of a hydraulically fractured well. No fracturing fluid reaches the aquifer without a permeable pathway. In the presence of a permeable pathway, 10 times more fracturing fluid reaches the aquifer if well production and capillary imbibition are not included in the model.

Matculevich, S., P. Neitaanmäki, and S. Repin A posteriori error estimates for time-dependent reaction-diffusion problems based on the Payne-Weinberger inequality Discrete Contin. Dyn. Syst. - Series A, 35/6, pp. 2659-2677, 2015. [Download PDF] [View Abstract]We consider evolutionary reaction-diffusion problems with mixed Dirichlet-Robin boundary conditions. For this class of problems, we derive two-sided estimates of the distance between any function in the admissible energy space and the exact solution of the problem. The estimates (majorants and minorants) are explicitly computable and do not contain unknown functions or constants. Moreover, it is proved that the estimates are equivalent to the energy norm of the deviation from the exact solution.

Luhmann, A.J., M. Covington, J. Myre, M. Perne, S.W. Jones, C.E. Alexander Jr., and M.O. Saar Thermal damping and retardation in karst conduits Hydrology and Earth System Sciences, 19/1, pp. 137-157, 2015. [Download PDF] [View Abstract]Water temperature is a non-conservative tracer in the environment. Variations in recharge temperature are damped and retarded as water moves through an aquifer due to heat exchange between water and rock. However, within karst aquifers, seasonal and short-term fluctuations in recharge temperature are often transmitted over long distances before they are fully damped. Using analytical solutions and numerical simulations, we develop relationships that describe the effect of flow path properties, flow-through time, recharge characteristics, and water and rock physical properties on the damping and retardation of thermal peaks/troughs in karst conduits. Using these relationships, one can estimate the thermal retardation and damping that would occur under given conditions with a given conduit geometry. Ultimately, these relationships can be used with thermal damping and retardation field data to estimate parameters such as conduit diameter. We also examine sets of numerical simulations where we relax some of the assumptions used to develop these relationships, testing the effects of variable diameter, variable velocity, open channels, and recharge shape on thermal damping and retardation to provide some constraints on uncertainty. Finally, we discuss a multitracer experiment that provides some field confirmation of our relationships. High temporal resolution water temperature data are required to obtain sufficient constraints on the magnitude and timing of thermal peaks and troughs in order to take full advantage of water temperature as a tracer.

Adams, B.M., T.H. Kuehn, J.M. Bielicki, J.B. Randolph, and M.O. Saar A comparison of electric power output of CO2 Plume Geothermal (CPG) and brine geothermal systems for varying reservoir conditions Applied Energy, 140, pp. 365-377, 2015. [Download PDF] [View Abstract]In contrast to conventional hydrothermal systems or enhanced geothermal systems, CO2 Plume Geothermal (CPG) systems generate electricity by using CO2 that has been geothermally heated due to sequestration in a sedimentary basin. Four CPG and two brine-based geothermal systems are modeled to estimate their power production for sedimentary basin reservoir depths between 1 and 5km, geothermal temperature gradients from 20 to 50°Ckm-1, reservoir permeabilities from 1×10-15 to 1×10-12m2 and well casing inner diameters from 0.14m to 0.41m. Results show that CPG direct-type systems produce more electricity than brine-based geothermal systems at depths between 2 and 3km, and at permeabilities between 10-14 and 10-13m2, often by a factor of two. This better performance of CPG is due to the low kinematic viscosity of CO2, relative to brine at those depths, and the strong thermosiphon effect generated by CO2. When CO2 is used instead of R245fa as the secondary working fluid in an organic Rankine cycle (ORC), the power production of both the CPG and the brine-reservoir system increases substantially; for example, by 22% and 20% for subsurface brine and CO2 systems, respectively, with a 35°Ckm-1 thermal gradient, 0.27m production and 0.41m injection well diameters, and 5×10-14m2 reservoir permeability.

Tutolo, B.M., A.T. Schaen, M.O. Saar, and W.E. Seyfried Jr. Implications of the redissociation phenomenon for mineral-buffered fluids and aqueous species transport at elevated temperatures and pressures Applied Geochemistry, 55, pp. 119-127, 2015. [Download PDF] [View Abstract]Aqueous species equilibrium constants and activity models form the foundation of the complex speciation codes used to model the geochemistry of geothermal energy production, extremophilic ecosystems, ore deposition, and a variety of other processes. Researchers have shown that a simple three species model (i.e., Na+, Cl?, and NaCl(aq)) can accurately describe conductivity measurements of concentrated NaCl and KCl solutions at elevated temperatures and pressures (Sharygin et al., 2002). In this model, activity coefficients of the charged species (e.g., Na+, K+, Cl?) become sufficiently low that the complexes must redisocciate with increasing salt concentration in order to meet equilibrium constant constraints. Redissociation decreases the proportion of the elements bound up as neutral complexes, and thereby increases the true ionic strength of the solution. In this contribution, we explore the consequences of the redissociation phenomenon in albite–paragonite–quartz (APQ) buffered systems. We focus on the implications of the redissociation phenomenon for mineral solubilities, particularly the observation that, at certain temperatures and pressures, calculated activities of charged ions in solution remain practically constant even as element concentrations increase from <1 molal to 4.5 molal. Finally, we note that redissociation has a similar effect on pH, and therefore aqueous speciation, in APQ-hosted systems. The calculations and discussion presented here are not limited to APQ-hosted systems, but additionally apply to many others in which the dominant cations and anions can form neutral complexes.

Garapati, N., J.B. Randolph, and M.O. Saar Brine displacement by CO2, energy extraction rates, and lifespan of a CO2-limited CO2-Plume Geothermal (CPG) system with a horizontal production well Geothermics, 55, pp. 182-194, 2015. [Download PDF] [View Abstract]Several studies suggest that CO2-based geothermal energy systems may be operated economically when added to ongoing geologic CO2 sequestration. Alternatively, we demonstrate here that CO2-Plume Geothermal (CPG) systems may be operated long-term with a finite amount of CO2. We analyze the performance of such CO2-limited CPG systems as a function of various geologic and operational parameters. We find that the amount of CO2 required increases with reservoir depth, permeability, and well spacing and decreases with larger geothermal gradients. Furthermore, the onset of reservoir heat depletion decreases for increasing geothermal gradients and for both particularly shallow and deep reservoirs.

Tutolo, B.M., A.J. Luhmann, X.-Z. Kong, M.O. Saar, and W.E. Seyfried Jr. CO2 sequestration in feldspar-rich sandstone: Coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties Geochimica Et Cosmochimica Acta, 160, pp. 132-154, 2015. [Download PDF] [View Abstract]To investigate CO2 Capture, Utilization, and Storage (CCUS) in sandstones, we performed three 150 °C flow-through experiments on K-feldspar-rich cores from the Eau Claire formation. By characterizing fluid and solid samples from these experiments using a suite of analytical techniques, we explored the coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties during CO2 sequestration in feldspar-rich sandstone. Overall, our results confirm predictions that the heightened acidity resulting from supercritical CO2 injection into feldspar-rich sandstone will dissolve primary feldspars and precipitate secondary aluminum minerals. A core through which CO2-rich deionized water was recycled for 52 days decreased in bulk permeability, exhibited generally low porosity associated with high surface area in post-experiment core sub-samples, and produced an Al hydroxide secondary mineral, such as boehmite. However, two samples subjected to ?3 day single-pass experiments run with CO2-rich, 0.94 mol/kg NaCl brines decreased in bulk permeability, showed generally elevated porosity associated with elevated surface area in post-experiment core sub-samples, and produced a phase with kaolinite-like stoichiometry. CO2-induced metal mobilization during the experiments was relatively minor and likely related to Ca mineral dissolution. Based on the relatively rapid approach to equilibrium, the relatively slow near-equilibrium reaction rates, and the minor magnitudes of permeability changes in these experiments, we conclude that CCUS systems with projected lifetimes of several decades are geochemically feasible in the feldspar-rich sandstone end-member examined here. Additionally, the observation that K-feldspar dissolution rates calculated from our whole-rock experiments are in good agreement with literature parameterizations suggests that the latter can be utilized to model CCUS in K-feldspar-rich sandstone. Finally, by performing a number of reactive transport modeling experiments to explore processes occurring during the flow-through experiments, we have found that the overall progress of feldspar hydrolysis is negligibly affected by quartz dissolution, but significantly impacted by the rates of secondary mineral precipitation and their effect on feldspar saturation state. The observations produced here are critical to the development of models of CCUS operations, yet more work, particularly in the quantification of coupled dissolution and precipitation processes, will be required in order to produce models that can accurately predict the behavior of these systems.

Tutolo, B.M., X.-Z. Kong, W.E. Seyfried Jr., and M.O. Saar High performance reactive transport simulations examining the effects of thermal, hydraulic, and chemical (THC) gradients on fluid injectivity at carbonate CCUS reservoir scales International Journal of Greenhouse Gas Control, 39, pp. 285-301, 2015. [Download PDF] [View Abstract]Carbonate minerals and CO2 are both considerably more soluble at low temperatures than they are at elevated temperatures. This inverse solubility has led a number of researchers to hypothesize that injecting low-temperature (i.e., less than the background reservoir temperature) CO2 into deep, saline reservoirs for CO2 Capture, Utilization, and Storage (CCUS) will dissolve CO2 and carbonate minerals near the injection well and subsequently exsolve and re-precipitate these phases as the fluids flow into the geothermally warm portion of the reservoir. In this study, we utilize high performance computing to examine the coupled effects of cool CO2 injection and background hydraulic head gradients on reservoir-scale mineral volume changes. We employ the fully coupled reactive transport simulator PFLOTRAN with calculations distributed over up to 800 processors to test 21 scenarios designed to represent a range of reservoir depths, hydraulic head gradients, and CO2 injection rates and temperatures. In the default simulations, 50 °C CO2 is injected at a rate of 50 kg/s into a 200 bar, 100 °C calcite or dolomite reservoir. By comparing these simulations with others run at varying conditions, we show that the effect of cool CO2 injection on reservoir-scale mineral volume changes tends to be relatively minor. We conclude that the low heat capacity of CO2 effectively prevents low-temperature CO2 injection from decreasing the temperature across large portions of the simulated carbonate reservoirs. This small thermal perturbation, combined with the low relative permeability of brine within the supercritical CO2 plume, yields limited dissolution and precipitation effects directly attributable to cool CO2 injection. Finally, we calculate that relatively high water-to-rock ratios, which may occur over much longer CCUS reservoir lifetimes or in materials with sufficiently high brine relative permeability within the supercritical CO2 plume, would be required to substantially affect injectivity through thermally-induced mineral dissolution and precipitation. Importantly, this study shows the utility of reservoir scale-reactive transport simulators for testing hypotheses and placing laboratory-scale observations into a CCUS reservoir-scale context.

Schaedle, P., N. Hubschwerlen, and H. Class Optimizing the modeling performance for safety assessments of nuclear waste repositories by approximating two-phase flow and transport by single-phase transport simulations Nuclear Technology/187(2), pp. 188-197, 2014. [Download PDF]

Deichmann, N., T. Kraft, and K.F. Evans Identification of faults activated during the stimulation of the Basel geothermal project from cluster analysis and focal mechanisms of the larger magnitude events Geothermics, 52, pp. 84-97, 2014. [Download PDF]

Ren, J., L. Zhang, J. Ezekiel, S. Ren, and S. Meng Reservoir Characteristics and Productivity Analysis of Tight Sand Gas in Upper Paleozoic Ordos Basin China Journal of Natural Gas Science and Engineering, 19, pp. 244-250, 2014. [Download PDF] [View Abstract]Abundant tight sands rich in natural gas, as a kind of unconventional energy source, have been discovered in the Ordos Basin, Central-North China, which can contribute greatly to the sustainable supply of natural gas in China. In this paper, the geological and petrophysical characteristics of a typical tight sand gas reservoir in the Upper Paleozoic of the Ordos Basin were investigated and correlated to the productivity of gas wells. Several important petrophysical relationships were revealed based on data from drill cuttings, core, and well logging, including porosity versus permeability, stress sensitivity to permeability, and rock density versus porosity. Their effects on well's productivity were discussed. The productivity of the targeted reservoir was analyzed and classified using a formation capacity method (K·H factor), and was compared with the production data of similar tight sand and low permeability blocks in the region, which can provide good reference for the field development.

Zhang, L., J. Ezekiel, D. Li, and J. Pei Potential Assessment of CO2 Injection for Heat Mining and Geological Storage in Geothermal Reservoirs of China Applied Energy, 122, pp. 237-246, 2014. [Download PDF] [View Abstract]Supercritical CO2 has good mobility and certain heat capacity, which can be used as an alternative of water for heat recovery from geothermal reservoirs, meanwhile trapping most of injected CO2 underground to achieve the environmental benefits. In this paper, different types of geothermal resources are assessed to screen reservoirs suitable for heat mining and geological storage by CO2 injection, in terms of geological properties, heat characteristics, storage applicability, and development prospects, etc. Hot dry rock, deep saline aquifer, and geopressured reservoir are selected as the potential sites for this study, mainly due to their relatively positive geological conditions for CO2 circulation and storage. Reservoir simulations are conducted to analyze the heat extracting capacity and storage efficiency of CO2 in the promising geothermal reservoirs. A simple calculation method is presented to estimate the potentials of heat mining and CO2 storage in the major prospective geothermal regions of China. The preliminary assessment results show that the recoverable geothermal potential by CO2 injection in China is around 1.55 × 1021 J with hot dry rocks as the main contributor. The corresponding CO2 storage capacity is up to 3.53 × 1014 kg with the deep saline aquifers accounting for more than 50%. CO2 injection for geothermal production is a more attractive option than pure CO2 storage due to its higher economic benefits in spite of that many technological and economic issues still need to be solved in the future.

Leal, A.M.M., M.J. Blunt, and T.C. LaForce Efficient chemical equilibrium calculations for geochemical speciation and reactive,transport modelling Geochimica et Cosmochimica Acta, 131, pp. 301-322, 2014. [Download PDF] [View Abstract]Chemical equilibrium calculations are essential for many environmental problems. It is also a fundamental tool for chemical kinetics and reactive transport modelling, since these applications may require hundreds to billions equilibrium calculations in a single simulation. Therefore, an equilibrium method for such critical applications must be very efficient, robust and accurate. In this work we demonstrate the potential effectiveness of a novel Gibbs energy minimisation algorithm for reactive transport simulations. The algorithm includes strategies to converge from poor initial guesses; capabilities to specify non-linear equilibrium constraints such as pH of an aqueous solution and activity or fugacity of a species; a rigorous phase stability test to determine the unstable phases; and a strategy to boost the convergence speed of the calculations to quadratic rates, requiring only few iterations to converge. We use this equilibrium method to solve geochemical problems relevant to carbon storage in saline aquifers, where aqueous, gaseous and minerals phases are present. The problems are formulated to mimic the ones found in kinetics and transport simulations, where a sequence of equilibrium calculations are performed, each one using the previous solution as the initial guess. The efficiency and convergence rates of the calculations are presented, which require an average of 1–2 iterations. These results indicate that critical applications such as chemical kinetics and reactive transport modelling can potentially benefit by using this multiphase equilibrium algorithm.

Buscheck, T.A., J.M. Bielicki, M. Chen, Y. Sun, Y. Hao, T.A. Edmunds, M.O. Saar, and J.B. Randolph Integrating CO2 Storage with Geothermal Resources for Dispatchable Renewable Electricity Energy procedia, 63, pp. 7619-7630, 2014. [Download PDF] [View Abstract]We present an approach that uses the huge fluid and thermal storage capacity of the subsurface, together with geologic CO2 storage, to harvest, store, and dispatch energy from subsurface (geothermal) and surface (solar, nuclear, fossil) thermal resources, as well as energy from electrical grids. Captured CO2 is injected into saline aquifers to store pressure, generate artesian flow of brine, and provide an additional working fluid for efficient heat extraction and power conversion. Concentric rings of injection and production wells are used to create a hydraulic divide to store pressure, CO2, and thermal energy. Such storage can take excess power from the grid and excess/waste thermal energy, and dispatch that energy when it is demanded, enabling increased penetration of variable renewables. Stored CO2 functions as a cushion gas to provide enormous pressure-storage capacity and displaces large quantities of brine, which can be desalinated and/or treated for a variety of beneficial uses. Geothermal power and energy-storage applications may generate enough revenues to justify CO2 capture costs.

Garapati, N., and B.J. Anderson Statistical Thermodynamics Model and Empirical Correlations for Predicting Mixed Hydrate Phase Equilibria Fluid Phase Equilibria, 373, pp. 20-28, 2014. [Download PDF] [View Abstract]Natural gas hydrate deposits contain CH4 along with other hydrocarbon gases like C2H6, C3H8 and non-hydrocarbon gases like CO2 and H2S. If CH4 stored in natural gas hydrates can be recovered, the hydrates would potentially become a cleaner energy resource for the future producing less CO2 when combusted than does coal. The production of CH4 from natural gas hydrate reservoirs has been predicted by reservoir simulators that implement phase equilibrium data in order to predict various production scenarios. In this paper two methods are discussed for calculating the phase equilibria of mixed hydrates. In the first method, the phase equilibrium is predicted using a ‘cell potential’ code, which is based on van der Waals and Platteeuw statistical mechanics, along with variable reference parameters to account for lattice distortion, and with temperature-dependent Langmuir constants proposed by Bazant and Trout. The method is validated by reproducing the existing phase equilibrium data of simple and mixed hydrates and the structural transitions that are known to occur, without the use of any fitting parameters. A computationally-simple method is to use empirical correlations of gas hydrate dissociation pressure with respect to temperature and gas-phase composition as they are easy to implement into the simulators. The parameters for the empirical expression were determined for the CH4–C2H6 mixed hydrate system by non-linear regression analysis of available experimental data and data obtained from the first method.

Garapati, N., J.B. Randolph, J.L. Valencia Jr., and M.O. Saar CO2-Plume Geothermal (CPG) Heat Extraction in Multi-layered Geologic Reservoirs Energy Procedia, 63, pp. 7631-7643, 2014. [Download PDF] [View Abstract]CO2-Plume Geothermal (CPG) technology involves injecting CO2 into natural, highly permeable geologic units to extract energy. The subsurface CO2 absorbs heat from the reservoir, buoyantly rises to the surface, and drives a power generation system. The CO2 is then cooled and reinjected underground. Here, we analyze the effects of multi-layered geologic reservoirs on CPG system performance by examining the CO2 mass fraction in the produced fluid, pore-fluid pressure buildup during operation, and heat energy extraction rates. The produced CO2 mass fraction depends on the stratigraphic positions of highly permeable layers which also affect the pore-fluid pressure drop across the reservoir.

Adams, B.M., T.H. Kuehn, J.M. Bielicki, J.B. Randolph, and M.O. Saar On the importance of the thermosiphon effect in CPG (CO2-Plume geothermal) power systems Energy, 69, pp. 409-418, 2014. [Download PDF] [View Abstract]CPG (CO2 Plume Geothermal) energy systems use CO2 to extract thermal energy from naturally permeable geologic formations at depth. CO2 has advantages over brine: high mobility, low solubility of amorphous silica, and higher density sensitivity to temperature. The density of CO2 changes substantially between geothermal reservoir and surface plant, resulting in a buoyancy-driven convective current – a thermosiphon – that reduces or eliminates pumping requirements. We estimated and compared the strength of this thermosiphon for CO2 and for 20 weight percent NaCl brine for reservoir depths up to 5 km and geothermal gradients of 20, 35, and 50 °C/km. We found that through the reservoir, CO2 has a pressure drop approximately 3–12 times less than brine at the same mass flowrate, making the CO2 thermosiphon sufficient to produce power using reservoirs as shallow as 0.5 km. At 2.5 km depth with a 35 °C/km gradient – the approximate western U.S. continental mean – the CO2 thermosiphon converted approximately 10% of the energy extracted from the reservoir to fluid circulation, compared to less than 1% with brine, where additional mechanical pumping is necessary. We found CO2 is a particularly advantageous working fluid at depths between 0.5 km and 3 km.

Tutolo, B.M., X.-Z. Kong, W.E. Seyfried Jr., and M.O. Saar Internal consistency in aqueous geochemical data revisited: Applications to the aluminum system Geochimica et Cosmochimica Acta, 133, pp. 216-234, 2014. [Download PDF] [View Abstract]Internal consistency of thermodynamic data has long been considered vital for confident calculations of aqueous geochemical processes. However, an internally consistent mineral thermodynamic data set is not necessarily consistent with calculations of aqueous species thermodynamic properties due, potentially, to improper or inconsistent constraints used in the derivation process. In this study, we attempt to accommodate the need for a mineral thermodynamic data set that is internally consistent with respect to aqueous species thermodynamic properties by adapting the least squares optimization methods of Powell and Holland (1985). This adapted method allows for both the derivation of mineral thermodynamic properties from fluid chemistry measurements of solutions in equilibrium with mineral assemblages, as well as estimates of the uncertainty on the derived results. Using a large number of phase equilibria, solubility, and calorimetric measurements, we have developed a thermodynamic data set of 12 key aluminum-bearing mineral phases. These data are derived to be consistent with Na+ and K+ speciation data presented by Shock and Helgeson (1988), H4SiO4(aq) data presented by Stefánsson (2001), and the Al speciation data set presented by Tagirov and Schott (2001). Many of the constraining phase equilibrium measurements are exactly the same as those used to develop other thermodynamic data, yet our derived values tend to be quite different than some of the others’ due to our choices of reference data. The differing values of mineral thermodynamic properties have implications for calculations of Al mineral solubilities; specifically, kaolinite solubilities calculated with the developed data set are as much as 6.75 times lower and 73% greater than those calculated with Helgeson et al. (1978) and Holland and Powell (2011) data, respectively. Where possible, calculations and experimental data are compared at low T, and the disagreement between the two sources reiterates the common assertion that low-T measurements of phase equilibria and mineral solubilities in the aluminum system rarely represent equilibrium between water and well-crystallized, aluminum-bearing minerals. As an ancillary benefit of the derived data, we show that it may be combined with high precision measurements of aqueous complex association constants to derive neutral species activity coefficients in supercritical fluids. Although this contribution is specific to the aluminum system, the methods and concepts developed here can help to improve the calculation of water–rock interactions in a broad range of earth systems.

Luhmann, A.J., X.-Z. Kong, B.M. Tutolo, N. Garapati, B.C. Bagley, M.O. Saar, and W.E. Seyfried Jr. Experimental dissolution of dolomite by CO2-charged brine at 100oC and 150 bar: Evolution of porosity, permeability, and reactive surface area Chemical Geology, 380, pp. 145-160, 2014. [Download PDF] [View Abstract]Hydrothermal flow experiments of single-pass injection of CO2-charged brine were conducted on nine dolomite cores to examine fluid–rock reactions in dolomite reservoirs under geologic carbon sequestration conditions. Post-experimental X-ray computed tomography (XRCT) analysis illustrates a range of dissolution patterns, and significant increases in core bulk permeability were measured as the dolomite dissolved. Outflow fluids were below dolomite saturation, and cation concentrations decreased with time due to reductions in reactive surface area with reaction progress. To determine changes in reactive surface area, we employ a power-law relationship between reactive surface area and porosity (Luquot and Gouze, 2009). The exponent in this relationship is interpreted to be a geometrical parameter that controls the degree of surface area change per change in core porosity. Combined with XRCT reconstructions of dissolution patterns, we demonstrate that this exponent is inversely related to both the flow path diameter and tortuosity of the dissolution channel. Even though XRCT reconstructions illustrate dissolution at selected regions within each core, relatively high Ba and Mn recoveries in fluid samples suggest that dissolution occurred along the core's entire length and width. Analysis of porosity–permeability data indicates an increase in the rate of permeability enhancement per increase in porosity with reaction progress as dissolution channels lengthen along the core. Finally, we incorporate the surface area–porosity model of Luquot and Gouze (2009) with our experimentally fit parameters into TOUGHREACT to simulate experimental observations.

Tutolo, B.M., A.J. Luhmann, X.-Z. Kong, M.O. Saar, and W.E. Seyfried Jr. Experimental observation of permeability changes in dolomite at CO2 sequestration conditions Environmental Science and Technology, 48/4, pp. 2445-2452, 2014. [Download PDF] [View Abstract]Injection of cool CO2 into geothermally warm carbonate reservoirs for storage or geothermal energy production may lower near-well temperature and lead to mass transfer along flow paths leading away from the well. To investigate this process, a dolomite core was subjected to a 650 h, high pressure, CO2 saturated, flow-through experiment. Permeability increased from 10–15.9 to 10–15.2 m2 over the initial 216 h at 21 °C, decreased to 10–16.2 m2 over 289 h at 50 °C, largely due to thermally driven CO2 exsolution, and reached a final value of 10–16.4 m2 after 145 h at 100 °C due to continued exsolution and the onset of dolomite precipitation. Theoretical calculations show that CO2 exsolution results in a maximum pore space CO2 saturation of 0.5, and steady state relative permeabilities of CO2 and water on the order of 0.0065 and 0.1, respectively. Post-experiment imagery reveals matrix dissolution at low temperatures, and subsequent filling-in of flow passages at elevated temperature. Geochemical calculations indicate that reservoir fluids subjected to a thermal gradient may exsolve and precipitate up to 200 cm3 CO2 and 1.5 cm3 dolomite per kg of water, respectively, resulting in substantial porosity and permeability redistribution.

Ma, Y., A. Scheuermann, D. Bringemeier, X.-Z. Kong, and L. Li Size distribution measurement for densely binding bubbles via image analysis Experiments in Fluids, 55/1860, 2014. [Download PDF] [View Abstract]For densely binding bubble clusters, conventional image analysis methods are unable to provide an accurate measurement of the bubble size distribution because of the difficulties with clearly identifying the outline edges of individual bubbles. In contrast, the bright centroids of individual bubbles can be distinctly defined and thus accurately measured. By taking this advantage, we developed a new measurement method based on a linear relationship between the bubble radius and the radius of its bright centroid so to avoid the need to identify the bubble outline edges. The linear relationship and method were thoroughly tested for 2D bubble clusters in a highly binding condition and found to be effective and robust for measuring the bubble sizes.

Matculevich, S., and S. Repin Computable estimates of the distance to the exact solution of the evolutionary reaction-diffusion equation Appl. Math. Comput., 247, pp. 329-347, 2014. [Download PDF] [View Abstract]We derive guaranteed bounds of distance to the exact solution of the evolutionary reaction-diffusion problem with mixed Dirichlet-Neumann boundary condition. It is shown that two-sided error estimates are directly computable and equivalent to the error. Numerical experiments confirm that estimates provide accurate two-sided bounds of the overall error and generate efficient indicators of local error distribution.

Kong, X.-Z., and M.O. Saar Numerical study of the effects of permeability heterogeneity on density-driven convective mixing during CO2 dissolution storage Int. J. Greenhouse Gas Control, 19, pp. 160-173, 2013. [Download PDF] [View Abstract]Permanence and security of carbon dioxide (CO2) in geologic formations requires dissolution of CO2 into brine, which slightly increases the brine density. Previous studies have shown that this small increase in brine density induces convective currents, which greatly enhances the mixing efficiency and thus CO2 storage capacity and rate in the brine. Density-driven convection, in turn, is known to be largely dominated by permeability heterogeneity. This study explores the relationship between the process of density-driven convection and the permeability heterogeneity of an aquifer during CO2 dissolution storage, using high-resolution numerical simulations. While the porosity is kept constant, the heterogeneity of the aquifer is introduced through a spatially varying permeability field, characterized by the Dykstra-Parsons coefficient and the correlation length. Depending on the concentration profile of dissolved CO2, we classify the convective finger patterns as dispersive, preferential, and unbiased fingering. Our results indicate that the transition between unbiased and both preferential and dispersive fingering is mainly governed by the Dykstra-Parsons coefficient, whereas the transition between preferential and dispersive fingering is controlled by the permeability correlation length. Furthermore, we find that the CO2 dissolution flux at the top boundary will reach a time-independent steady state. Although this flux strongly correlates with permeability distribution, it generally increases with the permeability heterogeneity when the correlation length is less than the system size.

Kong, X.-Z., and M.O. Saar DBCreate: A SUPCRT92-based program for producing EQ3/6, TOUGHREACT, and GWB thermodynamic databases at user-defined T and P Computers and Geosciences, 51, pp. 415-417, 2013. [Download PDF] [View Abstract]SUPCRT92 is a widely used software package for calculating the standard thermodynamic properties of minerals, gases, aqueous species, and reactions. However, it is labor-intensive and error-prone to use it directly to produce databases for geochemical modeling programs such as EQ3/6, the Geochemist's Workbench, and TOUGHREACT. DBCreate is a SUPCRT92-based software program written in FORTRAN90/95 and was developed in order to produce the required databases for these programs in a rapid and convenient way. This paper describes the overall structure of the program and provides detailed usage instructions.

Samrock, F., and A. Kuvshinov Tippers at island observatories: Can we use them to probe electrical conductivity of the Earth's crust and upper mantle? Geophysical Research Letters - AGU Journal, 40/5, pp. 824-828, 2013. [Download PDF] [View Abstract][1] For decades, time series of hourly-mean values of the geomagnetic field measured on a global network of observatories have been routinely used to recover the electrical conductivity distribution in midmantle depths. Nowadays, most observatories provide data in the form of minute-means. This allows for analysis of short-period geomagnetic variations, which, in principle, contain information about geoelectric structures in the crust and upper mantle. However, so far these data have been ignored for induction studies of the Earth due to a theoretical preconception. In this paper, we demonstrate that short-period responses (tippers) at island observatories, being large owing to the ocean effect, are also sensitive to 1-D structures and thus can be used for probing the Earth. This means that a huge amount of data that was not exploited hitherto for induction studies should be reconsidered as a useful source of information about geoelectric structures in oceanic regions where our knowledge is still very limited.

Helmig, R., B. Flemisch, M. Wolff, A. Ebigbo, and H. Class Model coupling for multiphase flow in porous media Advances in Water Resources, 51/7, pp. 52-66, 2013. [Download PDF] [View Abstract]Numerical models for flow and transport in porous media are valid for a particular set of processes, scales, levels of simplification and abstraction, grids etc. The coupling of two or more specialised models is a method of increasing the overall range of validity while keeping the computational costs relatively low. Several coupling concepts are reviewed in this article with a focus on the authors’ work in this field. The concepts are divided into temporal and spatial coupling concepts, of which the latter is subdivided into multi-process, multi-scale, multi-dimensional, and multi-compartment coupling strategies. Examples of applications for which these concepts can be relevant include groundwater protection and remediation, carbon dioxide storage, nuclear-waste disposal, soil dry-out and evaporation processes as well as fuel cells and technical filters.

Cunningham, A.B., E. Lauchnor, J. Eldring, E. Esposito, A.C. Mitchell, R. Gerlach, A.J. Phillips, A. Ebigbo, and L.H. Spangler Abandoned well CO2 leakage mitigation using biologically induced mineralization: current progress and future directions Greenhouse Gases: Science & Technology, 3, pp. 40-49, 2013. [Download PDF] [View Abstract]Methods of mitigating leakage or re-plugging abandoned wells before exposure to CO2are of high potential interest to prevent leakage of CO2 injected for geologic carbon sequestration in depleted oil and gas reservoirs where large numbers of abandoned wells are often present. While CO2resistant cements and ultrafine cements are being developed, technologies that can be delivered via low viscosity fluids could have significant advantages including the ability to plug small aperture leaks such as fractures or delamination interfaces. Additionally there is the potential to plug rock formation pore space around the wellbore in particularly problematic situations. We are carrying out research on the use of microbial biofilms capable of inducing the precipitation of crystalline calcium carbonate using the process of ureolysis. This method has the potential to reduce well bore permeability, coat cement to reduce CO2–related corrosion, and lower the risk of unwanted upward CO2 migration. In this spotlight, we highlight research currently underway at the Center for Biofilm Engineering (CBE) at Montana State University (MSU) in the area of ureolytic biomineralization sealing for reducing CO2 leakage risk. This research program combines two novel core testing systems and a 3-dimensional simulation model to investigate biomineralization under both radial and axial flow conditions and at temperatures and pressures which permit CO2 to exist in the supercritical state. This combination of modelling and experimentation is ultimately aimed at developing and verifying biomineralization sealing technologies and strategies which can successfully be applied at the field scale for carbon capture and geological storage (CCGS) projects. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd

Kissinger, A., R. Helmig, A. Ebigbo, H. Class, T. Lange, M. Sauter, M. Heitfeld, J. Klünker, and W. Jahnke Hydraulic fracturing in unconventional gas reservoirs: risks in the geological system, part 2 Environmental Earth Sciences, 70/8, pp. 3855-3873, 2013. [Download PDF] [View Abstract]Hydraulic fracturing is a method used for the production of unconventional gas resources. Huge amounts of so-called fracturing fluid (10,000–20,000 m3) are injected into a gas reservoir to create fractures in solid rock formations, upon which mobilised methane fills the pore space and the fracturing fluid is withdrawn. Hydraulic fracturing may pose a threat to groundwater resources if fracturing fluid or brine can migrate through fault zones into shallow aquifers. Diffuse methane emissions from the gas reservoir may not only contaminate shallow groundwater aquifers, but also escape into the atmosphere where methane acts as a greenhouse gas. The working group “Risks in the Geological System” as part of ExxonMobil’s hydrofracking dialogue and information dissemination processes was tasked with the assessment of possible hazards posed by migrating fluids as a result of hydraulic fracturing activities. In this work, several flow paths for fracturing fluid, brine and methane are identified and scenarios are set up to qualitatively estimate under what circumstances these fluids would leak into shallower layers. The parametrisation for potential hydraulic fracturing sites in North Rhine-Westphalia and Lower Saxony (both in Germany) is derived from literature using upper and lower bounds of hydraulic parameters. The results show that a significant fluid migration is only possible if a combination of several conservative assumptions is met by a scenario.

Lange, T., M. Sauter, M. Heitfeld, K. Schetelig, K. Brosig, W. Jahnke, A. Kissinger, R. Helmig, A. Ebigbo, and H. Class Hydraulic fracturing in unconventional gas reservoirs: risks in the geological system, part 1 Environmental Earth Sciences, 70/8, pp. 3839-3853, 2013. [Download PDF] [View Abstract]Hydraulic fracturing of unconventional gas reservoirs rapidly developed especially in the USA to an industrial scale during the last decade. Potential adverse effects such as the deterioration of the quality of exploitable groundwater resources, areal footprints, or even the climate impact were not assessed. Because hydraulic fracturing has already been practised for a long time also in conventional reservoirs, the expansion into the unconventional domain was considered to be just a minor but not a technological step, with potential environmental risks. Thus, safety and environmental protection regulations were not critically developed or refined. Consequently, virtually no baseline conditions were documented before on-site applications as proof of evidence for the net effect of environmental impacts. Not only growing concerns in the general public, but also in the administrations in Germany promoted the commissioning of several expert opinions, evaluating safety, potential risks, and footprints of the technology in focus. The first two publications of the workgroup “Risks in the Geological System” of the independent “Information and Dialogue process on hydraulic fracturing” (commissioned by ExxonMobil Production Deutschland GmbH) comprises the strategy and approaches to identify and assess the potential risks of groundwater contamination of the exploitable groundwater system in the context of hydraulic fracturing operations in the Münsterland cretaceous basin and the Lower Saxony Basin, Germany. While being specific with respect to local geology and the estimation of effective hydraulic parameters, generalized concepts for the contamination risk assessment were developed. The work focuses on barrier effectiveness of different units of the overburden with respect to the migration of fracking fluids and methane, and considers fault zones as potential fluid pathway structures.

Ebigbo, A., F. Golfier, and M. Quintard A coupled, pore-scale model for methanogenic microbial activity in underground hydrogen storage Advances in Water Resources, 61, pp. 74-85, 2013. [Download PDF] [View Abstract]Underground hydrogen storage (UHS) as a means of energy storage is an efficient way of compensating for seasonal fluctuations in the availability of energy. One important factor which influences this technology is the activity of methanogenic microorganisms capable of utilising hydrogen and carbon dioxide for metabolism and leading to a change in the stored gas composition. A coupled, pore-scale model is presented which aids in the investigation of the mechanisms that govern the conversion of hydrogen to methane, i.e. advective hydrogen flow, its diffusion into microbial biofilms of multiple species, and its consumption within these biofilms. The model assumes that spherical grains are coated by a film of residual water and treats the biofilm development within each film in a quasi one-dimensional manner. A sample simulation using the presented model illustrates the biofilm growth process in these films as well as the competition between three different microbial species: methanogens, acetogens, and acetotrophs.

Ma, J., D. Petrilli, and J.C. Manceau Core scale modelling of CO2 flowing: identifying key parameters and experiment fitting Energy Procedia, 37, pp. 5464-5472, 2013. [Download PDF] [View Abstract]In this study, we propose to evaluate CO2-brine characteristics using core flooding experiment results with magnetic resonance (MR) imaging and a 1D numerical modelling approach along with a perspective on the role of CO2-brine characteristics on storage efficiency at the reservoir scale. MRI can be used to understand the pore structure and the flow characteristic of the drainage process more directly. The relative permeability curve which is the key parameter to field scale simulation can be obtained by the experiments. 1D numerical modelling is conducted to understand the results observed experimentally and the associated processes by using the parameters measured during the experiments. The modelling can explain the observed differences with the experiment through a sensitivity analysis and propose several set of parameters allowing a good match between experiments and models (history matching). It is shown that the combination method between the experiments and the modelling is a suitable method to understand the mechanism of CO2 geological storage. Moreover, the experiments can provide the validation to the modelling which is the important tool to predict the CO2 migration underground.

Walsh, S.D.C., and M.O. Saar Developing extensible lattice-Boltzmann simulators for general-purpose graphics-processing units Communications in Computational Physics, 13/3, pp. 867-879, 2013. [Download PDF] [View Abstract]Lattice-Boltzmann methods are versatile numerical modeling techniques capable of reproducing a wide variety of fluid-mechanical behavior. These methods are well suited to parallel implementation, particularly on the single-instruction multiple data (SIMD) parallel processing environments found in computer graphics processing units (GPUs). Although recent programming tools dramatically improve the ease with which GPUbased applications can be written, the programming environment still lacks the flexibility available to more traditional CPU programs. In particular, it may be difficult to develop modular and extensible programs that require variable on-device functionality with current GPU architectures. This paper describes a process of automatic code generation that overcomes these difficulties for lattice-Boltzmann simulations. It details the development of GPU-based modules for an extensible lattice-Boltzmann simulation package – LBHydra. The performance of the automatically generated code is compared to equivalent purposewritten codes for both single-phase,multiphase, andmulticomponent flows. The flexibility of the new method is demonstrated by simulating a rising, dissolving droplet moving through a porous medium with user generated lattice-Boltzmann models and subroutines.

Gottardi, R., P.-H. Kao, M.O. Saar, and Ch. Teyssier Effects of permeability fields on fluid, heat, and oxygen isotope transport in extensional detachment systems Geochemistry, Geophysics, Geosystems, 14/5, pp. 1493-1522, 2013. [Download PDF] [View Abstract][1] Field studies of Cordilleran metamorphic core complexes indicate that meteoric fluids permeated the upper crust down to the detachment shear zone and interacted with highly deformed and recrystallized (mylonitic) rocks. The presence of fluids in the brittle/ductile transition zone is recorded in the oxygen and hydrogen stable isotope compositions of the mylonites and may play an important role in the thermomechanical evolution of the detachment shear zone. Geochemical data show that fluid flow in the brittle upper crust is primarily controlled by the large-scale fault-zone architecture. We conduct continuum-scale (i.e., large-scale, partial bounce-back) lattice-Boltzman fluid, heat, and oxygen isotope transport simulations of an idealized cross section of a metamorphic core complex. The simulations investigate the effects of crust and fault permeability fields as well as buoyancy-driven flow on two-way coupled fluid and heat transfer and resultant exchange of oxygen isotopes between meteoric fluid and rock. Results show that fluid migration to middle to lower crustal levels is fault controlled and depends primarily on the permeability contrast between the fault zone and the crustal rocks. High fault/crust permeability ratios lead to channelized flow in the fault and shear zones, while lower ratios allow leakage of the fluids from the fault into the crust. Buoyancy affects mainly flow patterns (more upward directed) and, to a lesser extent, temperature distributions (disturbance of the geothermal field by ~25°C). Channelized fluid flow in the shear zone leads to strong vertical and horizontal thermal gradients, comparable to field observations. The oxygen isotope results show δ18O depletion concentrated along the fault and shear zones, similar to field data.

Randolph, J.B., M.O. Saar, and J.M. Bielicki Geothermal energy production at geologic CO2 sequestration sites: Impact of thermal drawdown on reservoir pressure Energy Procedia, 37, pp. 6625-6635, 2013. [Download PDF] [View Abstract]Recent geotechnical research shows that geothermal heat can be efficiently mined by circulating carbon dioxide through naturally permeable rock formations -- a method called CO2 Plume Geothermal -- the same geologic reservoirs that are suitable for deep saline aquifer CO2 sequestration or enhanced oil recovery. This paper describes the effect of thermal drawdown on reservoir pressure buildup during sequestration operations, revealing that geothermal heat mining can decrease overpressurization by 10% or more.

Leal, A.M.M., M.J. Blunt, and T.C. LaForce A robust and efficient numerical method for multiphase equilibrium calculations: Application to CO2-brine-rock systems at high temperatures, pressures and salinities Advances in Water Resources, 62(Part C), pp. 409-430, 2013. [Download PDF] [View Abstract]We present a robust and efficient method for calculating chemical equilibria of general multiphase systems. The method is based on a stoichiometric approach, which uses Newton’s method to solve a system of mass-action equations coupled with a system of equilibrium constraints. A stabilisation procedure is developed to promote convergence of the calculation when a presupposed phase in the chemical system is absent in the equilibrium state. The formulation of the chemical equilibrium problem is developed by presuming no specific details of the involved phases and species. As a consequence, the method is flexible and general enough so that the calculation can be customised with a combination of thermodynamic models that are appropriate for the problem of interest. Finally, we show the use of the method to solve relevant geochemical equilibrium problems for modelling carbon storage in highly saline aquifers.

Luhmann, A.J., X.-Z. Kong, B.M. Tutolo, K. Ding, M.O. Saar, and W.E. Seyfried Jr. Permeability reduction produced by grain reorganization and accumulation of exsolved CO2 during geologic carbon sequestration: A new CO2 trapping mechanism Environmental Science and Technlogy, 47/1, pp. 242-251, 2013. [Download PDF] [View Abstract]Carbon sequestration experiments were conducted on uncemented sediment and lithified rock from the Eau Claire Formation, which consisted primarily of K-feldspar and quartz. Cores were heated to accentuate reactivity between fluid and mineral grains and to force CO2 exsolution. Measured permeability of one sediment core ultimately reduced by 4 orders of magnitude as it was incrementally heated from 21 to 150 °C. Water-rock interaction produced some alteration, yielding sub-?m clay precipitation on K-feldspar grains in the core’s upstream end. Experimental results also revealed abundant newly formed pore space in regions of the core, and in some cases pores that were several times larger than the average grain size of the sediment. These large pores likely formed from elevated localized pressure caused by rapid CO2 exsolution within the core and/or an accumulating CO2 phase capable of pushing out surrounding sediment. CO2 filled the pores and blocked flow pathways. Comparison with a similar experiment using a solid arkose core indicates that CO2 accumulation and grain reorganization mainly contributed to permeability reduction during the heated sediment core experiment. This suggests that CO2 injection into sediments may store more CO2 and cause additional permeability reduction than is possible in lithified rock due to grain reorganization.

Ma, J., R.N. Xu, and S. Luo Core-scale Experimental Study on Supercritical-Pressure CO2 Migration Mechanism during CO2 Geological Storage in Deep Saline Aquifers Journal of Engineering Thermophysics, 33, pp. 1971-1975, 2012. [View Abstract]Abstract To address the climate change and reduce the emission of CO2, CO2 storage in the deep saline aquifer is one of the promising technologies. The visualization experimental system was set up to investigate the CO2 migration mechanism during the displacement of supercritical CO2 and water inside the core rock. From the experimental system, the experiment measured porosity, calculated the relative permeability-water saturation curve the water distribution will be achieved. The porosity can be measured accurately using MR technique. The fraction of effective porosity and movable fluid can be calculated, according to the T2 curve from MR. The MRI for core slice with the injection ratio of CO2:H2O=3:1 shows remarkable buoyancy effect. Core-scale experimental study on supercritical-pressure CO2 migration mechanism during CO2 geological storage in deep saline aquifers. Available from: [accessed Jun 7, 2017].

Alexander, S.C., and M.O. Saar Improved characterization of small u for Jacob pumping test analysis methods Ground Water, 50/2, pp. 256-265, 2012. [Download PDF] [View Abstract]Numerous refinements have been proposed to traditional pumping test analyses, yet many hydrogeologists continue to use the Jacob method due to its simplicity. Recent research favors hydraulic tomography and inverse numerical modeling of pumping test data. However, at sites with few wells, or relatively short screens, the data requirements of these methods may be impractical within physical and fiscal constraints. Alternatively, an improved understanding of the assumptions and limitations of Theis and, due to their widespread usage, Jacob analyses, leads to improved interpretations in data-poor environments. A fundamental requirement of Jacob is a “small” value of u = f(r2/t), with radial distance, r, and pumping time, t. However, selection of a too stringent (i.e., too low) maximum permissible u-value, umax, results in rejection of usable data from wells beyond a maximum radius, rmax. Conversely, data from small radii, less than rmin, where turbulent- and vertical-flow components arise, can result in acceptance of inappropriate data. Usage of drawdown data from wells too close to the pumping well, and exclusion of data from wells deemed too far, can cause unrealistic aquifer transmissivity, permeability, and storativity determinations. Here, data from an extensive well field in a glacial-outwash aquifer in north-central Minnesota, USA, are used to develop a new estimate for umax. Traditionally quoted values for umax range from 0.01 to 0.05. Our proposed value for Jacob distance-drawdown analyses is significantly higher with umax up to 0.2, resulting in larger allowable rmax-values and a higher likelihood of inclusion of additional wells in such pumping test analyses.

Covington, M.D., A.J. Luhmann, C. Wicks, and M.O. Saar Process length scales and longitudinal damping in karst conduits Journal of Geophysical Research - Earth Surface, 117, F01025, 2012. [Download PDF] [View Abstract][1] Simple mathematical models often allow an intuitive grasp of the function of physical systems. We develop a mathematical framework to investigate reactive or dissipative transport processes within karst conduits. Specifically, we note that for processes that occur within a characteristic timescale, advection along the conduit produces a characteristic process length scale. We calculate characteristic length scales for the propagation of thermal and electrical conductivity signals along karst conduits. These process lengths provide a quantitative connection between karst conduit geometry and the signals observed at a karst spring. We show that water input from the porous/fractured matrix is also characterized by a length scale and derive an approximation that accounts for the influence of matrix flow on the transmission of signals through the aquifer. The single conduit model is then extended to account for conduits with changing geometries and conduit flow networks, demonstrating how these concepts can be applied in more realistic conduit geometries. We introduce a recharge density function, ϕR, which determines the capability of an aquifer to damp a given signal, and cast previous explanations of spring variability within this framework. Process lengths are a general feature of karst conduits and surface streams, and we conclude with a discussion of other potential applications of this conceptual and mathematical framework.

Covington, M., A.F. Banwell, J. Gulley, and M.O. Saar Quantifying the effects of glacier conduit geometry and recharge on proglacial hydrograph form Journal of Hydrology, 414-415, pp. 59-71, 2012. [Download PDF] [View Abstract]The configuration of glacier hydrological systems is often inferred from proxy data, such as hydrographs, that are collected in proglacial streams. Seasonal changes in the peakedness of hydrographs are thought to reflect changes in the configuration of the subglacial drainage system. However, the amount of information that proglacial hydrographs contain about drainage system configurations depends critically on the degree to which the drainage systems modify recharge hydrographs. If the drainage system does not modify recharge hydrographs, then proglacial hydrographs primarily reflect the recharge conditions produced by supraglacial inputs. Here, we develop a theoretical framework to determine the circumstances under which glacier drainage systems can modify recharge hydrographs and the circumstances under which recharge pulses pass through glaciers unchanged. We address the capability of single conduits, simple arborescent conduit networks, and linked cavity systems to modify diurnal recharge pulses. Simulations of discharge through large sets of such systems demonstrate that, unless large reservoirs or significant constrictions are present, the discharge hydrographs of simple glacial conduit systems are nearly identical to their recharge hydrographs. Conduit systems tend not to modify hydrographs because the changes in storage within englacial and subglacial conduit networks on short time scales are typically small compared to their ability to transmit water. This finding suggests that proglacial hydrographs reflect a variety of factors, including surface melt rate, surface water transfer, and subglacial water transfer. In many cases the influence of suglacial processes may be relatively minor. As a result, the evolution of proglacial hydrographs cannot be used unambiguously to infer changes in the structure or efficiency of englacial or subglacial hydrological systems, without accurate knowledge of the nature of the recharge hydrograph driving the flow.

Ebigbo, A., A. Phillips, R. Gerlach, R. Helmig, A.B. Cunningham, and H. Class Darcy-scale modeling of microbially induced carbonate mineral precipitation in sand columns Water Resources Research, 48/7, W07519, 2012. [Download PDF] [View Abstract][1] This investigation focuses on the use of microbially induced calcium carbonate precipitation (MICP) to set up subsurface hydraulic barriers to potentially increase storage security near wellbores of CO2 storage sites. A numerical model is developed, capable of accounting for carbonate precipitation due to ureolytic bacterial activity as well as the flow of two fluid phases in the subsurface. The model is compared to experiments involving saturated flow through sand-packed columns to understand and optimize the processes involved as well as to validate the numerical model. It is then used to predict the effect of dense-phase CO2 and CO2-saturated water on carbonate precipitates in a porous medium.

Evans, K.F., A. Zappone, T. Kraft, N. Deichmann, and F. Moia A survey of the induced seismic responses to fluid injection in geothermal and CO2 reservoirs in Europe Geothermics, 41, pp. 30-54, 2012. [Download PDF]

Saar, M.O. Review: Geothermal heat as a tracer of large-scale groundwater flow and as a means to determine permeability fields, special theme issue on Environmental Tracers and Groundwater Flow, editor-invited peer-reviewed contribution Hydrogeology Journal, 19, pp. 31-52, 2011. [Download PDF] [View Abstract]A review of coupled groundwater and heat transfer theory is followed by an introduction to geothermal measurement techniques. Thereafter, temperature-depth profiles (geotherms) and heat discharge at springs to infer hydraulic parameters and processes are discussed. Several studies included in this review state that minimum permeabilities of approximately 5 × 10−17 < kmin <10−15 m2 are required to observe advective heat transfer and resultant geotherm perturbations. Permeabilities below kmin tend to cause heat-conduction-dominated systems, precluding inversion of temperature fields for groundwater flow patterns and constraint of permeabilities other than being

Randolph, J.B., and M.O. Saar Coupling carbon dioxide sequestration with geothermal energy capture in naturally permeable, porous geologic formations: Implications for CO2 sequestration Energy Procedia, 4, pp. 2206-2213, 2011. [Download PDF] [View Abstract]Carbon dioxide (CO2) sequestration in deep saline aquifers and exhausted oil and natural gas fields has been widely considered as a means for reducing CO2 emissions to the atmosphere as a counter-measure to global warming. However, rather than treating CO2 merely as a waste fluid in need of permanent disposal, we propose that it could also be used as a working fluid in geothermal energy capture, as its thermodynamic and fluid mechanical properties suggest it transfers geothermal heat more efficiently than water. Energy production and sales in conjunction with sequestration would improve the economic viability of CO2 sequestration, a critical challenge for large-scale implementation of the technology. In addition, using CO2 as the working fluid in geothermal power systems may permit utilization of lower temperature geologic formations than those that are currently deemed economically viable, leading to more widespread utilization of geothermal energy. Here, we present the results of early-stage calculations demonstrating the geothermal energy capture potential of CO2-based geothermal systems and implications of such energy capture for the economic viability of geologic CO2 sequestration.

Randolph, J.B., and M.O. Saar Combining geothermal energy capture with geologic carbon dioxide sequestration Geophysical Research Letters, 38, L10401, 2011. [Download PDF] [View Abstract][1] Geothermal energy offers clean, renewable, reliable electric power with no need for grid-scale energy storage, yet its use has been constrained to the few locations worldwide with naturally high geothermal heat resources and groundwater availability. We present a novel approach with the potential to permit expansion of geothermal energy utilization: heat extraction from naturally porous, permeable formations with CO2 as the injected subsurface working fluid. Fluid-mechanical simulations reveal that the significantly higher mobility of CO2, compared to water, at the temperature/pressure conditions of interest makes CO2 an attractive heat exchange fluid. We show numerically that, compared to conventional water-based and engineered geothermal systems, the proposed approach provides up to factors of 2.9 and 5.0, respectively, higher geothermal heat energy extraction rates. Consequently, more regions worldwide could be economically used for geothermal electricity production. Furthermore, as the injected CO2 is eventually geologically sequestered, such power plants would have negative carbon footprints.

Randolph, J.B., and M.O. Saar Impact of reservoir permeability on the choice of subsurface geothermal heat exchange fluid: CO2 versus water and native brine Geothermal Resources Council (GRC) Transactions, 35, pp. 521-526, 2011. [View Abstract]Abstract Geothermal systems utilizing carbon dioxide (CO 2) as the subsurface heat exchange fluid in naturally porous, permeable geologic formations have been shown to provide improved geothermal heat energy extraction, even at low resource temperatures. Such systems, termed CO 2 Plume Geothermal (CPG) systems, have the potential to permit expansion of geothermal energy utilization while supporting rapid implementation through the use of existing technologies. Here, we explore CPG heat extraction as a function of reservoir permeability and in comparison to water and brine geothermal heat extraction. We show that for reservoir permeabilities below 2 x 10 -14 m 2 , CO 2 -based geothermal provides better electric power production efficiency than both water-and brine-based systems. Impact of reservoir permeability on the choice of subsurface geothermal heat exchange fluid: CO2 versus water and native brine (PDF Download Available). Available from: [accessed Jun 7, 2017].

Kong, X.-Z., M. Holzner, F. Stauffer, and W. Kinzelbach Time-resolved 3D visualization of air injection in a liquidsaturated refractive-index-matched porous medium Exp. Fluids, 50, pp. 1659-1670, 2011. [Download PDF] [View Abstract]The main goal of this work is to implement and validate a visualization method with a given temporal/spatial resolution to obtain the dynamic three-dimensional (3D) structure of an air plume injected into a deformable liquid-saturated porous medium. The air plume develops via continuous air injection through an orifice at the bottom of a loose packing of crushed silica grains. The packing is saturated by a glycerin-water solution having the same refractive index and placed in a rectangular glass container. By using high-speed image acquisition through laser scanning, the dynamic air plume is recorded by sequential tomographic imaging. Due to the overlap between adjacent laser sheets and the light reflection, air bubbles are multiply exposed in the imaging along the scanning direction. Four image processing methods are presented for the removal of these redundant pixels arising from multiple exposure. The respective results are discussed by comparing the reconstructed air plume volume with the injected one and by evaluating the morphological consistency of the obtained air plume. After processing, a 3D dynamic air flow pattern can be obtained, allowing a quantitative analysis of the air flow dynamics on pore-scale. In the present experimental configuration, the temporal resolution is 0.1 s and the spatial resolution is 0.17 mm in plane and about 1 mm out of plane of the laser sheet.

Covington, M.D., A.J. Luhmann, F. Gabrovsek, M.O. Saar, I. Willis, and C.M. Wicks Mechanisms of heat exchange between water and rock in karst conduits Water Resources Research, 47, W10514/10, 2011. [Download PDF] [View Abstract][1] Previous studies, motivated by understanding water quality, have explored the mechanisms for heat transport and heat exchange in surface streams. In karst aquifers, temperature signals play an additional important role since they carry information about internal aquifer structures. Models for heat transport in karst conduits have previously been developed; however, these models make different, sometimes contradictory, assumptions. Additionally, previous models of heat transport in karst conduits have not been validated using field data from conduits with known geometries. Here we use analytical solutions of heat transfer to examine the relative importance of heat exchange mechanisms and the validity of the assumptions made by previous models. The relative importance of convection, conduction, and radiation is a function of time. Using a characteristic timescale, we show that models neglecting rock conduction produce spurious results in realistic cases. In contrast to the behavior of surface streams, where conduction is often negligible, conduction through the rock surrounding a conduit determines heat flux at timescales of weeks and longer. In open channel conduits, radiative heat flux can be significant. In contrast, convective heat exchange through the conduit air is often negligible. Using the rules derived from our analytical analysis, we develop a numerical model for heat transport in a karst conduit. Our model compares favorably to thermal responses observed in two different karst settings: a cave stream fed via autogenic recharge during a snowmelt event, and an allogenically recharged cave stream that experiences continuous temperature fluctuations on many timescales.

Davis, M.A., S.D.C. Walsh, and M.O. Saar Statistically reconstructing continuous isotropic and anisotropic two-phase media while preserving macroscopic material properties Physical Review E, 83, 026706, 2011. [Download PDF] [View Abstract]We propose a method to generate statistically similar reconstructions of two-phase media. As with previous work, we initially characterize the microstructure of the material using two-point correlation functions (a subset of spatial correlation functions) and then generate numerical reconstructions using a simulated annealing method that preserves the geometric relationships of the material’s phase of interest. However, in contrast to earlier contributions that consider reconstructions composed of discrete arrays of pixels or voxels alone, we generate reconstructions based on assemblies of continuous, three-dimensional, interpenetrating objects. The result is a continuum description of the material microstructure (as opposed to a discretized or pixelated description), capable of efficiently representing large disparities in scale. Different reconstruction methods are considered based on distinct combinations of two-point correlation functions of varying degrees of complexity. The quality of the reconstruction methods are evaluated by comparing the total pore fraction, specific surface area of the percolating cluster, pore fraction of the percolating cluster, tortuosity, and permeability of the reconstructions to those of a set of reference assemblies. Elsewhere it has been proposed that two-phase media could be statistically reproduced with only two spatial correlation functions: the two-point probability function (the probability that two points lie within the same phase) and the lineal path function (the probability that a line between two points lies entirely within the same phase). We find that methods employing the two-point probability function and lineal path function are improved if the percolating cluster volume is also considered in the reconstruction. However, to reproduce more complicated geometric assemblies, we find it necessary to employ the two-point probability, two-point cluster, and lineal path function in addition to the percolating cluster volume to produce a generally accurate statistical reconstruction.

Randolph, J.B., and M.O. Saar Coupling geothermal energy capture with carbon dioxide sequestration in naturally permeable, porous geologic formations: A comparison with enhanced geothermal systems Geothermal Resources Council (GRC) Transactions, 34, pp. 433-438, 2010. [View Abstract]Geothermal energy offers clean, consistent, reliable electric power with no need for grid-scale energy storage, unlike wind and solar renewable power alternatives. However, geothermal energy is often underrepresented in renewable energy discussions and has considerable room for growth. New technology and methods will be critical for future investment, and rapid implementation of new techniques will be critical in ensuring geothermal energy plays a significant role in the future energy landscape world - wide. Here, we discuss a novel approach with the potential to permit expansion of geothermal energy utilization while supporting rapid implementation through the use of exist - ing technologies: geothermal heat use in naturally porous, permeable geologic formations with carbon dioxide as the working heat exchange fluid.

Genter, A., K.F. Evans, N. Cuenot, D. Fritsch, and B. Sanjuan The Soultz geothermal adventure: 20 years of research and exploration of deep crystalline fractured rocks for EGS development Comptes Rendus Geoscience, 342, pp. 502-516, 2010. [Download PDF]

Dasgupta, S., M.O. Saar, R.L. Edwards, C.-C. Shen, H. Cheng, and C.E. Alexander Jr. Three thousand years of extreme rainfall events recorded in stalagmites from Spring Valley Caverns, Minnesota Earth and Planetary Science Letters, 300, pp. 46-54, 2010. [Download PDF] [View Abstract]Annual layer analysis in two stalagmites collected from Spring Valley Caverns, southeastern Minnesota, reveals hydrological response of the cave to extreme rainfall events in the Midwest, USA. Cave-flooding events are identified within the two samples by the presence of detrital layers composed of clay sized particles. Comparison with instrumental records of precipitation demonstrates a strong correlation between these cave-flood events and extreme rainfall observed in the Upper Mississippi Valley. A simple model is developed to assess the nature of rainfall capable of flooding the cave. The model is first calibrated to the last 50-yr (1950–1998 A.D.) instrumental record of daily precipitation data for the town of Spring Valley and verified with the first 50 yr of record from 1900 to 1949 A.D. Frequency analysis shows that these extreme flood events have increased from the last half of the nineteenth century. Comparison with other paleohydrological records shows increased occurrence of extreme rain events during periods of higher moisture availability. Our study implies that increased moisture availability in the Midwestern region, due to rise in temperature from global warming could lead to an increase in the occurrence of extreme rainfall events.

Kong, X.-Z., W. Kinzelbach, and F. Stauffer Compaction and size segregation in a liquid-saturated grain packing due to pulsation effect during air injection Chem.Eng.Sci., 65, pp. 2680-2688, 2010. [Download PDF] [View Abstract]Injecting air into two-dimensional vertical liquid-saturated assemblies of grains causes a rearrangement of the grains. The interaction of the air flow injected at the bottom with the grains and the liquid leads to a mobilization of the grains, in which air channels migrate and grain clusters undergo shearing. The channel migration comes to a stop after some time, leaving one thin and stable preferential channel for air flow. Furthermore, the grain packing is compacted due to a rearrangement process caused by the pulsating movement of air channels. The compaction process is found to obey a slow exponential growth law. Additionally, the pulsation introduces size segregation in the packing. This is visualized by a set of tracing experiments showing that the coarser grains tend to accumulate at the downstream end of the preferential air pathway. However, the pulsating strength decreases sharply as the grain size increases. Therefore no segregation was observed in the coarse packing. A stabilization process of the preferential air channel can be described by a lower bound of critical channel size assuming the validity of Hagen–Poiseuille air flow inside the channel. Nevertheless, the channel size could not exceed an upper size which is determined by the capillary instability, assuming a quasi-static equilibrium of the dilating process of the air channel.

Kong, X.-Z., W. Kinzelbach, and F. Stauffer Morphodynamics during air injection into water-saturated movable spherical granulates Chem.Eng.Sci., 65, pp. 4652-4660, 2010. [Download PDF] [View Abstract]Laboratory air injection experiments in a two-dimensional vertically placed cell filled with a water-saturated packing of spherical glass beads show a particular transition of air flow patterns at different length scales, tree-like fingering with pore-scale drainage, a local fluidization of beads with finger-scale channeled air flow, and an oscillating irregular channel with finger-scale air flow, where the channel is formed by pushing beads aside. The tree-like pattern shows a parabola-like growth with vertical height, while both the top and the bottom lateral width of the pattern increase linearly for the small injection rate, then stay more or less constant for further increasing injection rates. By tracing the vertical position of maximum advance of the fluidized pattern with time, the evolution of the fluidized pattern is characterized by two dynamic regimes, the pattern not reaching or reaching the injection orifice, respectively, using small and medium injection rates. As the injection rate increases further, the fluidized pattern shows up immediately after air is injected. Via physical conceptual models using local forces or pressure gradient balances, we derive a series of characteristics that describe the width of the tree-like air plume, the width of a single channel, and the starting vertical position of the fluidized pattern.

van Noorden, T.L., I.S. Pop, A. Ebigbo, and R. Helmig An upscaled model for biofilm growth in a thin strip Water Resources Research, 46/6, W06505, 2010. [Download PDF] [View Abstract][1] The focus of this paper is the derivation of an effective model for biofilm growth in a porous medium and its effect on fluid flow. The starting point is a pore-scale model in which the local geometry of the pore is represented as a thin strip. The model accounts for changes in pore volume due to biomass accumulation. As the ratio of the width of the strip to its length approaches zero, we apply a formal limiting argument to derive a one-dimensional upscaled (effective) model. For a better understanding of the terms and parameters involved in the equations derived here, we compare these equations to a well-known core-scale model from the literature.

Ebigbo, A., R. Helmig, A.B. Cunningham, H. Class, and R. Gerlach Modelling biofilm growth in the presence of carbon dioxide and water flow in the subsurface Advances in Water Resources, 33/7, pp. 762-781, 2010. [Download PDF] [View Abstract]The concentration of greenhouse gases – particularly carbon dioxide (CO2) – in the atmosphere has been on the rise in the past decades. One of the methods which have been proposed to help reduce anthropogenic CO2 emissions is the capture of CO2from large, stationary point sources and storage in deep geological formations. The caprock is an impermeable geological layer which prevents the leakage of stored CO2, and its integrity is of utmost importance for storage security. Due to the high pressure build-up during injection, the caprock in the vicinity of the well is particularly at risk of fracturing. Biofilms could be used as biobarriers which help prevent the leakage of CO2 through the caprock in injection well vicinity by blocking leakage pathways. The biofilm could also protect well cement from corrosion by CO2-rich brine. The goal of this paper is to develop and test a numerical model which is capable of simulating the development of a biofilm in a CO2 storage reservoir. This involves the description of the growth of the biofilm, flow and transport in the geological formation, and the interaction between the biofilm and the flow processes. Important processes which are accounted for in the model include the effect of biofilm growth on the permeability of the formation, the hazardous effect of supercritical CO2 on suspended and attached bacteria, attachment and detachment of biomass, and two-phase fluid flow processes. The model is tested by comparing simulation results to experimental data.

Skjaelaaen, I., A. Ebigbo, M. Espedal, and R. Helmig A model for transport of hydrogen sulfide in oil- and water-saturated porous media Computing and Visualization in Science, 13/6, pp. 265-273, 2010. [Download PDF] [View Abstract]In several oilfields, reservoir souring by generation of hydrogen sulfide (H2S) occurs in secondary recovery during which seawater is injected into originally sweet reservoirs. At the production site, high concentrations of H2S can cause severe damage to both equipment and human personnel. Proper modeling of H2S concentration in produced fluids can be useful for decision-making during field development design. We present a model for the transport of H2S in an oil- and water-saturated, water-wet porous medium. The different retardation mechanisms for the H2S are described. For the adsorption of H2S to rock, we include two distinct phases of adsorption. In addition, we introduce a functional relationship between adsorption capacity and permeability. As H2S mixes with oil, fractions become immobile as part of the residual oil. Communicated by Gabriel wittum. This article is dedicated to the memory of our dear colleague, friend and mentor, Magne Espedal, who passed away during the preparation of this manuscript.

Walsh, S.D.C., and M.O. Saar Macroscale lattice-Boltzmann methods for low-Peclet-number solute and heat transport in heterogeneous porous media Water Resources Research, 46, W07517, 2010. [Download PDF] [View Abstract][1] This paper introduces new methods for simulating subsurface solute and heat transport in heterogeneous media using large-scale lattice-Boltzmann models capable of representing both macroscopically averaged porous media and open channel flows. Previous examples of macroscopically averaged lattice-Boltzmann models for solute and heat transport are only applicable to homogeneous media. Here, we extend these models to properly account for heterogeneous pore-space distributions. For simplicity, in the majority of this paper we assume low Peclet number flows with an isotropic dispersion tensor. Nevertheless, this approach may also be extended to include anisotropic-dispersion by using multiple relaxation time lattice-Boltzmann methods. We describe two methods for introducing heterogeneity into macroscopically averaged lattice-Boltzmann models. The first model delivers the desired behavior by introducing an additional time-derivative term to the collision rule; the second model by separately weighting symmetric and anti-symmetric components of the fluid packet densities. Chapman-Enskog expansions are conducted on the governing equations of the two models, demonstrating that the correct constitutive behavior is obtained in both cases. In addition, methods for improving model stability at low porosities are also discussed: (1) an implicit formulation of the model; and (2) a local transformation that normalizes the lattice-Boltzmann model by the local porosity. The model performances are evaluated through comparisons of simulated results with analytical solutions for one- and two-dimensional flows, and by comparing model predictions to finite element simulations of advection isotropic-dispersion in heterogeneous porous media. We conclude by presenting an example application, demonstrating the ability of the new models to couple with simulations of reactive flow and changing flow geometry: a simulation of groundwater flow through a carbonate system.

Walsh, S.D.C., and M.O. Saar Interpolated lattice-Boltzmann boundary conditions for surface reaction kinetics Physical Review E, 82, 066703, 2010. [Download PDF] [View Abstract]This paper describes a method for implementing surface reaction kinetics in lattice Boltzmann simulations. The interpolated boundary conditions are capable of simulating surface reactions and dissolution at both stationary and moving solid-fluid and fluid-fluid interfaces. Results obtained with the boundary conditions are compared to analytical solutions for first-order and constant-flux kinetic surface reactions in a one-dimensional half space, as well as to the analytical solution for evaporation from the surface of a cylinder. Excellent agreement between analytical and simulated results is obtained for a wide range of diffusivities, lattice velocities, and surface reaction rates. The boundary model’s ability to represent dissolution in binary fluid mixtures is demonstrated by modeling diffusion from a rising bubble and dissolution of a droplet near a flat plate.

Myre, J., S.D.C. Walsh, D.J. Lilja, and M.O. Saar Performance analysis of single-phase multiphase, and multicomponent lattice-Boltzmann fluid flow simulations on GPU clusters Concurrency and Computation: Practice and Experience, 23, pp. 332-350, 2010. [Download PDF] [View Abstract]Abstract The lattice-Boltzmann method is well suited for implementation in single-instruction multiple-data (SIMD) environments provided by general purpose graphics processing units (GPGPUs). This paper discusses the integration of these GPGPU programs with OpenMP to create lattice-Boltzmann applications for multi-GPU clusters. In addition to the standard single-phase single-component lattice-Boltzmann method, the performances of more complex multiphase, multicomponent models are also examined. The contributions of various GPU lattice-Boltzmann parameters to the performance are examined and quantified with a statistical model of the performance using Analysis of Variance (ANOVA). By examining single- and multi-GPU lattice-Boltzmann simulations with ANOVA, we show that all the lattice-Boltzmann simulations primarily depend on effects corresponding to simulation geometry and decomposition, and not on the architectural aspects of GPU. Additionally, using ANOVA we confirm that the metrics of Efficiency and Utilization are not suitable for memory-bandwidth-dependent codes. Copyright © 2010 John Wiley & Sons, Ltd.

Kong, X.-Z., W. Kinzelbach, and F. Stauffer Migration of air channels: an instability of air flow in mobile saturated porous media Chem.Eng.Sci., 64, pp. 1528-1535, 2009. [Download PDF] [View Abstract]A set of two-dimensional laboratory visualization experiments reveals a previously unrecognized gas-flow instability in a porous medium saturated with a glycerine–water solution. The medium is a non-fixed vertically placed packing of grains of crushed fused silica glass. The interaction of the injected air flow and the medium structure leads to mobilization of the medium and an instability, which causes the air channel to migrate. This instability is dominated by a dimensionless number α, which can be interpreted as a normalization of a critical velocity with a dipole velocity for saturated conditions. The channel migration appears as a sequence of previous channels collapsing and new channels opening. The channel migration comes to a stop after some time, leaving one thin and stable channel. The process is studied by calculating the cumulated lateral movement distance of a channel and the lateral width of the area affected by the migration, both scaled by α with an empirical power of 0.25. Another dimensionless number f is defined to qualify the migration under different grain size, height of bed, and air flow rate.

Stauffer, F., X.-Z. Kong, and W. Kinzelbach A stochastic model for air injection into saturated porous media Water Resour, 32, pp. 1180-1186, 2009. [Download PDF] [View Abstract]Air injection into porous media is investigated by laboratory experiments and numerical modelling. Typical applications of air injection into a granular bed are aerated bio-filters and air sparging of aquifers. The first stage of the dynamic process consists of air injection into a fixed or a quasi-fixed water-saturated granular bed. Later stages could include stages of movable beds as well, but are not further investigated here. A series of laboratory experiments were conducted in a two-dimensional box of the size 60 cm × 38 cm × 0.55 cm consisting of glass walls and using glass beads of diameter 0.4–0.6 mm as granular material. The development of the air flow pattern was optically observed and registered using a digital video camera. The resulting transient air flow pattern can be characterized as channelled flow in a fixed porous medium with dynamic tree-like evolution behaviour. Attempts are undertaken to model the air injection process. Multiphase pore-scale modelling is currently disregarded since it is restricted to very small scales. Invasion percolation models taking into account gravity effects are usually restricted to slow processes. On the other hand a continuum-type two-phase flow modelling approach is not able to simulate the observed air flow pattern. Instead a stochastic continuum-type approach is discussed here, which incorporates pore-scale features on a subscale, relevant for the immiscible processes involved. Consequently, the physical process can be modelled in a stochastic manner only, where the single experiment represents one of many possible realizations. However, the present procedure retains realistic water and air saturation patterns and therefore produces similar finger lengths and widths as observed in the experiments. Monte Carlo type modelling leads to ensemble mean water saturation and the related variance.

Covington, M., C.M. Wicks, and M.O. Saar A dimensionless number describing the effects of recharge and geometry on discharge from simple karst aquifers Water Resources Research, 45, W11410, 2009. [Download PDF] [View Abstract][1] The responses of karstic aquifers to storms are often used to obtain information about aquifer geometry. In general, spring hydrographs are a function of both system geometry and recharge. However, the majority of prior work on storm pulses through karst has not studied the effect of recharge on spring hydrographs. To examine the relative importance of geometry and recharge, we break karstic aquifers into elements according to the manner of their response to transient flow and demonstrate that each element has a characteristic response timescale. These fundamental elements are full pipes, open channels, reservoir/constrictions, and the porous matrix. Taking the ratio of the element timescale with the recharge timescale produces a dimensionless number, γ, that is used to characterize aquifer response to a storm event. Using sets of simulations run with randomly selected element parameters, we demonstrate that each element type has a critical value of γ below which the shape of the spring hydrograph is dominated by the shape of the recharge hydrograph and above which the spring hydrograph is significantly modified by the system geometry. This allows separation of particular element/storm pairs into recharge-dominated and geometry-dominated regimes. While most real karstic aquifers are complex combinations of these elements, we draw examples from several karst systems that can be represented by single elements. These examples demonstrate that for real karstic aquifers full pipe and open channel elements are generally in the recharge-dominated regime, whereas reservoir/constriction elements can fall in either the recharge- or geometry-dominated regimes.

Kopp, A., A. Ebigbo, A. Bielinski, H. Class, and R. Helmig Numerical simulation of temperature changes caused by CO2 injection in geological reservoirs AAPG Studies in Geology, 59/26, pp. 439-456, 2009. [Download PDF] [View Abstract]Injection of CO 2 into the subsurface for geological storage has an effect on the temperature of the storage formation and the CO 2 itself. Numerical investigations are an essential tool in describing the relevant processes that determine such changes and the impact they may have on the migration and the storage mechanisms of CO 2 in the subsurface. This chapter focuses on the numerical simulation of such thermal effects and their consequences. Simulating the temperature changes in a storage site can be of interest for temperature-based monitoring. Determining whether or how such thermal effects change the transport of CO 2 in the formation is important for the success of a CO 2 storage effort. In particular, this chapter examines a leakage scenario and how temperature changes could affect the leakage flow. The second part of the chapter presents results of a complex reservoir-scale simulation. The target formation forms an anticlinal structure at a depth of about 570-900 m (1870-2953 ft). Strong temperature effects can be expected because of the possible tran Numerical simulation of temperature changes caused by CO2 injection in geological reservoirs. Available from: [accessed May 3, 2017].

Class, H., A. Ebigbo, R Helmig, H.K. Dahle, J.M. Nordbotten, M.A. Celia, P. Audigane, M. Darcis, J. Ennis-King, and Y. Fan A benchmark study on problems related to CO2 storage in geologic formations Computational Geosciences, 13/4, Sp. Iss. SI, pp. 409-434, 2009. [Download PDF] [View Abstract]This paper summarises the results of a benchmark study that compares a number of mathematical and numerical models applied to specific problems in the context of carbon dioxide (CO2) storage in geologic formations. The processes modelled comprise advective multi-phase flow, compositional effects due to dissolution of CO2 into the ambient brine and non-isothermal effects due to temperature gradients and the Joule–Thompson effect. The problems deal with leakage through a leaky well, methane recovery enhanced by CO2 injection and a reservoir-scale injection scenario into a heterogeneous formation. We give a description of the benchmark problems then briefly introduce the participating codes and finally present and discuss the results of the benchmark study.

Walsh, S.D.C., H. Burwinkle, and M.O. Saar A new partial-bounceback lattice-Boltzmann method for fluid flow through heterogeneous media Computers and Geosciences, 35/6, pp. 1186-1193, 2009. [Download PDF] [View Abstract]Partial-bounceback lattice-Boltzmann methods employ a probabilistic meso-scale model that varies individual lattice node properties to reflect a material's local permeability. These types of models have great potential in a range of geofluid, and other science and engineering, simulations of complex fluid flow. However, there are several different possible approaches for formulating partial-bounceback algorithms. This paper introduces a new partial-bounceback algorithm and compares it to two pre-existing partial-bounceback models. Unlike the two other partial-bounceback methods, the new approach conserves mass in heterogeneous media and shows improvements in simulating buoyancy-driven flow as well as diffusive processes. Further, the new model is better-suited for parallel processing implementations, resulting in faster simulations. Finally, we derive an analytical expression for calculating the permeability in all three models; a critical component for accurately matching simulation parameters to physical permeabilities.

Walsh, S.D.C., M.O. Saar, P. Bailey, and D.J. Lilja Accelerating geoscience and engineering system simulations on graphics hardware Computers and Geosciences, 35/12, pp. 2353-2364, 2009. [Download PDF] [View Abstract]Many complex natural systems studied in the geosciences are characterized by simple local-scale interactions that result in complex emergent behavior. Simulations of these systems, often implemented in parallel using standard central processing unit (CPU) clusters, may be better suited to parallel processing environments with large numbers of simple processors. Such an environment is found in graphics processing units (GPUs) on graphics cards. This paper discusses GPU implementations of three example applications from computational fluid dynamics, seismic wave propagation, and rock magnetism. These candidate applications involve important numerical modeling techniques, widely employed in physical system simulations, that are themselves examples of distinct computing classes identified as fundamental to scientific and engineering computing. The presented numerical methods (and respective computing classes they belong to) are: (1) a lattice-Boltzmann code for geofluid dynamics (structured grid class); (2) a spectral-finite-element code for seismic wave propagation simulations (sparse linear algebra class); and (3) a least-squares minimization code for interpreting magnetic force microscopy data (dense linear algebra class). Significant performance increases (between 10×× and 30×× in most cases) are seen in all three applications, demonstrating the power of GPU implementations for these types of simulations and, more generally, their associated computing classes.

Walsh, S.D.C., and M.O. Saar Numerical Models of Stiffness and Yield Stress Growth in Crystal-Melt Suspensions Earth and Planetary Science Letters, 267/1-2, pp. 32-44, 2008. [Download PDF] [View Abstract]Magmas and other suspensions that develop sample-spanning crystal networks undergo a change in rheology from Newtonian to Bingham flow due to the onset of a finite yield stress in the crystal network. Although percolation theory provides a prediction of the crystal volume fraction at which this transition occurs, the manner in which yield stress grows with increasing crystal number densities is less-well understood. This paper discusses a simple numerical approach that models yield stress in magmatic crystalline assemblies. In this approach, the crystal network is represented by an assembly of soft-core interpenetrating cuboid (rectangular prism) particles, whose mechanical properties are simulated in a network model. The model is used to investigate the influence of particle shape and alignment anisotropy on the yield stress of crystal networks with particle volume fractions above the percolation threshold. In keeping with previous studies, the simulation predicts a local minimum in the onset of yield stress for assemblies of cubic particles, compared to those with more anisotropic shapes. The new model also predicts the growth of yield stress above (and close to) the percolation threshold. The predictions of the model are compared with results obtained from a critical path analysis. Good agreement is found between a characteristic stiffness obtained from critical path analysis, the growth in assembly stiffness predicted by the model (both of which have approximately cubic power-law exponents) and, to a lesser extent, the growth in yield stress (with a power-law exponent of 3.5). The effect of preferred particle alignment on yield stress is also investigated and found to obey similar power-law behavior.

Walsh, S.D.C., and M.O. Saar Magma yield stress and permeability: Insights from multiphase percolation theory Journal of Volcanology and Geothermal Research, 177, pp. 1011-1019, 2008. [Download PDF] [View Abstract]Magmas often contain multiple interacting phases of embedded solid and gas inclusions. Multiphase percolation theory provides a means of modeling assemblies of these different classes of magmatic inclusions in a simple, yet powerful way. Like its single phase counterpart, multiphase percolation theory describes the connectivity of discrete inclusion assemblies as a function of phase topology. In addition, multiphase percolation employs basic laws to distinguish separate classes of objects and is characterized by its dependency on the order in which the different phases appear. This paper examines two applications of multiphase percolation theory: the fi rst considers how the presence of bubble inclusions in fl uences yield stress onset and growth in a magma's crystal network; the second examines the effect of bi-modal bubble-size distributions on magma permeability. We fi nd that the presence of bubbles induces crystal clustering, thereby 1) reducing the percolation threshold, or critical crystal volume fraction, φ c , at which the crystals form a space-spanning network providing a minimum yield stress, and 2) resulting in a larger yield stress for a given crystal volume fraction above φ c . This increase in the yield stress of the crystal network may also occur when crystal clusters areformed due toprocesses otherthanbubble formation, such as heterogeneouscrystallization, synneusis, and heterogeneity due to deformation or fl ow. Further, we fi nd that bimodal bubble size distributions can signi fi cantly affect the permeability of the system beyond the percolation threshold. This study thus demonstrates that larger-scale structures and topologies, as well as the order in which different phases appear, can have signi fi cant effects on macroscopic properties in multiphase materials.

Wang, W.-J., X.-Z. Kong, and Z.-G. Zhu Friction and relative energy dissipation in sheared granular materials Phys.Rev. E, 75/041302, 2007. [Download PDF] [View Abstract]The oscillating cylinder of a low-frequency inverted torsion pendulum is immersed into layers of noncohesive granular materials, including fine sand and glass beads. The relative energy dissipation and relative modulus of the granular system versus the amplitude and immersed depth of the oscillating cylinder are measured. A rheological model based on a mesoscopic picture is presented. The experimental results and rheological model indicate that small slides in the inhomogeneous force chains are responsible for the energy dissipation of the system, and the friction of the grains plays two different roles in the mechanical response of sheared granular material: damping the energy and enhancing the elasticity.

Ebigbo, A., H. Class, and R. Helmig CO2 leakage through an abandoned well: problem- oriented benchmarks Computational Geosciences, 11/2, pp. 103-115, 2007. [Download PDF] [View Abstract]The efficiency and sustainability of carbon dioxide (CO2) storage in deep geological formations crucially depends on the integrity of the overlying cap-rocks. Existing oil and gas wells, which penetrate the formations, are potential leakage pathways. This problem has been discussed in the literature, and a number of investigations using semi-analytical mathematical approaches have been carried out by other authors to quantify leakage rates. The semi-analytical results are based on a number of simplifying assumptions. Thus, it is of great interest to assess the influence of these assumptions. We use a numerical model to compare the results with those of the semi-analytical model. Then we ease the simplifying restrictions and include more complex thermodynamic processes including sub- and supercritical fluid properties of CO2 and non-isothermal as well as compositional effects. The aim is to set up problem-oriented benchmark examples that allow a comparison of different modeling approaches to the problem of CO2 leakage.

Kong, X.-Z., M.-B. Hu, Q.-S. Wu, and Y.-H. Wu Kinetic energy sandpile model for conical sandpile development by evolving rivers Phys. Lett A, 348, pp. 77-81, 2006. [Download PDF] [View Abstract]In this Letter, a kinetic energy sandpile model, taking into account of grain inertia and the moving directions of the toppling grain, is developed and used to study the behaviour of sandpiles. In our model, the inertial effects are based on the toppling kinetic energy. The phenomenon of sandpile formation by revolving rivers is reproduced with the model, revolving velocity ω∼t^{−2/3} and ∼h^{−3/2}, where t is the simulation time and h is the height of the sandpile.

Kong, X.-Z., M.-B. Hu, Q.-S. Wu, and Y.-H. Wu Effects of bottleneck on granular convection cells and segregation Granular Matter, 8, pp. 119-124, 2006. [Download PDF] [View Abstract]Dry glass granular material confined to a 2D-chamber convects when it is subjected to vertical sinusoidal vibrations of sufficient intensity. Effects of the container geometry on convection pattern and segregation process are studied experimentally. Here we introduce a bottleneck into the ordinary rectangular chamber, with one sidewall bended inward, characterized by λ being the ratio of the length of the bottleneck to the length of the chamber’s base. The convection roll and segregation pattern are significantly affected by λ. For λ = 0.9, two different stabilized patterns co-exist, depending on initial granular distribution. The sloping angle of the free surface to the horizontal increases with increasing λ, and reaches its saturation at λ = 0.9. The angle of the interface of the segregation region to the horizontal also increases with increasing λ.

Kong, X.-Z., M.-B. Hu, Q.-S. Wu, and Y.-H. Wu Effects of vibration frequency on intruders’ position in granular bed Phys. Lett A, 356, pp. 267-271, 2006. [Download PDF] [View Abstract]In this Letter, we study the motion of multiple intruders in a vertically vibrated granular bed. From molecular dynamics simulations, it is found that the mean vertical position of the intruders, relative to the base of the container, is governed primarily by the vibration frequency for a fixed vibration acceleration. The intruders stay in the upper layer of the granular bed at low frequency, then sink into the granular bed at a certain depth as the frequency increases, but rise up again at high frequency. This implies that the mean position of the intruders in the granular bed can be controlled by decreasing or increasing the vibration frequency. A subsequent theoretical analysis is also conducted to explore the mechanism behind the phenomenon.

Edwards, R.A., B. Rodriguez-Brito, L. Wegley, M. Haynes, M. Breitbart, D.M. Petersen, M.O. Saar, S.C. Alexander, E.C. Alexander Jr., and F. Rohwer Using pyrosequencing to shed light on deep mine microbial ecology BMC Genomics, 2006. [Download PDF] [View Abstract] Background Contrasting biological, chemical and hydrogeological analyses highlights the fundamental processes that shape different environments. Generating and interpreting the biological sequence data was a costly and time-consuming process in defining an environment. Here we have used pyrosequencing, a rapid and relatively inexpensive sequencing technology, to generate environmental genome sequences from two sites in the Soudan Mine, Minnesota, USA. These sites were adjacent to each other, but differed significantly in chemistry and hydrogeology. Results Comparisons of the microbes and the subsystems identified in the two samples highlighted important differences in metabolic potential in each environment. The microbes were performing distinct biochemistry on the available substrates, and subsystems such as carbon utilization, iron acquisition mechanisms, nitrogen assimilation, and respiratory pathways separated the two communities. Although the correlation between much of the microbial metabolism occurring and the geochemical conditions from which the samples were isolated could be explained, the reason for the presence of many pathways in these environments remains to be determined. Despite being physically close, these two communities were markedly different from each other. In addition, the communities were also completely different from other microbial communities sequenced to date. Conclusion We anticipate that pyrosequencing will be widely used to sequence environmental samples because of the speed, cost, and technical advantages. Furthermore, subsystem comparisons rapidly identify the important metabolisms employed by the microbes in different environments.

Class, H., A. Bielinski, R. Helmig, A. Kopp, and A. Ebigbo Numerical simulation of CO2 storage in geological formations Chemie Ingenieur Technik, 78/4, pp. 445-452, 2006. [Download PDF] [View Abstract]Die Speicherung von Kohlendioxid in geologischen Formationen wird derzeit als ein möglicher Beitrag zur Reduktion von Treibhausgaskonzentrationen in der Atmosphäre diskutiert und untersucht. Ein wichtiges Werkzeug begleitend zu experimentellen Untersuchungsmethoden ist die numerische Simulation. Hier wird ein Einblick in die physikalischen bzw. thermodynamischen Vorgänge im Untergrund während und nach einer Injektion von Kohlendioxid gegeben sowie deren modellkonzeptionelle Beschreibung durch nichtisotherme Mehrphasensysteme dargestellt. Die elementaren Schritte der physikalisch/mathematischen Modellbildung und numerischen Lösung der daraus entstehenden Gleichungssysteme werden erklärt. Anhand von Simulationsbeispielen werden dominierende Prozesse während und nach einer Injektion diskutiert.

Saar, M.O., M.C. Castro, C.M. Hall, M. Manga, and T.P. Rose Quantifying magmatic, crustal, and atmospheric helium contributions to volcanic aquifers using all stable noble gases: Implications for magmatism and groundwater flow Geochemistry Geophysics Geosystems, 6/3, 2005. [Download PDF] [View Abstract][1] We measure all stable noble gases (He, Ne, Ar, Kr, Xe) in spring waters in the Oregon Cascades volcanic arc and in eastern Oregon, USA. We show that in order to estimate magmatic helium (He) contributions it is critical to simultaneously consider He isotopic ratios, He concentrations, and mixing of He components. Our component mixing analysis requires consideration of all measured noble gases but no other elements and is particularly insightful when strong dilution by air-saturated water has occurred. In addition, this approach can allow distinction between crustal and magmatic He components and facilitates their identification in deep groundwaters that have been diluted by near-surface water. Using this approach, we show that some cold springs on the eastern flanks of the Oregon Cascades exhibit He isotopic ratios that indicate significant magmatic He contributions comparable to those observed in thermal springs on the western flanks. Furthermore, while these magmatic He contributions are largest in deep groundwaters near the Cascades crest, greater magmatic excess He fractions than may be inferred from He isotopic ratios alone are present in all (deep) groundwaters including those at larger distances (>70 km) from the volcanic arc. We also suggest that excess He and heat discharge without dilution by air-saturated water may be restricted to spring discharge along faults.

Christiansen, L.B., S. Hurwitz, M.O. Saar, S.E. Ingebritsen, and P.A. Hsieh Seasonal seismicity at western United States volcanic centers Earth and Planetary Science Letters, 240, pp. 307-321, 2005. [Download PDF] [View Abstract]We examine 20-yr data sets of seismic activity from 10 volcanic areas in the western United States for annual periodic signals (seasonality), focusing on large calderas (Long Valley caldera and Yellowstone) and stratovolcanoes (Cascade Range). We apply several statistical methods to test for seasonality in the seismic catalogs. In 4 of the 10 regions, statistically significant seasonal modulation of seismicity (> 90% probability) occurs, such that there is an increase in the monthly seismicity during a given portion of the year. In five regions, seasonal seismicity is significant in the upper 3 km of the crust. Peak seismicity occurs in the summer and autumn in Mt. St. Helens, Hebgen Lake/Madison Valley, Yellowstone Lake, and Mammoth Mountain. In the eastern south moat of Long Valley caldera (LVC) peak seismicity occurs in the winter and spring. We quantify the possible external forcing mechanisms that could modulate seasonal seismicity. Both snow unloading and groundwater recharge can generate large stress changes of > 5 kPa at seismogenic depths and may thus contribute to seasonality.

Hu, M.-B., Q.-S. Wu, X.-Z. Kong, and Y.-H. Wu Discharge oscillation of particles from a vertical Pipe with capillary outlet Chinese Science Bulletin, 50, pp. 1076-1078, 2005. [Download PDF] [View Abstract]A new style of discharge process from a vertical open-top pipe with capillary outlet is reported. The outflux fluctuates greatly with time and the bulk condensed granular flow in the pipe shows stop-and-go motion when the filling height is above a threshold. When the filling height falls towards the threshold, led by a transitional stage, the outflux and the bulk movement become much stable. The upper surface dropping velocity variation is measured. A heuristic theory is proposed to understand the stop- and-go motion and the transitional behavior.

Kong, X.-Z., M.-B. Hu, Q.-S. Wu, and Y.-H. Wu Ring-like size segregation in vibrated cylinder with a bottleneck Phys. Lett A, 341, pp. 278-284, 2005. [Download PDF] [View Abstract]In this Letter, a ring-like segregation pattern of bi-dispersed granular material in a vibrated bottleneck-cylinder is presented. The driving frequency can greatly affect the strength and structure of the convection roll and segregation pattern. The position and height of the ring (cluster of big beads) can be adjusted by altering the vibration frequency. And a heuristic theory is developed to interpret the ring's position dependence on driving frequency.

Hu, M.-B., X.-Z. Kong, Q.-S. Wu, and Y.-H. Wu Granular segregation in a multi-bottleneck container: Mobility effect Int. J. Mod. Phys. B, 19, pp. 1793-1800, 2005. [Download PDF] [View Abstract]We investigate experimentally and via computer simulations the segregation pattern of binary granular mixtures in a vibrated container with bottlenecks. During the vibration the granular motion is more violent at the bottlenecks than at the bellies. Particles with more mobility congregate to the necks, while those with less mobility congregate to the bellies. We use discrete element simulations to reproduce the main characteristics of the experimental observations.

Hu, M.-B., X.-Z. Kong, Q.-S. Wu, and Y.-H. Wu Effects of container geometry on granular segregation pattern Chinese Physics, 14, pp. 1793-1800, 2005. [Download PDF] [View Abstract]In a set of vibrating quasi-two-dimensional containers with the right-hand sidewall bent inward, three new segregation patterns have been identified experimentally including a Two-Side segregation Pattern, a Left-hand Side segregation Pattern and a pattern where big particles aggregate to the upper left part of the container. In a container with small bending degree, either the two-side segregation pattern or the left-hand side segregation pattern is stable, which is determined by the initial distribution of particles.

Jellinek, M.A., M. Manga, and M.O. Saar Did melting glaciers cause volcanic eruptions in eastern California? Probing the mechanics of dike formation Journal of Geophysical Research, 109/B9, B09206, 2004. [Download PDF] [View Abstract][1] A comparison of time series of basaltic and silicic eruptions in eastern California over the last 400 kyr with the contemporaneous global record of glaciation suggests that this volcanism is influenced by the growth and retreat of glaciers occurring over periods of about 40 kyr. Statistically significant cross correlations between changes in eruption frequency and the first derivative of the glacial time series imply that the temporal pattern of volcanism is influenced by the rate of change in ice volume. Moreover, calculated time lags for the effects of glacial unloading on silicic and basaltic volcanism are distinct and are 3.2 ± 4.2 kyr and 11.2 ± 2.3 kyr, respectively. A theoretical model is developed to investigate whether the increases in eruption frequency following periods of glacial unloading are a response ultimately controlled by the dynamics of dike formation. Applying results from the time series analysis leads, in turn, to estimates for the critical magma chamber overpressure required for eruption as well as constraints on the effective viscosity of the wall rocks governing dike propagation.

Saar, M.O., and M. Manga Depth dependence of permeability in the Oregon Cascades inferred from hydrogeologic, thermal, seismic, and magmatic modeling constraints Journal of Geophysical Research, 109/B4, B04204, 2004. [Download PDF] [View Abstract][1] We investigate the decrease in permeability, k, with depth, z, in the Oregon Cascades employing four different methods. Each method provides insight into the average permeability applicable to a different depth scale. Spring discharge models are used to infer shallow (z < 0.1 km) horizontal permeabilities. Coupled heat and groundwater flow simulations provide horizontal and vertical k for z < 1 km. Statistical investigations of the occurrences of earthquakes that are probably triggered by seasonal groundwater recharge yield vertical k for z < 5 km. Finally, considerations of magma intrusion rates and water devolatilization provide estimates of vertical k for z < 15 km. For depths >0.8 km, our results agree with the power law relationship, k = 10−14 m2 (z/1 km)−3.2, suggested by Manning and Ingebritsen [1999] for continental crust in general. However, for shallower depths (typically z ≤ 0.8 km and up to z ≤ 2) we propose an exponential relationship, k = 5 × 10−13 m2 exp (−z/0.25 km), that both fits data better (at least for the Cascades and seemingly for continental crust in general) and allows for a finite near-surface permeability and no singularity at zero depth. In addition, the suggested functions yield a smooth transition at z = 0.8 km, where their permeabilities and their gradients are similar. Permeabilities inferred from the hydroseismicity model at Mount Hood are about one order of magnitude larger than expected from the above power law. However, higher permeabilities in this region may be consistent with advective heat transfer along active faults, causing observed hot springs. Our simulations suggest groundwater recharge rates of 0.5 ≤ uR ≤ 1 m/yr and a mean background heat flow of Hb ≈ 0.080–0.134 W/m2 for the investigated region.

Hu, M.-B., X.-Z. Kong, Q.-S. Wu, and Z.-G. Zhu Experimental study of energy absorption properties of granular materials under low frequency vibrations Int. J. Mod. Phys. B., 18, pp. 2708-2712, 2004. [Download PDF] [View Abstract]The low frequency vibration energy absorption properties of granular materials have been investigated on an Invert Torsion Pendulum (ITP). The energy absorption rate of granular material changes nonlinearly with amplitude under low frequency vibration. The frequency of ITP system increases a little with granular materials in the holding cup. The vibration frequency of ITP system does not change with time.

Saar, M.O., and M. Manga Seismicity induced by seasonal groundwater recharge at Mt. Hood, Oregon Earth and Planetary Science Letters, 214, pp. 605-618, 2003. [View Abstract]Groundwater recharge at Mt. Hood, Oregon, is dominated by spring snow melt which provides a natural large- amplitude and narrow-width pore-fluid pressure signal. Time delays between this seasonal groundwater recharge and seismicity triggered by groundwater recharge can thus be used to estimate large-scale hydraulic diffusivities and the state of stress in the crust. We approximate seasonal variations in groundwater recharge with discharge in runoff- dominated streams at high elevations. We interpolate the time series ofnumber ofearthquakes, N , seismic moment, M o , and stream discharge, Q , and determine cross-correlation coefficients at equivalent frequency bands between Q and both N and M o . We find statistically significant correlation coefficients at a mean time lag of about 151 days. This time lag and a mean earthquake depth of about 4.5 km are used in the solution to the pressure diffusion equation, under periodic (1 year) boundary conditions, to estimate a hydraulic diffusivity of U W 10 3 1 m 2 /s, a hydraulic conductivity ofabout K h W 10 3 7 m/s, and a permeability ofabout k W 10 3 15 m 2 . Periodic boundary conditions also allow us to determine a critical pore-fluid pressure fraction, P P / P 0 W 0.1, ofthe applied near-surface pore-fluid pressure perturbation, P 0 W 0.1 MPa, that has to be reached at the mean earthquake depth to cause hydroseismicity. The low magnitude of P P W 0.01 MPa is consistent with other studies that propose 0.01 9 P P 9 0.1 MPa and suggests that the state of stress in the crust near Mt. Hood could be near critical for failure. Therefore, we conclude that, while earthquakes occur throughout the year at Mt. Hood, elevated seismicity levels along pre-existing faults south of Mt. Hood during summer months are hydrologically induced by a reduction in effective stress

Saar, M.O., and M. Manga Continuum percolation for randomly oriented soft-core prisms Physical Review E, 65/056131, 2002. [Download PDF] [View Abstract]We study continuum percolation of three-dimensional randomly oriented soft-core polyhedra (prisms). The prisms are biaxial or triaxial and range in aspect ratio over six orders of magnitude. Results for prisms are compared with studies for ellipsoids, rods, ellipses, and polygons and differences are explained using the concept of the average excluded volume, ⟨vex⟩. For large-shape anisotropies we find close agreement between prisms and most of the above-mentioned shapes for the critical total average excluded volume, nc⟨vex⟩, where nc is the critical number density of objects at the percolation threshold. In the extreme oblate and prolate limits simulations yield nc⟨vex⟩≈2.3 and nc⟨vex⟩≈1.3, respectively. Cubes exhibit the lowest-shape anisotropy of prisms minimizing the importance of randomness in orientation. As a result, the maximum prism value, nc⟨vex⟩≈2.79, is reached for cubes, a value close to nc⟨vex⟩=2.8 for the most equant shape, a sphere. Similarly, cubes yield a maximum critical object volume fraction of φc=0.22. φc decreases for more prolate and oblate prisms and reaches a linear relationship with respect to aspect ratio for aspect ratios greater than about 50. Curves of φc as a function of aspect ratio for prisms and ellipsoids are offset at low-shape anisotropies but converge in the extreme oblate and prolate limits. The offset appears to be a function of the ratio of the normalized average excluded volume for ellipsoids over that for prisms, R=⟨¯vex⟩e/⟨¯vex⟩p. This ratio is at its minimum of R=0.758 for spheres and cubes, where φc(sphere)=0.2896 may be related to φc(cube)=0.22 by φc(cube)=1−[1−φc(sphere)]R=0.23. With respect to biaxial prisms, triaxial prisms show increased normalized average excluded volumes, ⟨¯vex⟩, due to increased shape anisotropies, resulting in reduced values of φc. We confirm that Bc=nc⟨vex⟩=2Cc applies to prisms, where Bc and Cc are the average number of bonds per object and average number of connections per object, respectively.

Saar, M.O., M. Manga, K.V. Cashman, and S. Fremouw Numerical models of the onset of yield strength in crystal-melt suspensions Earth and Planetary Science Letters, 187, pp. 367-379, 2001. [Download PDF] [View Abstract]The formation of a continuous crystal network in magmas and lavas can provide finite yield strength, τy, and can thus cause a change from Newtonian to Bingham rheology. The rheology of crystal–melt suspensions affects geological processes, such as ascent of magma through volcanic conduits, flow of lava across the Earth’s surface, melt extraction from crystal mushes under compression, convection in magmatic bodies, and shear wave propagation through partial melting zones. Here, three-dimensional numerical models are used to investigate the onset of ‘static’ yield strength in a zero-shear environment. Crystals are positioned randomly in space and can be approximated as convex polyhedra of any shape, size and orientation. We determine the critical crystal volume fraction, φc, at which a crystal network first forms. The value of φc is a function of object shape and orientation distribution, and decreases with increasing randomness in object orientation and increasing shape anisotropy. For example, while parallel-aligned convex objects yield φc=0.29, randomly oriented cubes exhibit a maximum φc of 0.22. Approximations of plagioclase crystals as randomly oriented elongated and flattened prisms (tablets) with aspect ratios between 1:4:16 and 1:1:2 yield 0.08<φc<0.20, respectively. The dependence of φc on particle orientation implies that the flow regime and resulting particle ordering may affect the onset of yield strength. φc in zero-shear environments is a lower bound for φc. Finally the average total excluded volume is used, within its limitation of being a ‘quasi-invariant’, to develop a scaling relation between τy and φ for suspensions of different particle shapes.

Saar, M.O., and M. Manga Permeability-porosity relationship in vesicular basalts. Geophysical Research Letters, 26/1, pp. 111-114, 1999. [Download PDF] [View Abstract]The permeability κ and porosity ϕ of vesicular basalts are measured. The relationship between κ and ϕ reflects the formation and emplacement of the basalts and can be related to the crystal and vesicle microstructure obtained by image analysis. Standard theoretical models relating κ and ϕ that work well for granular materials are unsuccessful for vesicular rocks due to the fundamental difference in pore structure. Specifically, κ in vesicular rocks is governed by apertures between bubbles. The difference between calculated and measured κ reflects the small size of these apertures with aperture radii typically O(10) times smaller than the mean bubble radii.

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Huang, P.W., and J.F. Wellmann Investigating Different Formulations for Hydrothermal Convection in Geothermal Systems , Proceedings World Geothermal Congress 2020, 2020. [View Abstract]Hydrothermal convection in porous media is an essential piece of physics in geothermal reservoirs, and understanding them leads to better development of geothermal energy. We analyze the validity of simulating hydrothermal convection using different formulations of partial differential equations. Using the Elder problem as a benchmark, we found out that the stream function formulation and the velocity formulation are a valid and efficient model of hydrothermal convection. The Nusselt number and entropy production are measurements of the quality of convective heat transfer. The Rayleigh number describes the physical properties of a porous media. We use simulations to investigate further the discrepancy in the Nusselt Rayleigh relationship found in previous experiments. The conclusion is that the multiple steady states of convection pattern in a 3D box are the main reason for the discrepancy found in the Nusselt-Rayleigh relationship.

Maldonado, S.B., J.B. Bielicki, M.W. Miranda, C. Howard, B.M. Adams, T.A. Buscheck, and M.O. Saar Geospatial estimation of the electric power potential in sedimentary basin geothermal resources using geologically stored carbon dioxide , World Geothermal Congress, 2020. [View Abstract]Sedimentary basins have emerged as potential candidates for geothermal development, in part because the aquifers within them are also the targets for the emplacement of carbon dioxide (CO2) to isolate it from the atmosphere. This geologically stored CO2 could be used as a geothermal heat extraction fluid and circulated between the CO2 storage reservoir and a surface power plant where it could be expanded in a turbine to produce electricity, and thus be a CO2 capture, utilization, and storage approach. The use of CO2 for geothermal heat mining has a number of thermophysical advantages over the use of native brine. Here, we used an integrated power cycle-well-reservoir modeling approach from our prior work to estimate the capacity of a CPG power plant as a function of important parameters of the aquifers into which CO2 would be emplaced. We then produced a reduced-form equation that predicts these estimated power generation capacities. In a case study of the continental United States, we applied this reduced-form equation to the relevant geospatial data for sedimentary basins and the aquifers and heat fluxes within them. While the availability of relevant data with high fidelity is limited, the results of this geospatial assessment suggest that there are large areas within the continental United States in which CPG power plants could be constructed and have power generation capacities on the order of those of other components of the electricity system. In particular, if other siting issues could be addressed, CPG developments in portions of Central Utah, Northwest and Southwest/South Central Colorado, Southwest and Central New Mexico, Eastern and Southern Arkansas, Northern Louisiana, West-Central Wyoming/Eastern Idaho, the central valley in California, Western Texas, and the Texas gulf coast may be able to have power generation capacities on the order of 100s of megawatts or more.

Fleming, M.R., B.M. Adams, and M.O. Saar Using sequestered CO2 as geothermal working fluid to generate electricity and store energy , World Geothermal Congress, 2020. [View Abstract]The CO2-Plume Geothermal (CPG) power system can operate either as a baseload power source or as a dispatchable generator, making power when it is needed on the electric grid. Unlike wind and solar, which are intermittent power sources that operate only when the wind blows or the sun shines, geothermal heat is always available and can be extracted as needed to generate electricity. As wind and solar begin to constitute a larger portion of the electricity provided to the grid, there is an increased need to provide flexible power generation that makes up the difference between demand and this varying renewable supply. Thus, CPG is a carbon-neutral, renewable, flexible power generator that can fulfill this need. Unlike most geothermal technologies, CPG can be extended to be an energy storage system, termed CO2 Plume Geothermal Energy Storage (CPGES). To create one version of a CPGES system, a second shallow reservoir is added to the CPG system. CO2 is stored in this shallow reservoir in an intermediate state after power is generated but before the energy-intensive parasitic loads, which reduce the power plant’s overall output. When the generation and parasitic stages are separated by time, nearly the full gross turbine electric generation can be sent to the grid when power is needed. Later, when electricity is cheap, power is taken from the grid and used to cool (and sometimes pump or compress) the CO2. Thus, CPG is expanded into CPGES, adding energy storage to the electric grid. In this work, we describe a new type of CPGES, termed Earth Battery Extension II (EBE II), which uses a large surface storage tank, or gasometer, to store the CO2 at near-atmospheric pressure. This permits up 260 MWe of electricity to be generated during the battery discharge phase compared to 2.5 MWe for CPG alone. Additionally, the new CPGES system can be configured to produce solid CO2 (dry ice) that can be sublimated at near atmospheric pressure, providing a -78 °C heat sink that can be used for cooling purposes in general and, specifically, to cryogenically capture CO2 from the air. This CO2 can, in turn, be used to develop more such CPGES systems. If no heat sink is desired, the turbine can be optimized by including (additional) stages that result in increased electric power output without dry ice formation.

Adams, B.M., D. Sutter, M. Mazzotti, and M.O. Saar Combining direct air capture and geothermal heat and electricity generation for net-negative carbon dioxide emissions , World Geothermal Congress, 2020. [View Abstract]In this work, Direct Air Capture (DAC) of CO2 is coupled directly with a sedimentary geothermal system with the goal of creating a stand-alone, carbon-negative CO2 capture system. An isobutane Organic Rankine Cycle (ORC) is used to generate electricity from a 2.5 km deep, 50 mD porous-media reservoir with a temperature ranging from 90°C to 140°C. The heat required by the DAC is extracted directly from the produced geothermal stream. A 1 km2 inverted 5-spot reservoir configuration is used. Four different system configurations are tested, including different ORC configurations and the use of an external electrical supply. We find that Direct Air Capture and a geothermal cycle can be coupled in a stand-alone power-island to capture up to 0.04 MtCO2 per year with a 140°C reservoir and the given 1 km2 reservoir configuration. When 12 MWe of external power is supplied, possibly from a nearby wind turbine farm or photovoltaic park, the same combined system can capture up to 0.18 MtCO2 per year. If a thirty year system lifetime is assumed, the systems can capture from 1.2 MtCO2 to 5.4 MtCO2 over the system lifetime. Also, as the 5-spot reservoir system is designed to be up-scaled by tiling additional 5-spots together into larger configurations without thermal or pressure interference, we considered coupling a 5 km x 5 km geothermal reservoir with Direct Air CO2 Capture as well. When using this 25 km2 reservoir, the stand-alone power island system captures 1.04 MtCO2 per year while the grid-connected system captures 4.63 MtCO2 per year using 282 MWe of externally supplied electricity. Additionally, we find that there is a temperature optimum for the geothermal resource at which the ORC and steam generator both provide the required proportions of heat and electricity to the DAC. At higher resource temperatures, additional electricity is needed for the DAC, either from a parallel ORC or from an external source. At lower resource temperatures, necessary additional heat for the DAC is provided via an isobutane Heat Pump (HP). With the current assumption of an adsorption-based DAC process whose heat demand is provided through saturated steam at atmospheric pressure, a temperature of 110°C is most ideally suited. Lastly, we find that most resource temperatures have an electric opportunity cost—the forgone electricity per tonne CO2—of approximately 700 kWe-h per tonne CO2.

Hefny, M., M. Hammed, A. Ebigbo, and M. O. Saar Reservoir characterization of the Nubian Sandstone for geothermal applications: petrophysical evaluation in the central Gulf of Suez (Egypt) , World Geothermal Congress , 2020. [View Abstract]The eastern coast of the central Gulf of Suez (cGOS) has a significant geothermal potential, as expressed by the occurrence of hot springs (i.e. Hammam Faraun and Hammam Moussa) and Nazzazat oil seeps. In addition, an evaluation of from the bottom-hole temperatures (BHT) in 172 offshore boreholes shows a marked increase in geothermal gradient in the area. We attribute this geothermal anomaly to the asymmetry of the Late Tertiary GOS structural configuration and repeated Carboniferous, Jurassic and Oligo-Miocene volcanic activities at the rift margins of cGOS. The present work provides a new dataset of petrophysical characteristics of Nubian Sandstone reservoirs as a first step in their evaluation as potential reservoirs for CO2 storage and/or geothermal energy production in the GOS basin, Egypt. The dataset comprises: a) property model of pre-Cenomanian Nubian Sandstone reservoirs from Ras-Budran offshore oil-fields, the eastern sides of the cGOS; and b) laboratory measurements, including quantitative mineralogical maps using QEMscan analysis, grain density, effective porosity, capillarity, and fluid permeability of outcrop samples using routine experimental techniques. These results can be used to build a reliable 3D reservoir model that can help to achieve a quantitative evaluation of the geothermal potential of the region.

Hassanjanikhoshkroud, N., M.G.C. Nestola, P. Zulian, C. von Planta, D. Vogler, H. Köstler, and R. Krause Thermo-Fluid-Structure Interaction Based on the Fictitious Domain Method: Application to Dry Rock Simulations , PROCEEDINGS, 45th Workshop on Geothermal Reservoir Engineering, pp. 1-12, 2020. [View Abstract]Enhanced Geothermal Systems(EGS) generate geothermal energy without the need for natural convective hydrothermal resources by enhancing permeability through hydraulic fracturing. EGS involve complex and highly nonlinear multiphysics processes and face several technical challenges that govern productivity and associated risks in a wide range of reservoir engineering problems.Numericalsimulations offer a unique opportunity to improve the hydraulic stimulation design and investigate their long-term performance. In this work, we present a thermo-hydro–mechanical coupling based on the fictitious domain method. Fictitious domain methodsinclude nonconforming mesh approaches that rely on the use of different discretizationsfor the solid and the fluid domain. We consider the governing equations of linear thermo-elasticity to model the heat transfer in dry and hot rocks.Navier-Stokes equations and heat convection equationsare adopted for describingthe thermal flux in an incompressible fluidflow. The two problems are coupled in a staggered manner through the variational transfer. Indeed, the use of the variational transfer allows simplifying the setup of fracture simulations with complex surfaces embedded in the fluid domain, thus avoiding the inconvenient mesh generation required bythe boundary fitted methods. The presented computational framework is validated by comparing with analytical solutions of two-dimensional benchmarks. Finally, we show that our framework provides high flexibility and cansimulatethe thermo-hydro–mechanical coupling between fluid and idealized fracture asperities.

Lima, M., P. Schädle, D. Vogler, M. Saar, and X.-Z. Kong A Numerical Model for Formation Dry-out During CO2 Injection in Fractured Reservoirs Using the MOOSE Framework: Implications for CO2-based Geothermal Energy Extraction , Proceedings of the World Geothermal Congress 2020, Reykjavík, Iceland, (in press). [View Abstract]Injection of supercritical carbon dioxide (scCO2) into geological reservoirs is involved in Carbon Capture, Utilization, and Storage (CCUS), such as geological CO2 storage, and Enhanced Geothermal Systems (EGS). The potential physico-chemical interactions between the dry scCO2, the reservoir fluid, and rocks may cause formation dry-out, where mineral precipitates due to continuous evaporation of water into the scCO2 stream. This salt precipitation may impair the rock bulk permeability and cause a significant decrease in the well injectivity. Formation dry-out and the associated salt precipitation during scCO2 injection into porous media have been investigated in previous studies by means of numerical simulations and laboratory experiments. However, few studies have focused on the dry-out effects in fractured rocks in particular, where the mass transport is strongly influenced by the fracture aperture distribution. In this study, we numerically model the dry-out processes occurring during scCO2 injection into brine-saturated single fractures and evaluate the potential of salt precipitation. Fracture aperture fields are photogrammetrically determined with fracture geometries of naturally fractured granite cores from the Deep Underground Geothermal (DUG) Lab at the Grimsel Test Site (GTS), in Switzerland. We use an open-source, parallel finite element framework to numerically model two-phase flow through a 2D fracture plane. Under in-situ reservoir conditions, the brine is displaced by dry scCO2 and also evaporates into the CO2 stream. The fracture permeability is calculated with the local cubic law. Additionally, we extend the numerical model by the Young-Laplace equation to determine the aperture-based capillary pressure. Finally, as future work, the precipitation of salt will be modelled by employing a uniform mineral growth approach, where the local aperture uniformly decreases with the increase in precipitated mineral volume. The numerical simulations assist in understanding the long-term behaviour of reservoir injectivity during subsurface applications that involve scCO2 injection, including CO2-based geothermal energy extraction.

Guglielmetti, L., P. Alt-Epping, D. Birdsell, F. de Oliveira, L. Diamond, T. Driesner, O. Eruteya, P. Hollmuller, et al., and M.O. Saar HEATSTORE SWITZERLAND: New Opportunities of Geothermal District Heating Network Sustainable Growth by High Temperature Aquifer Thermal Energy Storage Development , World Geothermal Congress, 2020. [View Abstract]HEATSTORE is a GEOTHERMICA ERA-NET co-funded project, aiming at developing High Temperature (~25°C to ~90°C) Underground Thermal Energy Storage (HT-UTES) technologies by lowering the cost, reducing risks, improving the performance, and optimizing the district heating network demand side management at 6 new pilot and demonstration sites, two of which are in Switzerland, plus 8 case studies. The European HEATSTORE consortium includes 24 contributing partners from 9 countries, composing a mix of scientific research institutes and private companies. The Swiss consortium, developing HEATSTORE in Switzerland, involves of two industrial partners (Services Industriels de Geneva - SIG and Energie Wasser Bern - EWB) and four academic partners (Universities of Geneva, Bern, Neuchâtel and ETH Zurich), with support from the Swiss Federal Office of Energy. The aims are to develop two demonstration projects for High Temperature Aquifer Thermal Energy Storage (HT-ATES) in the cantons of Geneva and Bern such that industrial waste heat can be converted into a resource. This paper presents the results of the first year of activities in the Swiss projects. The activities planned cover subsurface characterization, energy system analysis, surface implementation design, legal framework improvement and business modelling to ensure the sustainability of the projects. This approach is supported by large industrial investments for subsurface characterization. Two wells, down to 1200m below surface level (bsl) are being drilled in the Geneva area to tap potential targets in the carbonate Mesozoic units and at least three additional wells, down to 500m bsl will target the Molasse sediments in the Bern area next year. These wells allow subsurface exploration and characterization and will provide data, used for detailed THMC modelling to assess the thermal energy storage potential at the two sites in Switzerland. The results of such numerical modelling are combined with energy system analysis to quantify the waste heat availability and heat demand and hence optimize the production and injection operations. The outcomes of the coupled assessments will aid in designing the integration of the new installations into the district- heating network. Legal framework improvements, based on complete technical evaluation and on the best-practice sharing with the other European partners, will be an enabling tool to accelerate the implementation of the HT-ATES systems, while business modelling helps calibrate the economic feasibility of the projects and helps industrial partners to plan future investments.

Birdsell, D., and M. Saar Modeling Ground Surface Deformation at the Swiss HEATSTORE Underground Thermal Energy Storage Sites , Proceedings World Geothermal Congress, 2020. [View Abstract]High temperature (>25 °C) aquifer thermal energy storage (HT-ATES) is a promising technology to store waste heat and reduce greenhouse gas emissions by injecting hot water into the subsurface during the summer months and extracting it for district heating in the winter months. Nevertheless, ensuring the long-term technical success of an HT-ATES project is difficult because it involves complex coupling of fluid flow, heat transfer, and geomechanics. For example, ground surface deformation due to thermo- and poro- elastic deformation could cause damage to nearby infrastructure, and it has not been considered very extensively in the literature. The Swiss HEATSTORE consortium is a group of academic and industrial partners that is developing HT-ATES pilot projects in Geneva and Bern, Switzerland. Possible target formations at the Geneva site include: (a) fractured Cretaceous limestone aquifers interbedded within lower-permeability sedimentary rock and (b) Jurassic reef complex(es), also potentially fractured. In this work we offer numerical modeling support for the Geneva site. A site-specific, hydro-mechanical (HM) model is created, which uses input from the energy systems scenarios and 3D static geological modeling performed by other Swiss consortium partners. Results show that a large uplift (> 5 cm) is possible after one loading cycle, but a sensitivity analysis shows that uplift is decreased to ≤ 0.3 cm if the aquifer permeability is increased or an auxiliary well is included to balance inflow and outflow. Future work includes running coupled thermo-hydro-mechanical (THM) models for several loading and unloading cycles. The THM framework can help inform future decisions about the Swiss HT-ATES sites (e.g. the final site selection within the Geneva basin, well spacing, and operating temperature). It can also be applied to understand surface deformation in the context of geothermal energy, carbon sequestration, and at other ATES sites worldwide.

Niederau, J., M. Lauster, J. Bruckmann, C. Clauser, and D. Mueller Coupling Dynamic Heat Demands of Buildings with Borehole Heat Exchanger Simulations for Realistic Monitoring and Forecast , Proceedings World Geothermal Congress 2020, Reykjavik, Iceland, April 26 - May 2, 2020, 2020. [View Abstract]We present results of building performance simulations coupled with borehole heat exchanger (BHE) simulations for modeling the response of BHE-fields to varying heating power demands. We apply this method to an existing settlement in the Lower Rhine Embayment in Germany, called Neu-Teveren. Buildings in this former military settlement were built in the 1950s and will be extensively retrofitted in the coming years. Thus, it is a prime opportunity to model the impact of retrofitted buildings on the performance and longevity of BHEs. Our simulation results based on multi-year outdoor temperature records show that the cooling effect of the BHEs in the subsurface is about 3 K lower for retrofitted buildings. Further, a layout with one borehole heat exchanger per building can be efficiently operated over a time frame of 15 years, if the BHE-field layout considers regional groundwater flow. Due to northward groundwater flow, thermal plumes of reduced temperatures develop at each BHE, showing that BHEs in the southern part of the model affect their northern neighbors. Changing the layout of the BHE-field increases the performance of individual BHEs.

Niederau, J., J. F. Wellmann, and N. Börsing How the Spatial Continuity of Permeability Affects Hydrothermal Convection: A Study Using Entropy Production , Proceedings World Geothermal Congress 2020, Reykjavik, Iceland, April 26 - May 2, 2020, 2020. [View Abstract]With active convection in a reservoir, regions of upflow in convective systems can increase the geothermal energy potential of said reservoir; on the other hand, convection introduces uncertainty, because it is difficult to locate these regions of upflow. Several predictive criteria, such as the Rayleigh number, exist to estimate whether convection might occur under certain conditions. Once a convection system is established, diagnostic measures are needed for describing the convection pattern, e.g. the likely number of upwelling regions. We use the thermodynamic measure called entropy production to describe the influence of spatially heterogeneous permeability on a hydrothermal convection pattern in a hot sedimentary aquifer in the Perth Basin, Australia. To this end, we analyze the entropy production in multiple ensembles in a Monte Carlo study. Each ensemble contains several hundred realizations of spatially heterogeneous permeability. By observing the measure of entropy production, we see that the convection patterns in our models drastically change with the introduction and increase of a finite correlation length in permeability. An initial decrease of the average entropy production number with increasing lateral correlation length indicates that less ensemble members show convection. When neglecting the purely conductive ensembles in our analysis, no change in the convection pattern is seen for lateral correlation lengths larger 2000 m. Besides model dimensions, our results show that also the spatial anisotropy of important flow-parameters, such as permeability, is important to be considered if hydrothermal convection is likely to occur in a sedimentary geothermal reservoir system.

Niederau, J., A. Ebigbo, and M. O. Saar Characterization of Subsurface Heat-Transport Processes in the Canton of Aargau Using an Integrative Workflow , Proceedings World Geothermal Congress 2020, Reykjavik, Iceland, April 26 - May 2, 2020, 2020. [View Abstract]In a referendum in May 2017, Switzerland decided to phase out nuclear power in favor of further developing renewable energy sources. One of these energy sources is geothermal energy, which, as a base-load technology, fills a niche complementary to solar and wind energy. A known surface-heat-flow anomaly exists in the Canton of Aargau in Northern Switzerland. With measured specific heat-flow values of up to 140 mW m-2, it is an area of interest for deep geothermal energy exploration. In a pilot study, which started in late 2018, we want to characterize the heat-flow distribution in the vicinity of the anomaly in more detail to facilitate future assessment of the geothermal potential of this region. To achieve a complete characterization of the heat-flow values as well as their spatial uncertainty, we develop a workflow comprising: (i) assimilation and homogenization of different types of geologic data, (ii) development of a geological model with focus on heat transport, and (iii) numerical simulations of the dominant heat-transport processes. Due to its nature as a pilot study, the developed workflow needs to be integrative and adaptable. This means that data generated during the course of the project can easily be integrated in the modeling and simulation process, and that the generated workflow should easily be adaptable to other regions for potential future studies. One further goal of this project is that the generated models and simulations provide insights into the nature of the heat-flow anomaly in Northern Switzerland and to test the hypothesis that upward migration of deep geothermal fluids along structural pathways is the origin of this particular heat-flow anomaly.

Samrock, F., A.V. Grayver, B. Cherkose, A. Kuvshinov, and M.O. Saar Aluto-Langano Geothermal Field, Ethiopia: Complete Image Of Underlying Magmatic-Hydrothermal System Revealed By Revised Interpretation Of Magnetotelluric Data , Proceedings World Geothermal Congress 2020, 2020. [View Abstract]Aluto-Langano in the Main Ethiopian Rift Valley is currently the only producing geothermal field in Ethiopia and probably the best studied prospect in the Ethiopian Rift. Geoscientific exploration began in 1973 and led to the siting of an exploration well LA3 on top of the volcanic complex. The well was drilled in 1983 to a depth of 2144m and encountered temperatures of 320°C. Since 1990 Aluto has produced electricity, albeit with interruptions. Currently it is undergoing a major expansion phase with the plan to generate about 70MWe from eight new wells, until now two of them have been drilled successfully. Geophysical exploration at Aluto involved magnetotelluric (MT) soundings, which helped delineate the clay cap atop of the hydrothermal reservoir. However, until now geophysical studies did not succeed in imaging the proposed magmatic heat source that would drive the observed hydrothermal convection. For this study, we inverted 165 of a total of 208 MT stations that were measured over the entire volcanic complex in three independent surveys by the Geological Survey of Ethiopia and ETH Zurich, Switzerland. For the inversion, we used a novel 3-D inverse solver that employs adaptive finite element techniques, which allowed us to accurately model topography and account for varying lateral and vertical resolution. We inverted MT phase tensors. This transfer function is free of galvanic distortions that have long been recognized as an obstacle in MT inversion. Our recovered model shows, for the first time, the entire magmatic-hydrothermal system under the geothermal field. The up-flow of melt is structurally controlled by extensional rift faults and sourced by a lower crustal basaltic mush reservoir. Productive wells were all drilled into a weak fault zone below the clay cap. The productive reservoir is underlain by an electrically conductive upper-crustal feature, which we interpret as a highly crystalline rhyolitic mush zone, acting as the main heat source. Our results demonstrate the importance of a dense MT site distribution and state-of-the-art inversion tools in order to obtain reliable and complete subsurface models of high enthalpy systems below volcanic geothermal prospects.

Hefny, M. Nanoscale simulation of two fluid phases in a low-enthalpy geothermal system using synchrotron-based CT dataset: the case of the Nubian Sandstone (Egypt) , 2019. [View Abstract]The injection of CO2 into the highly permeable Nubian Sandstone of a depleted oil field in the central Gulf of Suez Basin, Egypt is an effective way to extract enthalpy from deep sedimentary basins while sequestering CO2, forming a so-called CO2-Plume Geothermal (CPG) system. The fluid displacement mechanism is capillarity-dominated, acting at brine-CO2 interfaces and at constant flow rate, eventually leading to the breakup of the CO2 phase into bubbles and ganglia, which become immobile (residual CO2 saturation). CO2 capillary trapping at the pore scale is a key process for maximizing capacity and ensuring the storage security at industrial scales. Based on the fluid displacement mechanisms, quasi-static pore-network modeling has been used to simulate the equilibrium positions of fluid-fluid interfaces, and thus determine the capillary pressure and relative permeability curves. Three-dimensional images with a voxel size of 650 nm3 of a Nubian Sandstone plug have been obtained using Synchrotron Radiation X-ray Tomographic Microscopy. From the images, topological properties of pores/throats were constructed. Using a pore-network model, we performed two sequential drainage-imbibition cycles of quasi-static fluid invasion to quantify the saturation of scCO2 at the point of breakthrough, the effect of initial scCO2 saturation and flow rate on the storage/trapping potential of Nubian Sandstone. These results will help in identifying preferred locations for CO2 injection to maximize storage. Further investigations will employ capillary pressure, relative permeability curves and pore-scale displacement mechanisms to predict the extent of CO2 plume migration at a representative geological scale and significantly enhance the outcomes of in-situ operations modeling using the available subsurface field data.

Hefny, M., C.-Z. Qin, A. Ebigbo, J. Gostick, M.O. Saar, and M. Hammed CO2-Brine flow in Nubian Sandstone (Egypt): Pore-Network Modeling using Computerized Tomography Imaging , European Geothermal Congress (EGC), 2019. [View Abstract]The injection of CO2 into the highly permeable Nubian Sandstone of a depleted oil field in the central Gulf of Suez Basin (Egypt) is an effective way to extract enthalpy from deep sedimentary basins while sequestering CO2, forming a so-called CO2-Plume Geothermal (CPG) system. Subsurface flow models require constitutive relationships, including relative permeability and capillary pressure curves, to determine the CO2-plume migration at a representative geological scale. Based on the fluid-displacement mechanisms, quasi-static pore-network modeling has been used to simulate the equilibrium positions of fluid-fluid interfaces, and thus determine the capillary pressure and relative permeability curves. 3D images with a voxel size of 650 nm3 of a Nubian Sandstone rock sample have been obtained using Synchrotron Radiation X-ray Tomographic Microscopy. From the images, topological properties of pores/throats were constructed. Using a pore-network model, we performed a cycle of primary drainage of quasi-static invasion to quantify the saturation of scCO2 at the point of a breakthrough with emphasis on the relative permeability–saturation relationship. We compare the quasi-static flow simulation results from the pore-network model with experimental observations. It shows that the Pc-Sw curve is very similar to those observed experimentally.

Ma, J., M.O. Saar, and X.-Z. Kong Estimation of Effective Surface Area: A Study on Dolomite Cement Dissolution in Sandstones , Proceedings World Geothermal Congress 2020, 2019.

Kyas, S., S. Repin, and U. Langer Space-Time Isogeometric Analysis of Parabolic Diffusion Problems in Moving Spatial Domains , Proceedings in Applied Mathematics and Mechanics, 2019. [View Abstract]This paper is devoted to locally stabilized space-time isogeometric (IgA) schemes for parabolic diffusion problems in moving spatial domains. It generalizes the results of our preceding works for problems in fixed spatial domains. We present functional a posteriori error estimates and study adaptive numerical procedures based on them and on truncated hierarchical B-splines.

Kyas, S., U. Langer, and S. Repin Adaptive space-time isogeometric analysis for parabolic evolution problems , Space-Time Methods: Applications to Partial Differential Equations, Radon Series on Computational and Applied Mathematics, pp. 155-199, 2019. [View Abstract]The paper proposes new locally stabilized space-time Isogeometric Analysis approximations to initial boundary value problems of the parabolic type. Previously, similar schemes (but weighted with a global mesh parameter) has been presented and studied by U. Langer, M. Neumüller, and S. Moore (2016). The current work devises a localized version of this scheme, which is suited for adaptive mesh refinement. We establish coercivity, boundedness, and consistency of the corresponding bilinear form. Using these fundamental properties together with standard approximation error estimates for B-splines and NURBS, we show that the space-time Isogeometric Analysis solutions generated by the new scheme satisfy asymptotically optimal a priori discretization error estimates. Error indicators used for mesh refinement are based on a posteriori error estimates of the functional type that has been introduced by S. Repin (2002) and later rigorously studied in the context of Isogeometric Analysis by U. Langer, S. Matculevich, and S. Repin (2017). Numerical results discussed in the paper illustrate an improved convergence of global approximation errors and respective error majorants. They also confirm the local efficiency of the error indicators produced by the error majorants.

Lima, M.M., P. Schädle, D. Vogler, M.O. Saar, and X.-Z. Kong Impact of Effective Normal Stress on Capillary Pressure in a Single Natural Fracture , European Geothermal Congress 2019, pp. 1-9, 2019. [View Abstract]Multiphase fluid flow through rock fractures occurs in many reservoir applications such as geological CO2 storage, Enhanced Geothermal Systems (EGS), nuclear waste disposal, and oil and gas production. However, constitutional relations of capillary pressure versus fluid saturation, particularly considering the change of fracture aperture distributions under various stress conditions, are poorly understood. In this study, we use fracture geometries of naturally-fractured granodiorite cores as input for numerical simulations of two-phase brine displacement by super critical CO 2 under various effective normal stress conditions. The aperture fields are first mapped via photogrammetry, and the effective normal stresses are applied by means of a Fast Fourier Transform (FFT)-based convolution numerical method. Throughout the simulations, the capillary pressure is evaluated from the local aperture. Two approaches to obtain the capillary pressure are used for comparison: either directly using the Young-Laplace equation, or the van Genuchten equation fitted from capillary pressure-saturation relations generated using the pore-occupancy model. Analyses of the resulting CO2 injection patterns and the breakthrough times enable investigation of the relationships between the effective normal stress, flow channelling and aperture-based capillary pressures. The obtained results assist the evaluation of two-phase flow through fractures in the context of various subsurface applications.

Rossi, E., S. Jamali, M.O. Saar, and Ph. Rudolf von Rohr Laboratory and field investigation of a combined thermo-mechanical technology to enhance deep geothermal drilling , 81st EAGE Conference & Exhibition 2019, pp. 1-4, 2019.

Rossi, E., M.A. Kant, O. Borkeloh, M.O. Saar, and Ph. Rudolf von Rohr Experiments on Rock-Bit Interaction During a Combined Thermo-Mechanical Drilling Method , 43rd Workshop on Geothermal Reservoir Engineering, SGP-TR-213, 2018. [View Abstract]The development of deep geothermal systems to boost global electricity production relies on finding cost-effective solutions to enhance the drilling performance in hard rock formations. Conventional drilling methods, based on mechanical removal of the rock material, are characterized by high drill bit wear rates and low rates of penetration (ROP) in hard rocks, resulting in high drilling costs, which account for more than 60% of the overall costs for a geothermal project. Therefore, alternative drilling technologies are investigated worldwide with the aim of improving the drilling capabilities and therewith enhancing the exploitation of deep geothermal resources. In this work, a promising drilling method, where conventional rotary drilling is thermally assisted by a flame-jet, is evaluated. Here, the thermal weakening of the rock material, performed by flame-jets, facilitates the subsequent mechanical removal performed by conventional cutters. The flame moves on the rock surface and thermally treats the material by inducing high thermal gradients and high temperatures, therewith reducing the mechanical properties of the rock. This would result in reduced forces on the drill bits, leading to lower bit wear rates and improved rates of penetration and therefore significantly decreasing the drilling costs, especially for deep-drilling projects. In this work, the feasibility of the proposed drilling method is assessed by comparing the rock-bit interaction in sandstone and granite under baseline and thermally treated conditions. Rock abrasivity, tool penetration and cutting forces are investigated to quantify the rock-bit interaction in granite and sandstone under baseline conditions and after the thermal treatment. The results highlights the dominant mechanisms regulating the rock removal. The removal performance of the tool in the granite material are found to be greatly enhanced by the thermal treatment both in terms of volume removed from the sample and worn volume at the tool’s tip. On the other hand, the sandstone material, after a thermal treatment, yields significantly lower wearing of the cutting tool. Thus, this results allow to draw important conclusions regarding the achievable drilling performances during the combined thermo-mechanical drilling method towards its application in the field.

von Planta, C., D. Vogler, X. Chen, M. Nestola, P. Zulian, M.O. Saar, and R. Krause Fluid-Structure Interaction with a parallel transfer operators to model hydro-mechanical processes in heterogeneous fractures , CouFrac 2019, pp. 1-4, 2018. [View Abstract]Contact mechanics and fluid flow in rough fractures are actively researched topics in reservoir engineering (e.g., enhanced geothermal systems, CO2 sequestration and oil- and gas-extraction) to estimate reservoir productivity or leak-off. Mechanical and fluid flow processes in reservoirs are often tightly coupled and exhibit a strongly non-linear behavior [1, 2]. Understanding hydro-mechanically coupled behavior in fractures is complicated further by highly variable fracture geometries [3, 4]. We present a simulation approach for hydro-mechanical processes in rough fracture geometries with variational parallel transfer operators. The contact problem at the boundary between the two rough fracture surfaces is solved using a finite element formulation of linear elasticity on an unstructured mesh. The contact formulation uses a mortar method with Lagrange multipliers and does not use a penalty parameter or other regularizations. For the Navier-Stokes formulation of the fluid we use a finite element formulation on a structured grid. Information between the meshes is transferred via the variational transfer operators, whereby the solid interacts with the fluid by enforcing velocity constraints at the solid-fluid interface and the fluid interacts with the solid by converting the fluid velocity into a pressure force acting on the solid.

Planta, C. , D. Vogler, M. Nestola, P. Zulian, and R. Krause Variational Parallel Information Transfer between Unstructured Grids in Geophysics - Applications and Solution Methods , PROCEEDINGS, 43rd Workshop on Geothermal Reservoir Engineering, pp. 1-13, 2018. [View Abstract]State of the art simulations of enhanced geothermal systems are multi-physics simulations, where different physical properties are often being modeled on different geometries or grids. For example, for the simulation of fluids many prefer finite difference or volume formulations on structured grids, whereas in mechanics finite element formulations on unstructured grids are often preferred. To transfer information between these various geometries, we use a generic variational transfer operator, which has previously been introduced as pseudo-L2-projection. In this paper, we demonstrate that this transfer operator can be particularly useful for geophysics with three applications: First, in contact simulations we often have non-matching surfaces at the contact boundary. Here, the transfer operator acts as a mortar projection and we show with high resolution rock fracture geometries from the Grimsel Test Site in Switzerland how the variational transfer operator is used to formulate the contact problem. Secondly, we present a three-dimensional fluid-structure simulation, computing water flow between two rock surfaces and simultaneous deformation with an immersed boundary approach. In this method the solid, which is formulated on an unstructured grid, interacts with the fluid, formulated on a structured grid, by means of weakly enforced velocity constraints at the interface between fluid and solid. And lastly, we show how the transfer operator can be used to effectively solve contact problem between rock bodies. Using our operator, we can generate nested multilevel hierarchies, enabling us to solve the problem with optimal complexity, thus extending the possible size of simulations immensely.

Matculevich, S., U. Langer, and S. Repin Functional Type Error Control for Stabilized Space-Time IgA Approximations to Parabolic Problems , Lecture Notes in Computer Science, 10665 LNCS, pp. 55-65, 2018. [Download PDF] [View Abstract]The paper is concerned with reliable space-time IgA schemes for parabolic initial-boundary value problems. We deduce a posteriori error estimates and investigate their applicability to space-time IgA approximations. Since the derivation is based on purely functional arguments, the estimates do not contain mesh dependent constants and are valid for any approximation from the admissible (energy) class. In particular, they imply estimates for discrete norms associated with stabilised space-time IgA approximations. Finally, we illustrate the reliability and efficiency of presented error estimates for the approximate solutions recovered with IgA techniques on a model example.

Myre, J.M., E. Frahm, D.J. Lilja, and M.O. Saar TNT: A solver for large dense least-squares problems that takes conjugate gradient from bad in theory, to good in practice , 32nd IEEE International Parallel and Distributed Processing Symposium, pp. 987-995, 2018. [Download PDF]

Myre, J.M., E. Frahm, D.J. Lilja, and M.O. Saar Solving Large Dense Least-Squares Problems: Preconditioning to Take Conjugate Gradient From Bad in Theory, to Good in Practice , IEEE International Parallel and Distributed Processing Symposium Workshops (IPDPSW), pp. 987-995, 2018. [Download PDF] [View Abstract]Since its inception by Gauss, the least-squares problem has frequently arisen in science, mathematics, and engineering. Iterative methods, such as Conjugate Gradient Normal Residual (CGNR), have been popular for solving sparse least-squares problems, but have historically been regarded as undesirable for dense applications due to poor convergence. We contend that this traditional “common knowledge” should be reexamined. Preconditioned CGNR, and perhaps other iterative methods, should be considered alongside standard methods when addressing large dense least-squares problems. In this paper we present TNT, a dynamite method for solving large dense least-squares problems. TNT implements a Cholesky preconditioner for the CGNR fast iterative method. The Cholesky factorization provides a preconditioner that, in the absence of round-off error, would yield convergence in a single iteration. Through this preconditioner and good parallel scaling, TNT provides improved performance over traditional least-squares solvers allowing for accelerated investigations of scientific and engineering problems. We compare a parallel implementations of TNT to parallel implementations of other conventional methods, including the normal equations and the QR method. For the small systems tested (15000 × 15000 or smaller), it is shown that TNT is capable of producing smaller solution errors and executing up to 16× faster than the other tested methods. We then apply TNT to a representative rock magnetism inversion problem where it yields the best solution accuracy and execution time of all tested methods.

Birdsell, D., H. Rajaram, and S. Karra Code development for modeling induced seismicity with flow and mechanics using a discrete fracture network and matrix formulation with evolving hydraulic diffusivity , 52nd US Rock Mechanics/Geomechanics Symposium and Discrete Fracture Network Engineering Conference, ARMA 18-565, 2018. [View Abstract]Injection-induced seismicity (IIS) depends on pore pressure, in-situ stress state, and fault orientation; generally occurs in basement rock that contains fractures and faults; and moves away from the injection well as a nonlinear diffusion process. Therefore, to numerically model IIS a code should incorporate flow and geomechanics, the presence of fractures and faults, and the capability for hydraulic diffusivity to evolve with effective stress and failure history. In this work, we introduce and verify a modeling framework that allows hydraulic diffusivity to evolve as fractures open and close. Details and challenges in code development are discussed, including how the Bandis model for normal fracture deformation can be used to calculate hydraulic diffusivity as a function of effective normal stress. The discrete fracture network and matrix (DFNM) model is implemented in PFLOTRAN such that hydraulic diffusivity has different constitutive relationships for fracture and matrix grid cells. This model is applied to understand the recent IIS near Greeley, Colorado, and its results are compared to: (a) a traditional DFNM model where hydraulic diffusivity cannot evolve and (b) an equivalent porous media (EPM) model where the effect of the fractures are averaged over a large region of rock. The new DFNM model predicts critical pressure will propagate farther from an injection well. This modeling framework shows promise for applications where fracture and matrix flow are important and hydraulic diffusivity is a function of pressure, stress, and/or shear failure history.

Deb, P., D. Vogler, S. Dueber, P. Siebert, S. Reiche, C. Clauser, R.R. Settgast, and K. Willbrand Laboratory Fracking Experiments for Verifying Numerical Simulation Codes , 80th EAGE Conference and Exhibition, pp. 1-4, 2018. [Download PDF] [View Abstract]Carefully designed and well monitored experiments are irreplaceable when it comes to producing reliable data sets for a detailed understanding of physical processes, such as hydraulic fracturing. While such experiments provide insight into the governing physical processes, numerical simulations provide additional information on system behaviour by enabling a straightforward study of parameter sensitivity. In this study, we focus on both these aspects. We report on results from (1) a benchmark experimental facility for performing hydraulic fracturing experiments on large rock samples in the laboratory under controlled conditions and (2) numerical simulations of these experiments using programs, which, in future, may be used for designing hydraulic stimulation layouts. We conduct series of experiments in order to ensure reproducibility and accuracy of the measurements. This experimental data set is then shared with several research institutes to be used for verifying their simulation software. Results from the simulation provide further insight regarding parameters, which contribute to uncertainties during measurements. Detailed study of the sensitive parameters help us to improve our experimental set up further and to perform future experiments under even better controlled conditions.

Vogler, D., R.R. Settgast, C.S. Sherman, V.S. Gischig, R. Jalali, J.A. Doetsch, B. Valley, K.F. Evans, F. Amann, and M.O. Saar Modeling the Hydraulic Fracture Stimulation performed for Reservoir Permeability Enhancement at the Grimsel Test Site, Switzerland 42nd Workshop on Geothermal Reservoir Engineering, Stanford University, Proc. of the 42nd Workshop on Geothermal Reservoir Engineering, Stanford University Stanford, CA, USA February 13-15, 2017, Proceedings of the 42nd Workshop on Geothermal Reservoir Engineering Stanford University, 2017. [View Abstract]In-situ hydraulic fracturing has been performed on the decameter scale in the Deep Underground rock Laboratory (DUG Lab) at the Grimsel Test Site (GTS) in Switzerland in order to measure the minimum principal stress magnitude and orientation. Conducted tests were performed in a number of boreholes, with 3–4 packer intervals in each borehole subjected to repeated injection. During each test, fluid injection pressure, injection flow rate and microseismic events were recorded amongst others. Fully coupled 3D simulations have been performed with the LLNL's GEOS simulation framework. The methods applied in the simulation of the experiments address physical processes such as rock deformation/stress, LEFM fracture mechanics, fluid flow in the fracture and matrix, and the generation of micro-seismic events. This allows to estimate the distance of fracture penetration during the injection phase and correlate the simulated injection pressure with experimental data during injection, as well as post shut-in. Additionally, the extent of the fracture resulting from simulations of fracture propagation and microseismic events are compared with the spatial distribution of the microseismic events recorded in the experiment.

Rossi, E., M. Kant, F. Amann, M.O. Saar, and P. Rudolf von Rohr The effects of flame-heating on rock strength: Towards a new drilling technology American Rock Mechanics Association (ARMA) Symposium, Proc. of the American Rock Mechanics Association (ARMA) Symposium San Francisco, USA June 25-28, 2017, Proceedings ARMA 2017, 2017. [View Abstract]The applicability of a combined thermo-mechanical drilling technique is investigated. The working principle of this method is based on the implementation of a heat source as a mean to either provoke thermal spallation on the surface or to weaken the rock material, when spallation is not possible. Thermal spallation drilling has already been proven to work in hard crystalline rocks, however, several difficulties hamper its application for deep resource exploitation. In order to prove the effectiveness of a combined thermo-mechanical drilling method, the forces required to export the treated sandstone material with a polycrystalline diamond compact (PDC) cutter are analyzed. The main differences between oven and flame treatments are studied by comparing the resulting strength after heat-treating the samples up to temperatures of \(650\, ^{\circ}C\) and for heating rates ranging from \(0.17 \,^{\circ}C/s\) to \(20 ^{\circ}C/s\). For moderate temperatures (\(300-450 \,^{\circ}C\)) the unconfined compressive strength after flame treatments monotonously decreased, opposed to the hardening behavior observed after oven treatments. Thermally induced intra-granular cracking and oxidation patterns served as an estimation of the treated depth due to the flame heat treatment. Therefore, conclusions on preferred operating conditions of the drilling system are drawn based on the experimental results.

Garapati, N., B.M. Adams, J.M. Bielicki, P. Schaedle, J.B. Randolph, T.H. Kuehn, and M.O. Saar A Hybrid Geothermal Energy Conversion Technology - A Potential Solution for Production of Electricity from Shallow Geothermal Resources , Energy Procedia, 114, pp. 7107-7117, 2017. [Download PDF] [View Abstract]Geothermal energy has been successfully employed in Switzerland for more than a century for direct use but presently there is no electricity being produced from geothermal sources. After the nuclear power plant catastrophe in Fukushima, Japan, the Swiss Federal Assembly decided to gradually phase out the Swiss nuclear energy program. Deep geothermal energy is a potential resource for clean and nearly CO2-free electricity production that can supplant nuclear power in Switzerland and worldwide. Deep geothermal resources often require enhancement of the permeability of hot-dry rock at significant depths (4-6 km), which can induce seismicity. The geothermal power projects in the Cities of Basel and St. Gallen, Switzerland, were suspended due to earthquakes that occurred during hydraulic stimulation and drilling, respectively. Here we present an alternative unconventional geothermal energy utilization approach that uses shallower, lower-temperature, naturally permeable regions, that drastically reduce drilling costs and induced seismicity. This approach uses geothermal heat to supplement a secondary energy source. Thus this hybrid approach may enable utilization of geothermal energy in many regions in Switzerland and elsewhere, that otherwise could not be used for geothermal electricity generation. In this work, we determine the net power output, energy conversion efficiencies, and economics of these hybrid power plants, where the geothermal power plant is actually a CO2-based plant. Parameters varied include geothermal reservoir depth (2.5-4.5 km) and turbine inlet temperature (100-220 °C) after auxiliary heating. We find that hybrid power plants outperform two individual, i.e., stand-alone geothermal and waste-heat power plants, where moderate geothermal energy is available. Furthermore, such hybrid power plants are more economical than separate power plants.

Vogler, D., R. Settgast, C. Annavarapu, P. Bayer, and F. Amann Hydro-Mechanically Coupled Flow through Heterogeneous Fractures 41st Workshop on Geothermal Reservoir Engineering Stanford University pp. SGP-TR-209, Proc. of the 41st Workshop on Geothermal Reservoir Engineering Stanford University pp. SGP-TR-209 Stanford, CA, USA February 13-15, 2017, Proceedings of the 41st Workshop on Geothermal Reservoir Engineering Stanford University, pp. SGP-TR-209, 2016. [View Abstract]Heterogeneous aperture distributions are an intrinsic characteristic of natural fractures. The presence of highly heterogeneous aperture distributions can lead to flow channeling, thus influencing the macroscopic behavior of the fluid flow. High-fidelity numerical simulation tools are needed for realistic simulation of fracture flow when such features are present. Here, focus is set on the role of mechanical fracture closure for fluid flow and appropriate simulation by a fully hydro-mechanically (HM) coupled numerical model. In a laboratory experiment, an artificial fracture in a granodiorite sample is created. During different sequential loading cycles, the development of fracture closure, contact area and contact stress are examined. Constant fluid flow rate injection into the center of the rough fracture is modelled to investigate the impact of fracture closure on the flow field and injection pressure. Results show that the numerical framework for heterogeneous fracture surfaces allows for reproducing experimental data of dry, mechanical tests at the laboratory scale, and it may offer advanced understanding and prediction of the behavior of reservoirs that are subject to high-pressure fluid injections.

Ahkami, M., K.H. Chakravarty, I. Xiarchos, K. Thomsen, and P.L. Fosbol Determining Optimum Aging Time Using Novel Core Flooding Equipment Society of Petroleum Engineers, SPE Bergen, Proc. of the Society of Petroleum Engineers, SPE Bergen Bergen, Norway 20 Apr 2016, 2016. [Download PDF]

Hommel, J., A. Ebigbo, R. Gerlach, A. B. Cunningham, R. Helmig, and H. Class Finding a Balance between Accuracy and Effort For Modeling Biomineralization , Energy Procedia, 97, pp. 379-386, 2016. [Download PDF]

Garapati, N., J. Randolph, S. Finsterle, and M.O. Saar Simulating Reinjection of Produced Fluids Into the Reservoir Stanford Geothermal workshop, Proc. of the Stanford Geothermal workshop Stanford, CA February 2016, Proceedings of 41st Workshop on Geothermal Reservoir Engineering, 2016. [View Abstract]ABSTRACT In order to maintain reservoir pressure and stability and to reduce reservoir s ubsidence, reinjection of produced fluids into the reservoir is common practice . Furthermore, studies by Karvounis and Jenny (2012 ; 2014), Buscheck et al. (2015), and Saar et al. (2015) found that preheating the working fluid in shallow reservoirs and then injecting the fluid into a deep reservoir can increase the reservoir life span, the heat extraction efficiency, and the economic gains of a geothermal power plant . We have modif ied the TOUGH2 simulator to enable the reinjection of produced fluids with the same chemical composition as the produced fluid and with either a prescribed or the production temperature . T he latter capability is useful, for example, for simulating injecti on of produced fluid into another (e .g., deeper) reservoir without energy extraction. Each component of the fluid mixture , produced from the production well , is reinjected into the reservoir as an individual source term. In the current study, we investigate a CO 2 - based geothermal system and focus on the effects of reinjecting small amounts of brine that are produced along with the CO 2 . Brine has a significantly smaller mobility (inverse kinematic viscosity) than supercritical CO 2 at a given temperature and thus accumulates near the injection well. Such brine accumulation reduces the relative permeability for the CO 2 phase, which in turn increases the pore - fluid pressure around the injection well and reduces the well in j ectivity index. For this reason, and as injection of two fluid phases is pr oblematic, we recommend removal of any brine from the produced fluid before the cooled CO 2 is reinjected into the reservoir. We also study the performance of a multi - level geothermal system (Karvounis and Jenny, 2012; 2014; Saar et al., 2015) by injection of preheated brine from a shallow reservoir (1.5 - 3 km) into a deep reservoir (5 km). We f i nd that preheating brine at the shallow reservoir extends the lifespan of the deep, hot reservoir, thereby increasing the total power production.

Buscheck, T.A., J.M. Bielicki, M. Chen, Y. Sun, Y. Hao, T.A. Edmunds, J.B. Randolph, and M.O. Saar Multi-Fluid Sedimentary Geothermal Energy Systems for Dispatchable Renewable Electricity World Geothermal Congress, Proc. of the World Geothermal Congress Melbourne 19-25April, Proceedings to the World Geothermal Congress, 2015. [View Abstract]Sedimentary geothermal resources typically have lower temperatures and energy densities than hydrothermal resources, but they often have higher permeability and larger areal extents. Consequently, spacing between injection and production wells is likely to be wider in sedimentary resources, which can result in more fluid pressure loss, increa sing the parasitic cost of powering the working fluid recirculation system, compared to hydrothermal systems . For hydrostatic geothermal resources , extracting heat requires that brine be lifted up production wells, such as with submersible pumps, which can consume a large portion of the electricity generated by the power plant. CO 2 is being considered as an alternative working fluid (also termed a supplemental fluid) because its advantageous thermophysical properties reduce this parasitic cost, and because of the synergistic benefit of geologic CO 2 sequestration (GCS). We expand on this idea by: (1) adding the option for multiple supplemental fluids (N 2 as well as CO 2 ) and injecting these fluids to create overpressured reservoir conditions , (2) utiliz ing up to three working fluids: brine, CO 2 , and N 2 for heat extraction, (3) using a well pattern designed to store supplemental fluid and pressure , and (4) time - shifting the parasitic load associated with fluid recirculation to provide ancillary services ( frequen cy regulat ion , load fo llowing , and spinning reserve) and bulk energy storage (BES) . Our approach uses concentric rings of horizontal wells to create a hydraulic divide to store supplemental fluid and pressure, much like a hydroelectric dam. While, as with any geothermal system, electricity production can be run as a base - load power source, p roduction wells can alternatively b e controlled like a spillway to supply power when demand is greatest. For conventional geothermal power, the parasitic power load for fluid recirculation is synchronous with gross power output. In contrast, our approach time - shift s much of this parasitic load, which is dominated by the power required to pressurize and inject brine . Th us, most of the parasitic load can be scheduled durin g minimum power demand or when, due to its inherent var iability, there is a surplus of renewable energy on the grid . Energy storage is almost 100 percent efficient because it is achieved by time - shifting the parasitic load. Consequently, net power can near ly equal gross power during peak demand so that geothermal energy can be used as a form of high - efficiency BES at large scales . A further benefit of our approach is that production rates (per well) can exceed the capacity of submersible pumps and thereby t ake advantage of the productivity of horizontal wells and better leverage we ll costs — which often constitute a major portion of capital costs . Our vision is a n efficient, dispatchable , renewable electricity system approach that facilitates deep market penet ration of all renewable energy sources: wind, solar, and geothermal, whi le utilizing and permanently storing CO 2 in a commercially viable manner

Saar, M.O., Th. Buscheck, P. Jenny, N. Garapati, J.B. Randolph, D. Karvounis, M. Chen, Y. Sun, and J.M. Bielicki Numerical Study of Multi-Fluid and Multi-Level Geothermal System Performance World Geothermal Congress 2015, Proc. of the World Geothermal Congress 2015 Melbourne, Australia April 19-25, 2015, Proceedings World Geothermal Congress 2015, 2015. [View Abstract]We introduce the idea of combining multi-fluid and multi-level geothermal systems with two reservoirs at depths of 3 and 5 km. In the base case, for comparison, the two reservoirs are operated independently, each as a multi-fluid (brine and carbon dioxide) reservoir that uses a number of horizontal, concentric injection and production well rings. When the shallow and the deep reservoirs are operated in an integrated fashion, in the shallow reservoir, power is produced only from the carbon dioxide (CO 2), while the brine is geothermally preheated in the shallow multi-fluid reservoir, produced, and then reinjected at the deeper reservoir's brine injectors. The integrated reservoir scenarios are further subdivided into two cases: In one scenario, both brine (preheated in the shallow reservoir) and CO 2 (from the surface) are injected separately into the deeper reservoir's appropriate injectors and both fluids are produced from their respective deep reservoir producers to generate electricity. In the other scenario, only preheated brine is injected into, and produced from, the deep reservoir for electric power generation. We find that integrated, vertically stacked, multi-fluid geothermal systems can result in improved system efficiency when power plant lifespans exceed ~30 years. In addition, preheating of brine before deep injection reduces brine overpressurization in the deep reservoir, reducing the risk of fluid-induced seismicity. Furthermore, CO2-Plume Geothermal (CPG) power plants in general, and the multi-fluid, multi-level geothermal system described here in particular, assign a value to CO2, which in turn may partially or fully offset the high costs of carbon capture at fossil-energy power plants and of CO2 injection, thereby facilitating economically feasible carbon capture and storage (CCS) operations that render fossil-energy power plants green. From a geothermal power plant perspective, the system results in a CO2 sequestering geothermal power plant with a negative carbon footprint. Finally, energy return on well costs and operational flexibility can be greater for integrated geothermal reservoirs, providing additional options for bulk and thermal energy storage, compared to equivalent, but separately operated reservoirs. System economics can be enhanced by revenues related to efficient delivery of large-scale bulk energy storage and ancillary services products (frequency regulation, load following, and spinning reserve), which are essential for electric grid integration of intermittently available renewable energy sources, such as wind and solar. These capabilities serve to stabilize the electric grid and promote development of all renewable energies, beyond geothermal energy. Numerical Study of Multi-Fluid and Multi-Level Geothermal System Performance (PDF Download Available). Available from: [accessed Jun 12, 2017].

Jalali, M.R., K.F. Evans, B.C. Valley, and M.B. Dusseault Relative Importance of THM Effects during Non-isothermal Fluid Injection in Fractured Media , 49th US Rock Mechanics / Geomechanics Symposium, 2015. [View Abstract]Rock mass treatment using fluid injection is common in various industrial applications, including enhanced recovery methods in the oil and gas industry, rock mass pre-conditioning in the mining industry, and heat extraction in geothermal systems. Non-isothermal fluid injection requires consideration of the thermomechanical perturbation as well as hydro-mechanical processes. Thermal effect is rarely included in injection analysis for geothermal application and thermal enhanced oil recovery methods, although with long times their impact becomes of first-order. In this paper, a fully-coupled, hybrid numerical model is implemented to study the effect of cold fluid injection into a conductive fracture under different injection/cooling schemes. The results show that the thermoelastic effect soon overwhelms the hydroelastic effect adjacent to the injection source, whereas far from the injection point, hydroelastic effect dominates because the pressure front always moves faster than the cold front. In addition, the fracture becomes more susceptible to shear failure in the presence of both thermoelastic and hydroelastic induced stresses for the case of cold fluid injection. The magnitude of the changes implies that an appropriate thermo-hydromechanical (THM) model is an essential key to address the physical behavior and potential impairment of fracture conductivity under thermal stimulation.

Garapati, N., J.B. Randolph, and M.O. Saar Superheating Low-Temperature Geothermal Resources to Boost Electricity Production 40th Geothermal Reservoir Engineering Workshop 2015, Proc. of the 40th Geothermal Reservoir Engineering Workshop 2015 Stanford, CA, USA January 26-28, 2015, Proceedings of the 40th Workshop on Geothermal Reservoir Engineering 2015, 2, pp. 1210-1221, 2015. [View Abstract]Low-temperature geothermal resources (<150°C) are typically more effective for direct use, i.e., district heating, than for electricity production. District or industrial heating, however, requires that the heat resource is close to residential or industrial demands in order to be efficient and thus economic. However, if a low-temperature geothermal resource is combined with an additional or secondary energy source that is ideally renewable, such as solar, biomass, biogas, or waste heat, but could be non-renewable, such as natural gas, the thermodynamic quality of the energy source increases, potentially enabling usage of the combined energy sources for electricity generation. Such a hybrid geothermal power plant therefore offers thermodynamic advantages, often increasing the overall efficiency of the combined system above that of the additive power output from two stand-alone, separate plants (one using geothermal energy alone and the other using the secondary energy source alone) for a wide range of operating conditions. Previously, fossil superheated and solar superheated hybrid power plants have been considered for brine/water based geothermal systems, especially for enhanced geothermal systems. These previous studies found, that the cost of electricity production can typically be reduced when a hybrid plant is operated, compared to operating individual plants. At the same time, using currently-available high-temperature energy conversion technologies reduces the time and cost required for developing other less-established energy conversion technologies. Adams et al. (2014) found that CO 2 as a subsurface working fluid produces more net power than when brine systems are employed at low to moderate reservoir depths, temperatures, and permeabilities. Therefore in this work, we compare the performance of hybrid geothermal power plants that use brine or, importantly, CO 2 (which constitutes the new research component) as the subsurface working fluid, irrespective of the secondary energy source used for superheating, over a range of parameters. These parameters include geothermal reservoir depth and superheated fluid temperature before passing through the energy conversion system. The hybrid power plant is modeled using two software packages: 1) TOUGH2 (Pruess, 2004), which is employed for the subsurface modeling of geothermal heat and fluid extraction as well as for fluid reinjection into the reservoir, and 2) Engineering Equation Solver (EES), which is used to simulate well bore fluid flow and surface power plant performance. We find here that for geothermal systems combined with a secondary energy source (i.e., a hybrid system), the maximum power production for a given set of reservoir parameters is highly dependent on the configuration of the power system. The net electricity production from a hybrid system is larger than that from the individual plants combined for all scenarios considered for brine systems and for low-grade secondary energy resources for CO 2 based geothermal systems. Superheating of Low-Temperature Geothermal Working Fluids to Boost Electricity Production: Comparison between Water and CO2 Systems (PDF Download Available). Available from: [accessed Jun 12, 2017].

Ziegler, M., B. Valley, and K.F. Evans Characterization of natural fractures and fracture zones of the Basel EGS reservoir inferred from geophysical logging of the Basel-1 well World Geothermal Congress (WGC) 2015, Proc. of the World Geothermal Congress (WGC) 2015 Melbourne , Australia April 19-25, 2015, Proceedings World Geothermal Congress 2015, pp. 31003, 2015. [View Abstract]The development of a geological model for the reservoir of an Enhanced Geothermal System (EGS) provides an essential fram e- work for geomechanical models that simulate reservoir behaviour during the stimulation and production phases. The geological model describes the spatial distribution and scaling of discontinuities within the reservoir as well as lithological variatio ns. In this paper we analyse logging data from the 5 km deep well, Basel - 1, located in Switzerland to investigate th e natural fractures and zones characterised by high fracture frequency in the crystalline basement. The logs extend from 2.6 km depth, about 100 m be low the weathered palaeo - surface of the granite, to a depth of 5.0 km, and include acoustic televiewer (UBI ), density and p - wave veloc i- ty. The results of drill cuttings analysis were also available. Two previous analyses of the UBI log have been made. Considerable differences in the distributions of natural fractures in the crystalline basement were found from the three analyses. The differences in large part reflect the difficulty in distinguishing natural from drilling - induced fractures. Poor quality images in the open - hole section below 4.7 km resulting from stick - slip motion of the UBI sonde were radically i mproved by applying a novel correction method using accelerometer data. This led to fewer natural fractures in the open section than recogni s ed in earlier studies. Fracture frequency decreases with depth from 3.1 fractures/m near the top of the logged sect ion to 0.3 fractures/m below 3.0 km . Or ientation cluster analyses revealed a complex pattern of up to 6 potential fracture sets along the well, some of which may be conjugate pairs. Only set 1 (steeply dipping to W – SW) is present along the entire imaged bo rehole, the other sets occurring over limited sections of the hole. The mean orientation of set 1 does not coincide with prominent NNE - striking Rhenish lineaments (faults) of first - and second - order in the Basel area , but strikes subparallel to the maximum principal horizontal stress. Fractures belonging to set 1 are spatially clustered and form locali s ed zones of high fracture frequency. Zone lengths ranged up to 100 m, but were more typically tens of metres, and below 4 .0 km the zones consisted predominan tly of fractures belonging to set 1. Zones of high fracture freque n- cy did not necessarily coincide with low density or low p - wave velocity anomalies, as might be expected from fracture zones with damage or higher porosity.

Garapati, N., J.B. Randolph, J.L. Valencia Jr., and M.O. Saar Design of CO2-Plume Geothermal (CPG) subsurface system for various geologic parameters Fifth International Conference on Coupled Thermo-Hydro-Mechanical-Chemical (THMC) Processes in Geosystems: Petroleum and Geothermal Reservoir Geomechanics and Energy Resource Extraction, Proc. of the Fifth International Conference on Coupled Thermo-Hydro-Mechanical-Chemical (THMC) Processes in Geosystems: Petroleum and Geothermal Reservoir Geomechanics and Energy Resource Extraction Salt Lake City, UT 2015, Proceedings of the Fifth International Conference on Coupled Thermo-Hydro-Mechanical-Chemical (THMC) Processes in Geosystems: Petroleum and Geothermal Reservoir Geomechanics and Energy Resource Extraction, 2015. [View Abstract]Recent geotechnical research shows that geothermal heat can be efficiently mined by circulating carbon dioxide through naturally permeable rock formations -- a method called CO2 Plume Geothermal -- the same geologic reservoirs that are suitable for deep saline aquifer CO2 sequestration or enhanced oil recovery. This paper describes the effect of thermal drawdown on reservoir pressure buildup during sequestration operations, revealing that geothermal heat mining can decrease overpressurization by 10% or more. Geothermal Energy Production at Geologic CO2 Sequestration sites: Impact of Thermal Drawdown on Reservoir Pressure (PDF Download Available). Available from: [accessed Jun 12, 2017].

Birdsell, D., H. Rajaram, D. Dempsey, and H. Viswanathan Numerical Model of Hydraulic Fracturing Fluid Transport in the Subsurface with Pressure Transient and Density Effects , 49th US Rock Mechanics/Geomechanics Symposium, 2015. [View Abstract]Understanding the transport of hydraulic fracturing (HF) fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated. Pressure gradients, permeable pathways such as faults or improperly abandoned wellbores, and the density contrast of the HF fluid to the surrounding brine could encourage upward HF fluid migration. In contrast, very low shale permeability and well production may work to keep HF fluid at depth and remove it from the subsurface. Single-phase flow and transport simulations are performed to quantify how much HF fluid is removed via the wellbore and how much reaches overlying aquifers. If a permeable pathway connects the shale reservoir to the overlying drinking water aquifer, the pressure transient due to injection and the density contrast allows rapid upward plume migration at early times, but well production reverses the direction of flow and removes a large amount of HF fluid from the subsurface. We present estimates of HF fluid migration to shallow aquifers during the first 1,000 years and show that the pressure transient from well operations should be included in subsequent numerical models while buoyancy may be neglected depending on depth and permeability.

Valley, B., and K.F. Evans Estimation of the stress magnitudes in Basel Enhanced Geothermal System World Geothermal Conference 2015, Proc. of the World Geothermal Conference 2015 Melbourne, Australia April 19-24, 2015, Proceedings World Geothermal Congress 2015, 2015. [View Abstract]The in - situ state of stress plays a major role in determining the response of the rock mass to hydraulic stimulation injections used to develop heat - exchangers in low - permeability EGS reservoirs. As such, stress and its heterogeneity must be speci fied in any geomechanical model of the s tim ulation process. This paper presents the results of an evaluation of stress magnitude s in the granitic EGS reservoir in Basel, Switzerland. The profile of minimum principal horizontal stress, Shmin, is constrained by hydraulic tests, but the magnitude of the maximum horizontal principal stress, SHmax is uncertain. Here we derive estimates for SHmax by analysing breakout width data from an acoustic televiewer log run the 5 km deep borehole BS - 1. Some 81% of the bore hole below the granite top at 2.42 km is affected by b reakouts, which is favourable for examining the depth trends of the estimates . A primary objective of the analysis was to evaluate the impact of four different failure criteria on the SHmax magnitude es timates. The criteria where Rankine, Mohr - Coulomb, Mogi - Coulomb, and Hoek - Brown 3D. All were parametrized using strength data from a single multi - stage triaxial compressive test on a core plug taken from near the well bottom . A numerical approach was emplo yed to derive SHmax magnitude from the estimated breakout widths , taking into account all stress components at the borehole wall including the remnant thermal stress arising from the cooling of the borehole wall by the drilling. Previous studies of breakou t width have shown that large, small - scale fluctuations are associated with fractures, which reflect variations in strength or stress, or both. At larger scales, breakout width tends to decrease with depth. Assuming there is no significant systematic chang e in the strength characteristics of the rock along the length of the hole, for which there is no evidence, the large - scale trend has the consequence of implying a small gradient of the SHmax profile. This result is independent of the failure criterion, an d also of the profile of Shmin used in the analysis. The absolute values of SHmax depend upon the failure criterion used. Criteria that consider the strengt hening effect of the intermediate stress (Mogi - Coulomb and Hoek - Brown 3D) yield profiles that violat e frictional limits on the strength of the crust above 4 km, whereas the profiles of the Mohr - Coulomb and Rankine criteria do not (the latter two are essentially identical for the case where pore pressure and wellbore pressure are equal and in the range of Shmin and SHmax relevant for our analyses ). The Mohr - Coulomb/Rankine criteria profiles indicate a trend in SHmax from favoring strike - slip faulting above 4200 m to strike - slip/normal faulting below. This is reasonably consistent with f ocal mechanisms recorded during the reservoir stimulation which show a mix of strike - slip and normal faulting throughout the depth range considered.

Ezekiel, J., Y. Wang, and Y. Liu Case Study of Air Injection IOR Process for a Low Permeability Light Oil Reservoir in Eastern China SPE Annual Caspian Technical Conference and Exhibition, Proc. of the SPE Annual Caspian Technical Conference and Exhibition Astana, Kazakhstan 12-14 November 2014, Proceeding SPE Annual Caspian Technical Conference and Exhibition, 2014. [Download PDF] [View Abstract] Air Injection into oil reservoirs specially offers unique technical and economic opportunities for secondary and/or tertiary oil recovery in light oil reservoirs with low permeability, in which conventional water injection techniques have been unsuccessful and/or uneconomical. This paper provides a comprehensive overview on the oxidation reactions and improved oil recovery (IOR) processes of air injection into low permeability light oil reservoir based on detailed analysis of some field projects and reservoir simulation case study carried out on a largely dipping, low permeability light oil reservoir, the Q131 oil block located in Eastern China to analyze the characteristics and processes of air injection. Kinetic models of low temperature oxidation (LTO) reactions were designed and used in the reservoir simulation study to predict oxygen consumption in the reservoir, examine the reaction schemes, IOR mechanisms, and the thermal effect of oxidation reactions occurring during the air injection process. The results of the study including temperature effects, oxygen concentration, oil saturation, gas breakthrough, GOR, and cumulative oil produced were outlined and discussed in details. An average of increased oil recovery factor of more than 45% OOIP was achieved when using a maximum of 60000m3/day air injection rate and no oxygen breakthrough was observed at the production wells.

Matculevich, S., P. Neitaanmäki, and S. Repin Guaranteed error bounds for a class of Picard-Lindelöf iteration methods , Numerical Methods for Diff. Eq., Opt., and Tech. Problems, Comput. Methods Appl. Sci., 27, pp. 151-168, 2013. [Download PDF] [View Abstract]We present a new version of the Picard-Lindelöf method for ordinary differential equations (ODEs) supplied with guaranteed and explicitly computable upper bounds of an approximation error. The upper bounds are based on the Ostrowski estimates and the Banach fixed point theorem for contractive operators. The estimates derived in the paper take into account interpolation and integration errors and, therefore, provide objective information on the accuracy of computed approximations.

Hommel, J., A. B. Cunningham, R. Helmig, A. Ebigbo, and H. Class Numerical Investigation of Microbially Induced Calcite Precipitation as a Leakage Mitigation Technology , Energy Procedia, 40, pp. 392-397, 2013. [Download PDF]

Bailey, P., J. Myre, S.C.D. Walsh, D.J. Lilja, and M.O. Saar Accelerating Lattice Boltzmann Fluid Flow Simulations Using Graphics Processors 38th International Conference on Parallel Processing (ICPP), Proc. of the 38th International Conference on Parallel Processing (ICPP) , IEEE, pp. 550-557, 2009. [Download PDF] [View Abstract]Lattice Boltzmann Methods (LBM) are used for the computational simulation of Newtonian fluid dynamics. LBM-based simulations are readily parallelizable; they have been implemented on general-purpose processors, field-programmable gate arrays (FPGAs), and graphics processing units (GPUs). Of the three methods, the GPU implementations achieved the highest simulation performance per chip. With memory bandwidth of up to 141 GB/s and a theoretical maximum floating point performance of over 600 GFLOPS, CUDA-ready GPUs from NVIDIA provide an attractive platform for a wide range of scientific simulations, including LBM. This paper improves upon prior single-precision GPU LBM results for the D3Q19 model by increasing GPU multiprocessor occupancy, resulting in an increase in maximum performance by 20%, and by introducing a space-efficient storage method which reduces GPU RAM requirements by 50% at a slight detriment to performance. Both GPU implementations are over 28 times faster than a single-precision quad-core CPU version utilizing OpenMP.

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Rossi, E. Combined Thermo-Mechanical Drilling Technology to Enhance Access to Deep Geo-Resources, Dissertation, ETH Zürich, 2020. [Download PDF]

Li, S Inverse Modeling of Discrete Fracture Network Using Trans-Dimensional MCMC Method, MSc Thesis, ETH Zürich, 2018.

Ezzat, M. Advanced neoclassical impurity transport modelling with its experimental comparison for TJ-II , MSc Thesis, Ghent University, 51 pp., 2018. [View Abstract]The absence of the disruptive instabilities, increasing of the confinement time with the ECRH heating and the steady-state operation make stellarator concept as a competitive candidate for future fusion reactors as the tokamak. Impurity accumulation in the core though is one of the stellarator drawbacks because it dilutes the plasma and increases the radiation losses contributing to the plasma collapse. Neoclassical theory predicts a non-ambipolar transport of different species electrons and ions in stellarators due to magnetic field ripples that are produced by the three-dimensional coils structure. Non-ambipolar transport creates, depending on the collisionality of each species, inward (outward) an ambipolar radial electric field for ion (electron) root regimes. Ion root regime has been predicted for the future stellarator reactor scenarios, which imply very likely impurity accumulation. However, outward transport has been observed during an improved confinement regime so-called \textit{HDH mode} at W7-AS (\textit{K. McCormick 2002}) and the \textit{impurity hole} at LHD (\textit{K. Ida 2009}) but without without satisfactory theoritical explanation. Historically, neoclassical treatment considers only the radial component of the electric field, which is a good approximation for the bulk species, but not for the higher charge species like impurity. Recent approaches have considered the tangential component due to the variation of the electrostatic potential within a flux surface which is more important for high charge impurity (see \[\textit{J. M. {Garc\'ia-Rega\~na} 2013}\] and reference therein). Advanced modelling of this variational part has been done here for TJ-II plasma introducing a parameter which can be measured indirectly. Direct measurement of the variational part is non-trivial and had been carried only for plasma edge (\textit{M. A. Pedrosa 2015}). Here, the indirect measurements cover the whole cross section by constructing the radiation map at two toroidal planes in TJ-II that carried impurity density distribution and in sequence the variational electrostatic part. Linearized impurity-ion collision operator (in \textit{I. Calvo 2018-Arxiv}) had been employed for impurity simulation because it is higher collisional instead of the usual pitch angel scattering operator (in \textit{C. D. Beidler 2011}) for bulk species with low collisions.

Delort, J. Numerical Methods to Account for Permeability Changes due to Mineral Precipitation/Dissolution Processes in Large Scale Reactive Transport Simulations, MSc Thesis, ENS Lyon, 2018. [Download PDF] [View Abstract]Permeability-Porosity based relationships were tested for their discretization dependency in large scale continuum reactive transport models of fluid flow in porous media. A new description of permeability on the cell face, independent from porosity, defined at the cell nodes, was proposed. This new description led to three models for permeability update to due mineral precipitation/dissolution processes. All these permeability update methods were tested thanks to a SNIA reactive transport 1D modeling of injection of a CO2 saturated brine in a porous rock made of 90% Quartz and 10% Calcite bulk volume. Dissolution and permeability enhancement was observed at the entrance whereas precipitation and porosity reduction due to calcite re-precipitation was observed in the middle of the domain, where a temperature step was empirically imposed. Our proposed methods to account for permeability reduction did not predict clogging in a more sensitive way than usual permeability-porosity based methods. They were all found to be equally dependent to the simulation discretization parameters. Further work needs to be done to better constrain this dependency and calibrate our methods.

Naets, I. Visualizing contact areas in roughfractures using 3D printing, MSc Thesis, ETH Zurich, 2018.

Faber, M. Usage Of Inter-Site Electromagnetic Transfer Functions In Exploration For Geothermal Resources, MSc Thesis, IDEA League Joint Master’s in Applied Geophysics: ETH Zurich, 85 pp., 2018. [View Abstract]Considering the rising demand for sustainable and clean energy resources, geothermal energy is a renewable source that is becoming increasingly attractive for exploration. It is best available in volcanic regions in the form of high-enthalpy hydrothermal systems which demonstrate characteristic subsurface conductivity structures. The magnetotelluric method (MT) is an optimal geophysical method for geothermal investigations due to its ability of characterizing a reservoir based on its electrical properties up to a depth of several kilometres. A magnetotelluric survey is based on the simultaneous measurement of horizontal total electric and magnetic field at each acquisition station. The electrical conductivity of the underlying material can be determined from the linear relationship between the horizontal components of the measured electric and magnetic field variations, using a so-called transfer function which corresponds to the impedance tensor. Compared to other geophysical methods, the field setup of a MT survey is cheap. However, the induction coils used as magnetic stations are very expensive compared to the electrodes used as electric field stations. Furthermore, induction coils are impractical to set up in the field which makes a traditional MT field setup with full magnetotelluric stations at each measurement point tedious and time intensive. In order to ensure progress in the domain of geothermal exploration, this study proposes a new MT acquisition survey layout based on the computation of the inter-site transfer function. The so-called inter-site setup aims at reducing the necessary number of magnetic field stations with respect to the electric field stations and thus reducing the cost and improving the efficiency of a MT survey without losing information about the subsurface. Different inter-site survey layouts are tested in which the inter-site transfer functions relate the magnetic field value at a chosen full MT base site with the electric field values of all the belonging telluric measurement stations. 3-D models are produced on the basis of high enthalpy hydrothermal systems and implemented in a forward modeling process to produce synthetic local and inter-site impedance and phase tensor responses. The phase tensor is determined from the impedance tensor but is resistant to galvanic distortions, however not sensitive to absolute conductivity subsurface values. The obtained local and inter-site data are then compared for their similarity. The different base station layouts are tested for different model complexities in order to evaluate their applicabilty range and are investigating whether if local and inter-site transfer functions are able to reach good agreement for various subsurface structures. Two main trends can be observed from the similarity data: (1) increasing model complexity decreases the agreement between local and inter-site data for the same base station layout, (2) for a given model complexity, the similarity depends on the location respective to subsurface conductivity structures and the number of base stations. Overall similarity results obtained for complex models suggest that the inter-site field setup is well suited for the investigation of geothermal targets. The significance of the obtained forward modelling similarity results remains to be confirmed by extended impedance and/or phase tensor inversion.

Birdsell, D. An Investigation of Subsurface Fluid Injections Related to Oil and Gas Development: Modeling Hydraulic Fracturing Fluid Migration and Induced Seismicity, Dissertation, 161 pp., 2018. [View Abstract]Unconventional oil and gas development is made economically feasible by horizontal drilling and hydraulic fracturing, and it can produce large volumes of wastewater that are injected into disposal wells. In the first part of this dissertation, we investigate the potential for fracturing fluid to migrate from the target formation to contaminate shallow drinking water aquifers. We present semianalytical calculations for capillary imbibition into shale, which can sequester up to 95% of the fracturing fluid, thus preventing migration to aquifers. Next, we present a numerical model of fracturing fluid migration, which is the first to combine injection and production, imbibition, and buoyancy. In the absence of a permeable pathway between the injection formation and the aquifer, no contamination occurs. In the presence of a permeable pathway, well suction and capillary imbibition significantly reduce the amount of fracturing fluid reaching the aquifer compared to scenarios that do not account for suction and imbibition. In part two, we present a new framework for modeling basin-scale injection-induced seismicity (IIS). The framework incorporates flow and geomechanics, the presence of fractures and faults, and the capability for hydraulic diffusivity to evolve with effective stress and earthquake history, which is modeled by the Mohr-Coulomb failure criteria. The model is implemented in the massively-parallel code PFLOTRAN, which is important to capture the large length scales (\( \sim \)10 km) and many fractures and faults (100-1000s). Applications of this model: (a) put constrains on the hydraulic diffusivity in basement rock, which may have been too large in previous modeling studies; (b) explain the heterogeneity of earthquake locations; and (c) capture the variations in critical pressure that cause earthquakes, based on stress state and fault orientation. In part three, we show work that verifies and increases the accuracy of subsurface simulators, which is important for the continued investigation of fluid migration, IIS, and other subsurface phenomena. First, we derive a porosity-pressure relationship for general subsurface flow codes (GSFs) which accounts for the relative velocity between pore fluid and rock matrix. Simulations using this relationship show excellent agreement between GSFs and the groundwater flow equation. Next we verify the fully-coupled flow and geomechanics implementation in PFLOTRAN by comparing to an analytical solution.

Huang, P.W. Investigating different formulations for hydrothermal convection in geothermal systems, MSc Thesis, RWTH Aachen, 2017. [View Abstract]Hydrothermal convection in porous media is an important piece of physics in geothermal reservoirs, and understanding them leads to better development of geothermal energy. We analyze the validity of simulating hydrothermal convection using different formulations of partial differential equations. Using the Elder problem as a physical benchmark, we found out that the stream function formulation and the velocity formulation are a valid and efficient model of hydrothermal convection. The Nusselt number and entropy production are measurements of the quality of convective heat transfer. The Rayleigh number describes the physical properties of a porous media. We use simulations to further investigate the discrepancy in the Nusselt Rayleigh relationship found in previous experiments. The conclusion is that the multiple steady states of convection pattern in a 3D box are the main reason for the discrepancy in the Nusselt Rayleigh relationship.

Hobé, A. Fluid Flow in Fracture Networks: A Graph Theory Approach, MSc Thesis, 32 pp., 2017. [View Abstract]Fluid flow in fractured rock is important for many societal applications, including geothermal energy, radioactive waste disposal in crystalline rock, oil/gas production, and tunnel drainage. An accurate description of such flow, however, requires a significant amount of data on various properties of the fractures, which is typically very scarce at depth. Two methods are presented here, which use graph theory algorithms in combination with stochastic discrete fracture networks (DFN) to conduct rapid calculations of flow rates. The results of this proof-of-concept investigation for a large number of DFNs of varying fracture density show that both methods have a relatively high accuracy as compared with the results obtained by explicitly solving partial differential equations for flow using an established fluid flow simulator. The low computational costs of these methods could facilitate much-needed in-depth analyses of the propagation of uncertainty in fracture and fracture-network properties to uncertainty in flow rates.

Dambly, L. On the direct measurement of the shear modulus in transversely isotropic rocks using the uniaxial compression test, MSc Thesis, 31 pp., 2017. [View Abstract]This paper presents a novel method to directly measure the out-of-plane shear modulus of transversely isotropic rocks by using a single cylindrical specimen subjected to uniaxial compression. This simple methodology relies on the measurement of strain at single or multiple points around the sample, and using those in an explicit formula to directly determine the shear modulus. In addition to the shear modulus, the plane of symmetry, the Young's moduli and the Poisson's ratio transverse to the plane of isotropy can be determined from this single test. Several experimental setups are proposed depending on whether the plane of symmetry is known or needs to be determined. Experimental results on three granitic samples show that the measured plane of symmetry is significantly deviated from the one apparent from the visual inspection of the foliation plane. In addition, the Saint-Venant formula is found to give an error of more than 10% for samples exhibiting a higher anisotropy ratio than 1.85.

Lima, M.M. The effect of sulfate ions on the performance of smartwater flooding applied to carbonate reservoirs, MSc Thesis, University of Campinas, 197 pp., 2016. [View Abstract]The "low salinity water" technology is an enhanced oil recovery method that is gaining attention from researchers and the oil industry. For carbonate reservoirs, which are normally oil wet or mixed wet, this technique is even more attractive. Studies have shown that the sulfate ion plays a unique role on the technique, acting as a catalyzer for the multiple ion exchanges that promote the wettability alteration. The concentration of the ions Na+ and Cl-, as well as the temperature, are considered important parameters. In many cases, seawater is already an optimized water for the process. Traditionally, however, in the Brazilian pre-salt oil fields, the desulfated seawater is used as the injecting water ¿ as a precaution to avoid reservoir incrustation or souring. To justify the replacement of the injection water by seawater, substantial laboratory data using reservoir carbonate rocks are still required. The present work aims to evaluate the effect of the sulfate ion in the injection water using carbonate plugs, as well as to analyze the influence of the Na+ and Cl- ions and the temperature. At the laboratory, nine water-flooding tests, at 65°C and 100°C, and six spontaneous imbibition tests, at 65°C and 90°C, were performed with different water compositions. The water-flooding tests were divided in three groups. The first group contained four tests that were selected in order to investigate the effects of the initially chosen parameters (SO42-, Na+, Cl- and temperature). The second and third groups were elaborated because the injection rates and the rock samples¿ lithology could influence the oil recovery. Therefore, other five water-flooding tests were performed to study these new perceived variables. The results have shown that an increase in the sulfate concentration of the injection water, up to the amount present in seawater, can recover up to an additional 8% OOIP, after the injection of desulfated seawater. The effect of the non-active Na+ and Cl- ions was also observed. The influence of relatively small variations (less than an order of magnitude) of the flow capillary number on the remaining oil saturation in carbonate rocks was clearly observed, indicating a very small or even inexistent critical capillary number. It was noticed that the minimum temperature for the effect of the potential ions depends on the type of test performed. In spontaneous imbibition tests, it was not possible to distinguish the effect of increasing sulfate concentrations in water at 65°C ¿ which is in agreement with literature results. However, in the water-flooding tests, at 65°C it was already possible to notice the effect of the sulfate ion on the oil recovery. Finally, the overall results open up the suggestion to analyze the viability of changing the composition of the injection water currently being used in many Brazilian pre-salt oil fields, given the seawater’s potential of saving money by simplifying the operating plant and promoting oil production

Rossi, E. A Feasibility Study of a Combined Mechanical-Thermal Drilling System, MSc Thesis, ETH Zurich, 81 pp., 2016. [View Abstract]In order to foster deep geothermal energy exploitation, a substantial reduction of the drilling costs is required. Spallation drilling is an alternative non-contact technique which would eliminate bit’s wearing related issues and increase the rate of perforation in hard crystalline rocks. However, its applicability is quite challenging and, furthermore, the spallation mechanisms do not work in soft rock formations. Therefore, a hybrid technique combining conventional mechanical and spallation drilling could be the sought breakthrough in the drilling research. Here, a flame-jet heats up the surface weakening the rock material which is then exported by PDC drill bits. An important advantage of this technique is the smoothening effect on the mechanical properties of the rock formation. Although literature presents a large amount of experimental studies about rock strength variation due to oven treatments, no investigations were found for the effects of flame thermal treatments. Therefore, an ad-hoc study is needed in order to precisely assess the consequences on the final strength of the material. Thus, in this work, the influences of temperature (until 650 ◦C), heating rate (from 0.17 to 40 ◦C/s) and confinement (until 150 Nm of tightening torque) on the material’s strength for Rohrschach sandstone and Grimsel granite are investigated. Material’s strength is measured by means of the scratch test and petrographic thin sections are used to vali- date the results. Data showed that the flame treatments lead to a monotonous decrease of strength with temperature, differently to the oven treatment where an initial increase of strength is observed. Regarding the final drilling application, two optimal operating conditions in terms of heating rate and maximum temperature are found. Besides, important variations of thermal diffusivity, conductivity and heat capacity with temperature are measured. The observed irreversible decay after the first heating cycle was justified by remarkable thermal cracking phenomena. Furthermore, an analytic approach based on Green’s functions has been developed in order to model the heat transfer phenomena for moving heat sources.

Vogler, D. Hydro-mechanically coupled processes in heterogeneous fractures: experiments and numerical simulations, Dissertation, ETH Zurich, 169 pp., 2016. [View Abstract]Enhanced Geothermal Systems (EGS), CO2-sequestration, oil- and gas reservoirs rely on an in-depth understanding of geomechanics and fluid flow in the subsurface to achieve production targets. In Switzerland, EGS are commonly targeted for deep basement formations of crystalline rock, as these are deep enough underground to provide high temperatures. In crystalline rock, fluid flow through fractures dominates transport processes, while mechanical behavior strongly depends on fracture topography and strength. This work focusses on fracture behavior in crystalline rock, such as granite and granodiorite, by investigating: (1) Differences in fracture topography linked to fracture size and nature; (2) Hydromechanically coupled processes in heterogeneous fractures in experiments on the laboratory scale; and (3) Hydro-mechanically coupled processes in heterogeneous fractures in simulations on the laboratory and field scale, supported by laboratory experiments. All rock specimens in this work are granite or granodiorite specimens obtained from the Grimsel Test Site (GTS), Switzerland. Fracture topography is studied by overcoring mode I and mode II fractures from core material and by subjecting intact specimens to Brazilian tests. This yields a range of fractures of various nature with sizes between 1 to 30 cm. Fracture topography is compared with the JRC, Z2 measure, fractal dimensions (Hausdorff and Box count dimension) and correlation functions (Two point correlation function and lineal path function) to quantify and compare roughness with a large range of parameters. Additionally, surface roughness is compared to specimen tensile strengths. Results show a clear distinction of natural shear and artificial tensile fractures, as measured with the Z2 measure. Fracture roughness appears to be linked to specimen size when comparing whole fracture sizes. Computing local roughness on small surface patches (e.g. 1 cm x 1 cm) yields smoother surfaces for large fractures, further indicating that fracture roughness is scale dependent and that this scale dependency can be traced down to scales significantly smaller than the whole fracture. The scale of the specimen has an influence on the probable fracture propagation path and therefore the tensile strength, which leads to different surface roughnesses of the induced tensile fracture. As specimen sizes increase, the tensile strength decreases and the fracture roughness increases. In summary, fractures of different nature and size can be distinguished by surface roughness measures, indicating that fracture origin has significant influence on surface topography. This is especially important, as fracture topography is linked to fracture conductivity and strength. Laboratory tests on granodiorite specimen were performed to investigate the relation of fluid flow rate, injection pressure, confining stress and fracture aperture during testing. Cylindrical specimen were overcored from natural tensile and shear fractures and subjected to a fluid pressure gradient across the fracture to sustain a constant flow rate. The specimens were tested in mated configuration and with shear offset in the fracture between 1 and 6 mm. Additionally, specimen fracture surfaces were scanned before and after testing to study the relationship of fracture transmissivity evolution during testing and surface deformation. Confining stress varied between 1 and 68 MPa for 5 to 10 cycles, yielding changes in transmissivity of up to three orders of magnitude. Shear offset of specimens lead to transmissivity increase of up to three orders of magnitude. Specimens experienced strongly damaged fracture surface and gouge production, which reduced transmissivity up to one order of magnitude for subsequent load cycles. While fracture surface roughness increased during testing, this effect was especially pronounced for specimens with shear offset. Almost all tests show hysteretic behavior during individual load cycles, indicating stress path dependent behavior of transmissivity. The experimental results qualitatively demonstrate and quantify mechanisms commonly encountered in EGS reservoir fractures. To further system understanding and predictive capabilities, a novel numerical model was tied into the GEOS framework to compute fully hydro-mechanically coupled processes in heterogeneous fractures. The model is compared against three experimental test sets investigating cylindrical granodiorite specimens with axial loads between 0.25 and 10 MPa for: (i) dry fracture closure; (ii) contact stress evolution in fractures during normal loading; and (iii) constant fluid flow rate injection into the fracture center. The non-linear behavior or fracture normal closure and fluid injection pressure increase with increasing axial load is replicated by the numerical model, by using the fracture aperture fields obtained from photogrammetry scans as model input. The numerical model captures contact stress evolution with axial load increase and shows a linear increase in contact area with axial load. Study of flow field simulations show an early onset of channeling, for axial loads as low as 2 MPa. Additionally, simulations of a field scale domain (100 m x 100 m x 40 m), with a 100 x 100 m fracture plane are performed. Pre-existing natural fractures were scanned, to use their aperture field to generate a synthetic aperture field for the fracture plane. In the next step, vertical stresses of 8.3 MPa, corresponding to the host rock of the fracture origin at the GTS are applied to the system. This yields the unique aperture field corresponding to the given stress state. Fluid is subsequently injected with constant pressure head into the fracture center with pressures between 0.01 and 8.7 MPa. While heterogeneous flow paths and pressure diffusion can be observed, the model additionally allows to observe heterogeneous fracture opening due to lowered effective normal stresses during injection. Further, the hydro-mechanically coupled analysis of the velocity and pressure field shows a deviation of the pressure distribution from linear diffusion for increased injection pressures, once hydro-mechanical contact between the fluid and the rock mass is established. Fluid pressure induced fracture opening is shown to strongly depend on aperture magnitude before injection and aperture magnitudes of the surrounding fracture region. Thereby, the model captures mechanical and hydraulic behavior of the laboratory tests, while providing unique insights for heterogeneous fracture behavior under compression and high pressure fluid injection. In summary, this work attempts to scrutinize heterogeneous fractures, and especially related hydro-mechanical processes. This is done by investigating possible bias by specimen fracture nature and size selection for testing. Hydro-mechanical processes are studied in experiments, which aim to replicate reservoir conditions, and showcase the impact of specific fractures, stress paths and gouge production. Finally, this work presents an approach to incorporate the observed phenomena in a numerical framework, which is tested against specifically designed laboratory tests. This work combines laboratory scale investigations by employing the framework to perform fully hydro-mechanically coupled simulations of a field scale fracture with heterogeneous aperture distribution, which yields quantitative results of fracture opening during high-pressure injection. The presented work thereby contributes to further understanding of fracture processes, which characterize behavior of Enhanced Geothermal Systems and other subsurface phenomena.

Kittilä, A. Groundwater flow paths in the bedrock fracture zones revealed by using the stable isotopes of oxygen and hydrogen in the Talvivaara mine gypsum pond area, Northeastern Finland, MSc Thesis, University of Helsinki, 68 pp., 2015. [View Abstract]Bedrock fracturing is considerably extensive and distinct in Finland, and the fractures that are open, conductive and interconnected usually control the groundwater flow paths in fractured bedrock. This highlights the importance of knowing the locations and hydraulic connections of water conducting fracture zones particularly in mining areas, because they can transport adverse substances outside the mining area. In this study, it is focused on examining possible hydraulic connections of bedrock groundwater by using the stable isotopes of oxygen (δ18O) and hydrogen (δ2H). The study was carried out in the Talvivaara mining area in Northeastern Finland alongside a project from the Geological Survey of Finland (GTK). After November 2012, when a leakage of acidic, metal-containing waste water occurred in the gypsum ponds, there was an urgent need to study the groundwater transport routes in the bedrock fractures. The aim was to find hydraulic connections between surface water and groundwater, and to study the flow of the groundwater in the fracture zones based on the different isotopic characteristics of waters from different sources and isotopic similarities. Most of the materials used in this study were obtained from the results of the project from the GTK. These materials included geophysical interpretations of the locations and water content of the main fracture zones and the results from the geochemical analyzes. Together with the interpretations of groundwater flow direction based on hydraulic heads these materials formed a frame for this study. The isotope composition of 39 water samples from bedrock wells, shallow wells and surface water was analyzed using cavity ring-down spectroscopy (CRDS) method. The surface waters were clearly distinguished based on their evident evaporation signal, but no significant such a signal was observed in the bedrock and shallow groundwaters. However, similarities between groundwater from different depths of same well were found, in addition to similarities between different wells along same fracture zones. Although the isotopes did not indicate surface water contamination, groundwater contamination with smaller amounts of water is possible, in which case the changes in isotope composition are not yet significant, while certain elements have elevated concentrations. A NE-SW oriented fracture zone passing in the center of the study area was concluded to have the most important role in collecting and transporting groundwater outside the mining area. More detailed interpretations would require regular sampling for a longer period of time to better distinguish naturally and artificially induced changes both in the isotopic but also geochemical compositions. Also the usage of packer tests possibly together with pumping tests would be useful in obtaining more comprehensive image of the groundwater flow in the fracture zones and their hydraulic connections.

Adams, B.M. On the power performance and integration of carbon-dioxide plume geothermal (CPG) electrical energy production., Dissertation, University of Minnesota,, 168 pp., 2015. [View Abstract]CO2 Plume Geothermal (CPG) energy is a method for producing electricity from heat extracted from hot rock layers or reservoirs deep within the earth's crust. CPG is differentiated from other geothermal technologies by several factors: 1) CPG uses CO2 as the primary geologic working fluid instead of brine, 2) CPG utilizes naturally permeable porous reservoirs to extract heat, such as saline aquifers or depleted hydrocarbon reservoirs, 3) CPG is deep--a CPG reservoir must have a depth of 1 km to maintain CO2 in its supercritical state; though depths of 2 to 5 km are more common, and 4) CPG utilizes reservoirs at common geologic temperature gradients, unlike traditional hydrothermal which utilizes shallow reservoirs of unusually high temperature. Thus, CPG is intended to be integrated into an existing CO2 sequestration site affording an economic return on CO2 capture expenses by providing carbon-neutral, dispatchable electricity. Even when CPG is used as a base-load power source, it correlates well with electrical demand, unlike wind and solar (Chapter 5). Typically, CPG configurations consist of one or more injection and production wells. These wells link the surface plant with the porous reservoir to create a fluid circuit. Cooled fluid is injected at the surface, heated within the reservoir, and then returned to the surface at higher temperature and pressure which can then be used to create electricity. The variation in CO2 density between injection and production wells creates a thermosiphon which can drive circulation of CO2 without the use of pumps (Chapter 2). The geologic CO2 can be passed directly through a turbine, called a direct system, or heat can be extracted and used to power an Organic Rankine Cycle, called an indirect system. Either system may be used to generate electricity, although a direct system will nearly always produce more electricity than the indirect system. With reservoirs at moderate depth and temperature, these direct systems will also produce more electricity than comparable brine hydrothermal systems (Chapter 3). The reservoir well spacing and diameter affect the average power and longevity of a CPG system. For every combination of well diameter and reservoir depth, temperature, permeability, and thickness, an optimum spacing between the central injection well and a circumferential collection well will provide the greatest power output over time; placing the collection well too close to the injection well depletes the reservoir too quickly while spacing it too far away increases pressure losses, decreasing the overall power (Chapter 4). Likewise, the selection of too small a well diameter will limit mass flowrate, and thus power, while an oversized well diameter may quickly deplete the reservoir and provide no additional benefit (Chapters 3 & 4). This research has provided a significantly deeper understanding of CPG power systems and their operation. The impact of this work is to establish a basis of CPG research to be used in several ways. It can directly inform industrial developments, such as a green-field implementation of CPG or the long-term planning of a CPG-ready Carbon Capture and Storage site. This work may also be the basis for future economic or policy analyses that can further argue for the development of CPG. Thus, this work will help enable CPG as part of the 21st century energy portfolio.

Samrock, F. Constraints on the source of unrest at the Aluto-Langano geothermal field, Ethiopia, inferred from 3-D interpretation of MT measurements, Dissertation, ETH Zurich, 155 pp., 2015. [Download PDF] [View Abstract]The global energy demand is ever rising and renewable energies are considered to be a major contributor to any future energy mix. A promising candidate is geothermal energy as it is carbon-neutral and readily available in regions that may have no access to con- ventional energy resources. Geothermal power generation is most attractive in volcanic regions with ready access to shallow high enthalpy systems. As for instance in Iceland and New Zealand, where a well established infrastructure allows profitable exploitation of geothermal resources accounting in a large part for the local energy production. One of the privileged regions possessing a remarkable, but so far largely untapped geothermal potential is the East African Rift system (EARS). The EARS is an active continental break-up zone hosting numerous young volcanic systems with most of them concentrated along its eastern branch between Mozambique and Ethiopia. Considerable progress in geothermal exploration along the EARS is so far limited to Kenya and Ethiopia, where first geothermal power plants have been installed during the 90s. Currently several geothermal projects are in progress in these regions and a considerable development of the renewable energy sector is expected in the near future. One plant is under construc- tion at Corbetti volcano in Ethiopia, once completed it is estimated to generate over 1000 MW electric power and hereby meant to be Africa’s largest geothermal power plant ( Reykjavik Geothermal , 2014). Recently the International Renewable Energy Agency (IRENA) presented a strategy to build a Clean Energy Corridor stretching from Ethiopia to South Africa to exploit the excellent renewable energy potential along the EARS focusing on hydro, geothermal, solar and wind power ( IRENA Headquarters , 2013). The aim of this project is to meet the increasing energy demand of the rapidly growing economies in East Africa by mas- sive investment in renewable energy. It is worth noting that the advantage of geothermal sources compared to other renewable sources like wind, solar and hydro power is their in- dependence from weather conditions and their constant output with availability around the clock. The region of interest addressed in this study is the Main Ethiopian Rift System, which encompasses a number of volcanoes that have been identified as potential high enthalpy geothermal systems in the past ( Endeshaw , 1988). Some of them are known to be actively deforming with reoccurring periods of uplift and setting as indicated by satellite observations ( Biggs et al. , 2011). One of the regions where temporal changes take place is the Aluto-Langano volcanic complex. It hosts Ethiopia’s currently only producing geothermal power plant, which taps a geothermal system with fluid temper- atures exceeding 350 ◦ C ( Gianel li and Teklemariam , 1993). The observed periods of uplift at Aluto took place in 2004 and 2008, they affected a region of around 100 km 2 and were followed by periods of subsidence. The power plant is located in the center of the deforming region where the maximum amplitudes of unrest occur. This state of play clearly raises the question of the unrest’s implication on the plant in terms of productivity and geohazard. The working hypothesis is that the causative source for the deformation is either in the hydrothermal reservoir, in a deeper magmatic system or in coupled magmatic-hydrothermal system. The aim of this thesis is to discriminate between the different scenarios and to delin- eate the nature of the deforming source. In order to do this we conducted magnetotel- luric (MT) measurements. This geophysical induction method uses natural occurring time-varying electromagnetic fields to decipher subsurface electrical conductivities and is especially sensitive to high conducting zones, as hydrothermal and magmatic reservoirs usually are ( Mu ̃noz , 2014). Furthermore it easily covers the necessary exploration depth down to approximately 10 km. In the past years MT has been successfully implemented in geothermal research and has proved to be a reliable and cost-efficient method in iden- tifying high enthalpy geothermal systems on the basis of subsurface conductivities. This is supported by recent and ongoing developments of efficient computational numerical methods, which make it capable to interpret and to invert for MT data in a fully 3-D manner. The study addressed in this thesis involved the whole process of organizing and plan- ning a field campaign, including logistics and customs clearance. The field measurements in Ethiopia were conducted together with a team of scientists from Addis Ababa Uni- versity, ETH Zurich, the Geological Survey of Ethiopia and local people from the survey region. In total we installed 46 MT sites covering the extent of the Aluto volcanic complex. The acquired data were processed, modeled and interpreted in context of in- terdisciplinary studies previously conducted at the Aluto volcanic complex and in the Main Ethiopian Rift System. Our recovered 3-D models reveal an electrical resistivity distribution, which is in accord with the conceptual reservoir model of a high enthalpy geothermal system, where a low resistive clay cap overlies the more resistive upflow zone ( Johnston et al. , 1992). Our models provide no evidence for an active magmatic sys- tem, this is why we conclude that the source of unrest is most likely situated within the shallower part of the hydrothermal system. In order to put constraints on possible mechanisms that might trigger the cyclic periods of uplift and setting we studied pub- lications on the analysis of well data and fluids from Aluto that were mainly published in the 90s. These studies consistently report major changes over time in the hydrother- mal regime of the geothermal field and reveal complex water-rock interaction processes taking place in at least the upper 2.6 km of the reservoir as known from well logs (e.g. Gizaw , 1993; Teklemariam et al. , 1996). On the basis of these findings we argue in favor of two different kinematic mechanisms that might trigger the observed unrest: The first mechanism is related to the hydro-mechanical behavior of clay minerals and their ten- dency to swell and shrink when exposed to changes of water saturation and pore water chemistry ( de Siqueira et al. , 1999; Xu et al. , 2006). The second mechanism we refer to is thermoelastic expansion of fractured rock consequent to forced advection of hot fluids ( Bonafede , 1991; Troiano et al. , 2011). All in all it is very likely that fluids act as causal agent driving kinematic mechanism that finally result in the observed ground level oscillations. v Based on geomagnetic transfer functions, which provide information on lateral resis- tivity contrasts we conclude that the dominating occurrence of melt is most likely at lower crustal depths along a N-S elongated off-axis zone of volcanism west of the Main Ethiopian Rift System rather than under the Aluto volcanic complex. This interesting finding is well constraint by previous magnetotelluric and seismic studies ( Whaler and Hautot , 2006; Bastow et al. , 2011; Kim et al. , 2012) and it clearly shows the impor- tance of making a regional MT survey in order to fully understand the thermal regime in the rifting zone. Understanding the plumbing system associated with the volcanoes in this region could also have a major impact on geothermal exploration and on the future deployment of geothermal power plants in Ethiopia. Widespread development of geothermal energy in the rift could meet a major part of the local energy demand resulting in a vast benefit for the Ethiopian nation.

Ahkami, M. Smart waterflooding of petroleum reservoirs: verifying the models with experimental data, MSc Thesis, 129 pp., 2015. [View Abstract]Water-flooding is one of the most well-known methods of Enhanced Oil Recovery (EOR)[42]. So far, it is widely accepted that a certain percentage of oil (known as residual oil) will remain in the porous media regardless of amounts of injected water. This oil is claimed to be trapped by complicated physical mechanisms (Chatzis et al. 1983[15]). Many EOR methods are claimed to be able to free up residual oils and increase the oil recovery. One of the promising methods is the modification of injection brine composition. Its application was discussed by Yildiz and Morrow 1996[48] based on the works of Jadhunadan and Morrow 1995[21]. Tang and Morrow 1999[41] observed an increase in oil recovery in the case of different salinities between injection brine and connate water. Zhang and Morrow 2006[54] performed flooding experiments with mixed-wet cores and reported an increase in both tertiary and secondary mode, however they indicate that the reservoir rocks show better response than outcrops, Zhang et al. 2007[55] water flooded two consolidated reservoir cores with cycles of high salinity forma- tion brine of 29.690 ppm, low salinity brine of 1.479 ppm and two concentrations of sodium chloride. They reported that injection of low salinity brine increases both secondary and tertiary recovery, the presence of divalent ions also increases the recovery. The effect of water composition on the oil recovery was also investi- gated within the near well-bore region of a reservoir by Web et al. 2004[46], they injected 10-15 pore volumes of high salinity brine into the ’volume of interest’ to obtain residual oil saturation, which was followed by the injection of diluted water. The log measurement of saturation showed 25 - 50% reduction in residual oil saturation and certifies the experimental investigations. Yousef et al. 2011[49] also certified the previous experimental observations and reported the effect of water salinity and ion composition on oil recovery. They tagged it as “Smart Wa- terFlood” which will be used in this work. Smart WaterFlooding EOR will be mentioned as Sw-EOR in this work. The authors reported a change in pressure drop and also a change in effluent’s pH during the Sw-EOR, they also mentioned rock properties, oil properties, ion composition, divalent ions concentration, pres- ence of clay contents and mobile particles, initial wettability, and temperature as the affecting factors on Sw-EOR. The mechanism or mechanisms of the Sw-EOR is not fully understood and have brought a discussion in the literature. Several authors have proposed different chemical and physical mechanisms but none of them have been globally accepted. These contradictions can be due to the varia- tion in experimental materials and procedures as the oil production is affected by complicated chemical and physical interactions of rock/connate water/injecting water/oil. The proposed mechanisms and experimental investigation of Sw-EOR are dis- cussed in chapter 2, it will be discussed in section 2.1.8 and section 2.2.5 that the suggested mechanism can be divided in two main categories, where the former ex- plains the wettability alteration and the later emphasizes the importance of other mechanisms such as increase in sweep efficiency by fine formation and precipita- tion. One upcoming question is whether they have equal weight on the increase in oil recovery or one of the mechanisms overweights the other one. In addition as will be discussed in chapter 2, experimental observations showed an increase in pressure difference and a decrease in residual oil saturation; the former is be- lieved to be an effect of fine formation and the later is explained by wettability alteration of the rock. In this manner, this study is to develop a numerical model- ing of the Sw-EOR with arbitrary number of reactions to represent the mentioned mechanisms. The governing equations and the implicit approach of solving equations is described in chapter 3. Further it is illustrated that one reaction is dedicated to the formation of particles and rock dissolution where ions in the injecting water and formation water participate in a reaction to form particles. Then produced particles precipitate and in this way modify the local porosity of the system. This affects the absolute permeability of the system as well. The reaction is a two way reaction so either precipitation or dissolution can happen in the system based on reaction rates and reactant concentrations. The second reaction is dedicated to wettability alteration, however the numerical modeling of wettability is not easily possible because of its complex concept and algorithm. Wettability alteration is modeled through the decrease in residual oil saturation. The reaction is assumed to happen between an ion in the injecting water and a mineral on the rock surface. Its product changes the rock wettability by decreasing the residual oil saturation. The modification of residual oil saturation results in the change of relative permeability of oil and relative permeability of water as well. Numerical solution of different scenarios are illustrated in chapter 4. Oil re- covery and other determining factors such as saturation and pressure are illustrated to compare the different waterflooding scenarios. The normal water flooding with- out any reaction is brought in section 4.1 while wettability alteration and fine formation are investigated in section 4.2 and section 4.3 respectively. Formation of particles are also divided into three scenarios where injection water compo- sition and reaction rates vary from each other, detailed explanation of each part is brought in section 4.3. Finally a cyclic waterflooding experiment is simulated where five pore volumes of water injection with wettability alteration came after five pore volumes of water injection without any reaction. Numerical results show the potential of wettability alteration to increase oil recovery both in secondary mode and tertiary mode. However its effect on pres- sure gradient is not completely correlated with the wettability modification rate and increase in oil recovery. On the other hand, formation and precipitation in a homogeneous medium such as the one in this study does not contribute to oil recovery. While it increases pressure gradient and this increase is correlated with precipitation rate and amount.

Ezekiel, J.C. Screening of Gas Injection Techniques for IOR in Low Permeability Reservoirs: Case Study of Q131 block in Liaohe, MSc Thesis, China University of Petroleum, 177 pp., 2014. [View Abstract]The targeted oil reservoir (Q131 block) is in Liaohe oilfield, Northeast of China, which is featured with light oil, very low permeability and sensitive to formation water, but with thick oil layers and large dip angle. The original reservoir pressure is of 33.4 MPa, reservoir temperature of 98-112 °C, and the OOIP is 2.53x106 m3. The viscosity of crude at reservoir condition is of 0.5mPa.s. Low primary production and failed water flooding experience have shown that the sweep efficiency and hence oil recovery factor in the block is very low due to lack of reservoir energy and poor water injectivity as a result of the low permeability and high heterogeneity of the reservoir block. Gravity Stabilized gas injection method has been optioned as an alternative tertiary improved oil recovery (IOR) technique to increase and/or maintain reservoir pressure, increase sweeping and displacement efficiencies to improve oil recovery. For this purpose, three different gas injection techniques, namely Air injection, Carbon dioxide (CO2) injection and Nitrogen (N2) injection are carefully selected in this study to be investigated and screened, as possible IOR methods for application in the low permeable Q131 light oil block. Real-time field data from this oil block were reviewed and summarized to evaluate feasibility of gas injection IOR technologies. The feasibility study of using these 3 gas injection methods for IOR was combined with literature review, theoretical analysis, knowledge of mechanisms of gas injection techniques, field application experiences, laboratory experimental studies, PVT analysis, history matching of the field’s production data, building a generic reservoir static model and reservoir numerical simulation of the 3 different gas injection techniques using the compositional CMG-STARS simulation software. Further work is carried out on screening the corresponding effects of the injected gases on increasing reservoir pressure, improving oil recovery, project economics, and their associated potential safety, corrosion and risk factors For the air injection process, kinetic models of low temperature oxidation (LTO) reactions for the air injection process were designed through laboratory experiments and used in the reservoir simulation study. The displacement experiments of air flooding indicate that air injection can achieve iii relatively higher oil recovery than water injection in the block and the oil recovery increased proportionally with increasing dip of the reservoir. The LTO experimental results over a temperature range of 130-170°C of a typical light oil indicate that LTO reaction is effective to completely consume O2 and have a high CO2 conversion rate of about 58%. The results of the reservoir numerical simulation study show that the cumulative produced oil increases as the gas injection rate increases, up to an optimum level and no oxygen breakthrough was observed at the production wells. For a 30 years period of injection using a base case 30000m3/day of gas injection rate (4 injectors), an incremental oil recovery factors of 35%, 36.5%, 35.5% of OOIP were achieved in air, CO2 and N2 injection simulations respectively, with CO2 having the best production performance. Early gas break-through and high GOR in producers are important factors to influence the gas injection performance. Sensitivity study indicates that shut-in of wells with high GOR can effectively reduce gas production, and a better reservoir performance of incremental oil recovery factors of about 33.5%, 36%, and 34% OOIP can be achieved for the respective gas injections in just an average period of 20 years. This is very favorable to the project economics. Preliminary economic and safety screening analyses, also confirmed that the 3 injection techniques are feasible and they all are profitable, i.e. positive net profit value, with air injection proving to be the most attractive of the three methods in terms of net profit due to its low cost of operation. Safety operations and corrosion controls of the different gas injection techniques are discussed in details and some recommendations are also provided for better IOR process in low permeability oil reservoirs.

Leal, A.M.M. Computational methods for geochemical modelling: Applications to carbon dioxide sequestration, Dissertation, Imperial College London, 152 pp., 2014. [View Abstract]Geochemical modelling is fundamental for solving many environmental problems, and specially useful for modelling carbon storage into deep saline aquifers. This is because the injected greenhouse gas perturbs the reservoir, causing the subsurface fluid to become acidic, and consequently increasing its reactivity with the formation rock. Assessment of the long term fate of carbon dioxide, therefore, requires accurate calculations of the geochemical processes that occur underground. For this, it is important to take into account the major water-gas-rock effects that play important roles during the gas storage and migration. These reactive processes can in general be formulated in terms of chemical equilibrium or chemical kinetics models. This work proposes novel numerical methods for the solution of multiphase chemical equilibrium and kinetics problems. Instead of adapting or improving traditional algorithms in the geochemical modelling literature, this work adopts an approach of abstracting the underlying mathematics from the chemical problems, and investigating suitable, modern and efficient methods for them in the mathematical literature. This is the case, for example, of the adaptation of an interior-point minimisation algorithm for the calculation of chemical equilibrium, in which the Gibbs energy of the system is minimised. The methods were developed for integration into reactive transport simulators, requiring them to be accurate, robust and efficient. These features are demonstrated in the manuscript. All the methods developed were applied to problems relevant to carbon sequestration in saline aquifers. Their accuracy was assessed by comparing, for example, calculations of pH and CO2 solubility in brines against recent experimental data. Kinetic modelling of carbon dioxide injection into carbonate and sandstone saline aquifers was performed to demonstrate the importance of accounting for the water-gas-rock effects when simulating carbon dioxide sequestration. The results demonstrated that carbonate rocks, for example, increase the potential of the subsurface fluid to dissolve even more mobile CO2. Computational methods for geochemical modelling: Applications to carbon dioxide sequestration (PDF Download Available). Available from: [accessed May 30, 2017].

Vogler, D. A comparison of different model reduction techniques for model calibration and risk assessment, MSc Thesis, University of Stuttgart, 62 pp., 2013. [View Abstract]Many engineering systems represent challenging classes of complex dynamic systems. Lacking information about their systems properties leads to model uncertainties up to a level where quantification of uncertainties may become the dominant question in modeling, simulation andapplication tasks. Uncertainty quantification is the prerequisite for probabilistic risk assessment and related tasks. The current work will present recent approaches for these challenges based on response surface techniques, which reduce massively the initial complex model. The reduction is achieved by a regression-like analysis of model output with orthonormal polynomials that depend on the model input parameters. This way, the model response to changes in uncertain parameters, design or control variables is represented by polynomials for each model prediction of interest. This technique is known as polynomial chaos expansion (PCE) in the field of stochastic PDE solutions. The reduced model represented by the response surface is vastly faster than the original complex one, and thus provides a promising starting point for follow-up tasks: uncertainty quantification, model calibration and probabilistic risk assessment. Obviously, a response surface can be constructed in different ways. Methods for constructing the response surface can demand only a minimum number of model evaluations, but as well may ask for many model evaluations to achieve a better quality of the involved projection integrals. The scope of the current work is to test and compare different integration rules, i.e., methods to choose the sets of parameter values for which the model has to be evaluated. To test and compare the different methods, their accuracy in uncertainty quantification, model calibration and risk assessment will be measured against brute-force reference computations based on the original model. As illustrative example, we consider a study from the field of CO2 storage in the subsurface.

Ma, J. Researches on the Migration of Supercritical CO2 on Geological Storage Conditions , MSc Thesis, Tsinghua University, 86 pp., 2013. [View Abstract]In order to mitigate the global warming, development of the technologies for CO2 storage is very necessary. CO2 storage in geological formations especially in deep saline aquifers is considered a promising way. As an important basis, the mechanisms of supercritical CO2 and water two phase flow of in porous media is not yet fully developed. Therefore, to have a better understanding of CO2 migration aquifers, this thesis investigates characteristic functions of multiphase fluid flow migration and the influences of formation heterogeneity and dissolution conditions by using experimental and numerical methods. As important functions of describing multiphase displacement processes in porous media, relative permeability and capillary pressure curves from different core samples are obtained, which provide essential parameters for numerical modeling. This paper also successfully extended relative permeability curves by analyzing capillary pressure experimental data. A 1D modeling approach using multiphase transport code TOUGH2 proposes several set of parameters allowing a good match between experiments and models. Sensitivities of ‘end effect’, capillary pressure, permeability, residual gas and water are analyzed using the same model. A series of experiments are performed to study the influence of the CO2 exsolution, calcite dissolution and precipitation. It shows the effect on permeability due to CO2 exsolution triggered by pressure drop is predictable. Calcite precipitates in different forms depending on chemical conditions, which has a much obvious influence on low permeability rocks than high permeability ones by blocking the pores and/or throats.

Vogler, D. Investigation of transport phenomena in a highly heterogeneous porous medium, MSc Thesis, Oregon State University, 70 pp., 2012. [View Abstract]This work focuses on solute mass transport in a highly heterogeneous two-region porous medium consisting of spherical low-hydraulic conductivity inclusions, embedded in a high-hydraulic conductivity matrix. The transport processes occuring in the system are described by three distinct time scales. The first time scale reflects the characteristic time for convective transport in the high-conductivity matrix. The second time scale reflects the characteristic time for diffusive transport in the low-conductivity inclusions. The third time scale reflects the characteristic time for convection within the inclusions. Two Péclet numbers can be defined that compare the time scales and provide qualitative insight into the net transport behavior in two-region media. To model this system, four different representations were developed: (1) a Darcy-scale model that involved direct microscale computation over the entire domain of the experimental system, (2) a direct microscale simulation computed on a simplified domain that had similar geometric parameters (e.g. volume fraction of inclusions) as the complete domain for the experimental system, (3) a volume averaged model (after Chastanet and Wood [2008]) which uses a constant mass transfer coefficient and (4) a volume averaged model which employs a time-dependent mass transfer coefficient. Two different experimental conditions were investigated: a high flow rate, and a low flow rate. Detailed understanding of the experimental system was developed, which led to accurate prediction of the system’s behavior for the higher flow rate. Accurate early time fit of the data was achieved for the experiment with the lower flow rate, while late time behavior between the models and experimental data diverged. Further investigations of the experimental system were conducted to examine possible sources of errors that could lead to an inaccurate description of the system’s properties. Additional mixing within the system, inhomogeneous distribution of the effective diffusion coefficient and imprecise initial estimates of the hydraulic parameters are all possible explanations for the inaccurate model representation of the system’s behavior for the lower flow rate case.

Schaedle, P. Rechenzeitoptimierung bei numerischen Sicherheitsabschätzungen für Atommüll-Endlager, MSc Thesis, University of Stuttgart, 82 pp., 2012. [View Abstract]Seit Mitte der 50er Jahre werden Atomkraftwerke gebaut und somit auch Abfälle produziert, die grosse Schwierigkeiten in der Handhabung und Lagerung mit sich bringen. Das grösste Problem liegt nicht in der Menge der Abfälle, sondern in der langanhaltenden schädlichen Strahlendosis die von den Abfällen abgegeben wird. Die momentan wissenschaftlich und wirtschaftlich vorherrschende Meinung ist, den Müll in geologischen Tiefenlagern zu deponieren. In der Vergangenheit wurden bereits unterschiedliche Gesteinsformationen als Endlagerstätten untersucht, getestet und auch eingesetzt. Viele haben sich jedoch als ungeeignet für die Deponierung atomaren Abfalls herausgestellt. Derzeit werden in Frankreich und der Schweiz Tonformationen als potenzielle Endlagerstandorte untersucht. Die geringe Durchlässigkeit und gleichermassen die über einen langen Zeitraum anhaltende Verschlusswirkung in Bezug auf Druck und Temperatur zeichnen eine Tonformation gegenüber Salz- oder Kristallinformationen aus. Zur Beurteilung eines potenziellen Endlagerstandorts werden geologische Erkundungen und Studien durchgeführt. Nach der ersten Erkundungsphase gilt es den Standort genauer zu untersuchen und das Gestein hinsichtlich seines Verhaltens während der Einlagerung zu verstehen. Um letztendlich eine Aussage über die Leistungsfähigkeit und Sicherheit eines potenziellen Endlagers machen zu können, sind auf Grundlage der zuvor gewonnenen Erkenntnisse umfangreiche numerische Simulationen nötig. Im Rahmen dieser Simulationen müssen unter anderem in der näheren Umgebung der Einlagerungsstollen hochaufgelöste Detailmodelle erstellt werden. Diese Modelle stellen die komplexen physikalischen Prozesse dar, die während der Einlagerung und nach dem Verschluss des Stollens ablaufen. Um möglichst viele Erkenntnisse über eventuelle Ereignisse oder Parameterunsicherheiten zu sammeln, müssen zusätzlich zu diesen Detailmodellen, deterministische und probabilistische Sicherheitsanalysen durchgeführt werden. Diese Arbeit wird bei der AF-Consult Switzerland AG im Rahmen eines durch die französische ANDRA (Agence nationale pour la gestion des déchets radioactifs – Französische nationale Agentur für die Entsorgung radioaktiver Abfälle) beauftragten Projekts durchgeführt. Dieses befasst sich mit der Optimierung und der effektiveren Umsetzung der Simulationen zur Berechnung der Radionuklidausbreitung. Durch die Projektvorgaben der ANDRA wird sich diese Arbeit an dem Endlagerkonzept der ANDRA orientieren. Die erarbeiteten numerischen Methoden sind aber gleichermassen auf andere Konzepte und Aufgabenstellungen anwendbar. Ein im Zusammenhang mit dieser Arbeit verfasster Konferenzbeitrag für das „TOUGH Symposium 2012“ wurde mit dem „Karsten Pruess Student Paper Award“ ausgezeichnet.

Kong, X.-Z. Experimental investigation of air injection in saturated unconsolidated porous media, Dissertation, ETH Zurich, 128 pp., 2010. [Download PDF] [View Abstract]The work described in this thesis is primarily concerned with the construction and study of laboratory scale models for the process of air injection into liquid-saturated grain packing. Experiments, both in two-dimensional (2D) and three-dimensional (3D) setups, were carried out using water-saturated packings of glass beads and/or packings of crashed fused silica glass grains saturated with a glycerin-water solution. High resolution digital images of the invasion patterns were recorded and analyzed. During air injection into a vertically-placed 2D glass bead packing saturated with water, three stages were identified, a tree-like pattern, a fluidized pattern, and a migrating single-channel pattern. The expansion of the tree-like pattern behaves in a diffusion-like manner as the air branches advance upward randomly, and finally reach a more or less constant width. The starting position of the fluidized pattern was quantitatively estimated via balancing the pressure forces between the effective stress due to the weight of the grains and the pressure resistance on the displaced fluid combined with the capillary pressure. Four dynamic regimes were distinguished: regime (i) where the fluidization stops somewhere between the top of the packing and the injection orifice, a transition regime (ii), regime (iii) where the fluidization reaches the injection orifice, and regime (iv) where the deformation of the packing appears as soon as the air is injected. A critical injection rate \(Q_{f}\) is defined to identify the transition regime. The value of \(Q_{f}\) can be determined via \(Q_{a}\), where \(Q_{a}\) is calculated as averaged flux per channel. The regime (iv) is characterized by a characteristic injection rate \(Q_{c}\), which is estimated by balancing the pressure gradient of the air flow and the overburden pressure gradient of the medium. The phenomenon of the migrating channel is measured quantitatively in two parts, before and after breakthrough. Before breakthrough, the characteristic measurements concern maximum vertical advance, maximum horizontal advance, air volumetric fraction, ratio of total surface area to volume, specific surface area of the air phase, and box-counting dimension. After breakthrough, the characteristic measurements focus on mean horizontal position of air channel, horizontal shifting distance, lateral movement distance, and lateral movement width. Before breakthrough, the maximum vertical height of the air structure approximately advances linearly with time. The maximum horizontal advance reaches a maximum value and then levels off for the rest of the time. Air volumetric fraction decreases monotonically with time, and finally levels off asymptotically to an approximate constant. In all cases, the air volumetric fraction for packings of small grains is larger than that for packings of large grains. The ratio of total surface area to volume varies in time similarly to the air volumetric fraction. However, the ratio of total surface area to volume can clearly be grouped according to the grain size, which is also true for the specific surface area of the air phase. Both can be scaled with the Bond number with a power of -0.5. After breakthrough, the migration process is studied by analyzing the mean horizontal position, horizontal shifting distance, lateral movement distance, and lateral movement width of the air channel. The results indicate that over 99% of the horizontal shifting distance is less than 10 mm. Furthermore, the its probability density function indicates that the air channel oscillates more frequently in the packing of small grains than in the packing of large grains. The interaction of the air flow with the grains and the liquid leads to a mobilization of the grains, in which air channels migrate and grain clusters undergo shearing. The channel migration comes to a stop after some time, leaving one thin and stable preferential channel for air flow. Assuming Hagen-Poiseuille’s formula to be applicable, the size of the preferential channel should exceed a lower threshold \(D_{ch}\) so that a mechanical equilibrium at the channel interface is maintained, but it should stay below an upper threshold \(D_{max}\) so that a stable air channel is sustained. A rearrangement of the grains is observed which is caused by a pulsation effect. It induces a compaction process, in which the individual grains are disassembled from the region of non-zero shear rate and then reassembled into the compacted clusters of the region of zero shear rate. It also induces a size segregation process, in which smaller grains move into the spaces beneath larger grains. By using high-speed image acquisition through laser scanning, the 3D dynamic air plume is recorded by sequential tomographic imaging. Due to the overlap between adjacent laser sheets and the light reflection, air bubbles are multiply exposed in the imaging along the scanning direction. A “curvature” method, based on a threshold on the curvature of grey-value in scanning direction, is proposed to remove the redundant pixels. The respective results are discussed by comparing the reconstructed air plume volume with the injected one and by evaluating the morphological consistency of the obtained air plume. The reconstructed air plume is further investigated with respect to its growth characteristics, such as breakthrough, air volume fraction, and air channel migration.

Leal, A.M.M. Four phase equilibrium and partitioning calculations for sequestration of carbon dioxide and hydrogen sulfide in deep saline aquifers, MSc Thesis, University of Wyoming, 213 pp., 2010. [View Abstract]A fully compositional numerical simulator for sequestration of CO2 and H2S in deep saline aquifers requires a large number of flash equilibrium calculations at each time step. Therefore, a robust and efficient flash solver is needed in order to resolve the thermodynamic equilibrium of systems composed of brine, CO2, and H2S, i.e., determine the equilibrium phases that emerge in the system, resolve the partitioning of the species among them, and calculate the phase molar fractions. In this work, we develop two fast and accurate flash equilibrium methods, the GeometricFlash and the ProgressiveFlash, for Brine-CO2 and Brine-CO2-H 2S systems. The temperature and pressure ranges are 12-100°C and 1-600 bar for Brine-CO2 systems and 12-100°C and 1-200 bar for Brine-CO2-H2S systems. In this study brine is a solution of NaCl in water. A serial and GPU-based parallel implementations of the above-mentioned flash equilibrium methods are developed for the computation of hundreds of thousands of flash equilibrium calculations. A performance analysis of the computations shows that the progressive flash is faster than the geometric one when executed in serial (on CPUs), while the geometric flash is slightly faster when solved on GPUs. Average speedups of 180 and 300 were achieved with the use of GPUs for the progressive and geometric flash method, respectively. Four phase equilibrium and partitioning calculations for sequestration of carbon dioxide and hydrogen sulfide in deep saline aquifers (PDF Download Available). Available from: [accessed May 30, 2017].

Samrock, F. Elektrisch hochleitfähige makroskopische Strukturen - ein alternatives Modell zur Erklärung scheinbarer Mantelanisotropie unter der känozoischen Vulkanprovinz Deutschlands, MSc Thesis, Georg-August-Universität zu Göttingen, 163 pp., 2010. [View Abstract]Die känozoische Vulkanprovinz Deutschlands ist eine Region, die während des Känozoi- kums im Tertiär bis hinein ins Quartär Schauplatz aktiven Vulkanismuses war. Die hierbei entstandenen Vulkane erstrecken sich über ca. 300 km entlang einer Reihe von der Eifel im Westen Deutschlands über den Vogelsberg, die Rhön bis zur Heldburger Gangschar in Teilen Thüringens und Bayerns (Wedepohl und Baumann, 1999). Der Vogelsberg in Hessen stellt mit rund 2500 km2 das größte zusammenhängende Vulkangebiet Mitteleuropas dar (Walter, 1995). Entsprechend ihrer interessanten geologischen Vergangenheit ist die känozoische Vulkan- provinz, die neben ihrer vulkanischen Aktivität von einer sich über gesamt Europa er- streckenden Riftstruktur durchkreuzt wird (Ziegler, 1992), langwährender Untersuchungs- gegenstand geophysikalischer Forschung mit verschiedensten Explorationsmethoden. Ein Schwerpunkt liegt hierbei in den Methoden der geophysikalischen Tiefenforschung, die es erlauben Aussagen über die Struktur und die Dynamik des Mantels zu treffen. Seismo- logische Messungen konzentrieren sich auf die Region des Rheinischen Schildes. Mit der Durchführung des großangelegten Eifel Plume Projekts in den Jahren 1997 - 1998 erhoffte man sich anhand seismologischer Messungen klärende Antworten auf die kontrovers dis- kutierte Plumehypothese zu finden. Zwar konnten unter der Eifel seismische low-velocity Anomalien nachgewiesen werden (Ritter u. a., 2001; Keyser, Ritter und Jordan, 2002), eine Klärung der Plumehypothese steht jedoch weiter aus. Die Hypothese an sich stößt vieler- orts auf Ablehnung (Meyer und Foulger, 2007). Die Analyse der Aufspaltung von Scherwellen (SKS-Scherwellen-Splitting ) ergab Hinweise auf eine seismische Anisotropie unter dem Rheinischen Schild. Eine Tiefenauflösung, mit der die Quellregion der Anisotropie bestimmt werden könnte, ist mit dieser Methode nicht möglich (Savage, 1999). Mit hoher Wahrscheinlichkeit liegt sie jedoch im oberen Mantel, da die Kruste aufgrund ihrer geringen Mächtigkeit zu keiner signifikanten Aufspaltung von Scherwellen führt (Walker u. a., 2005). Als Ursache für die seismische Anisotropie gel- ten Olivinkristalle, die aufgrund von durch den Mantelfluss induzierten Spannungsfeldern ausgerichtet werden (Zhang und Karato, 1995). Die Olivinkristalle sind bezüglich der Lauf- zeiten seismischer Wellen entlang ihrer kristallographischen Achsen anisotrop (Kumazawa und Anderson, 1969). Olivin stellt mit ca. 70% den mineralogischen Hauptbestandteil des Mantels dar. Neben seismologischen Untersuchungen war und ist die känozoische Vulkanprovinz Unter- suchungsgegenstand der elektromagnetischen Tiefenforschung. Hinweise auf die Existenz eines Eifelplumes konnte aber auch diese bisher nicht erbringen (z.B. Kuras, 1998). Jedoch konnte mit Hilfe der Magnetotellurik im gesamten Gebiet der känozoischen Vulkanprovinz eine teils tiefenabhängige Anisotropie der elektrischen Leitfähigkeit σ festgestellt werden (Hönig, 1998; Bahr u. a., 2000; Leibecker u. a., 2002; Gatzemeier und Moorkamp, 2005, u.a.). Eine Eigenschaft, die die Magnetotellurik auszeichnet, ist ihre genauere Tiefenauf- lösung, die auf den periodenabhängigen Eindringtiefen der magnetischen und elektrischen Feldvariationen beruht. Die tiefenabhängige Anisotropie untergliedert sich in zwei Berei- che - die Kruste und den Mantel. Generell ist die elektromagnetische Streichrichtung, d.h. die Richtung der hohen Leitfähigkeit, in Kruste und Mantel nicht identisch. Die elek- tromagnetische Streichrichtung in der Kruste orientiert sich vornehmlich an geologischen Großstrukturen, wie den Terrangrenzen. Als verantwortlicher Leitfähigkeitsmechanismus kommen hier in erster Linie vernetzte leitfähige Phasen, wie salinare Fluide oder Graphit, in Frage. Sie konzentrieren sich in krustalen Kluft- und Risssystemen, die sich entlang einer durch die tektonische Spannung vorgegebenen Vorzugsrichtung ausbilden. Das Hauptaugenmerk dieser Arbeit liegt auf der Struktur und der Dynamik des oberen Mantels. Dessen elektromagnetische Streichrichtung liegt unter der känozoischen Vulkan- provinz mit großer Konsistenz in Ost-West-Richtung entlang der Aufreihung der vulka- nischen Gebiete (Gatzemeier, 2001). Nach Norden hin ist eine Änderung der elektroma- gnetischen Streichrichtung auf Nord-Süd zu beobachten, während der Anisotropiefaktor im Süden Deutschlands deutlich schwächer wird (Moorkamp, 2003). Der Anisotropiefaktor ist das Verhältnis der elektrischen Leitfähigkeit σI in Streichrichtung und der elektrischen Leitfähigkeit σ⊥ senkrecht zur Streichrichtung. Die elektromagnetische Anisotropie im obe- ren Mantel wurde bisher hauptsächlich mit der Diffusion von Wasserstoffionen H+ in Olivin erklärt (Bahr u. a., 2000; Gatzemeier, 2001; Gatzemeier und Moorkamp, 2005, u.a.). Ähn- lich wie die seismische Anisotropie in Olivin ist dessen auf der Diffusion von H+-Ionen beruhende elektrische Leitfähigkeit bezüglich seiner kristallographischen Achsen anisotrop (Karato, 1990). Ferner stimmt die Richtung der hohen Leitfähigkeit mit der Richtung der hohen seismischen Geschwindigkeit überein. So ist die weltweit vielfach beobachtete Über- einstimmung von seismischer und elektrischer Anisotropie (Simpson, 2001; Gatzemeier und Moorkamp, 2005; Walker u. a., 2005) anhand einer gemeinsamen Grundlage, nämlich der Ausrichtung von Olivin, erklärbar. Jedoch gibt es hierfür auch Gegenbeispiele: Hamilton, Jones, Evans u. a. (2006) beobachteten keine Übereinstimmung von seismischer und elektri- scher Anisotropie in Südafrika. Sie schlossen daraus, dass die für die seismische Anisotropie verantwortliche Region entweder in größeren Tiefen liegt oder dass die hierfür verantwortli- chen Mechanismen keine signifikanten elektrischen Eigenschaften aufweisen. Die Forschung auf diesem Gebiet ist also längst nicht abgeschlossen. Jüngste, erste direkte Labormessun- gen der Leitfähigkeit von Olivin brachten sogar völlig gegensätzliche Ergebnisse zutage (Wang u. a., 2006; Yoshino u. a., 2006). Ungeachtet dessen liegt die elektrische Anisotropie im oberen Mantel unter der känozo- ischen Vulkanprovinz mit einem Anisotropiefaktor von A = σI/σ⊥ > 100 in einem Be- reich, der mit der H+-Diffusion in Olivin nicht erklärbar ist. Vernetzte partielle silikatische Schmelzen entfallen als alternative Erklärung. Sie besitzen zwar eine höhere Leitfähigkeit, ihr notwendiger Anteil von 10% im oberen Mantel kommt aus Gründen der Stabilität je- doch nicht in Frage. Jüngste Forschungen an karbonatischen Schmelzen ergaben, dass deren Leitfähigkeit drei Größenordnungen über der silikatischer Schmelzen liegt (Gaillard u. a., 2008a). Damit genü- gen bereits geringe Mengen, um hohe Leitfähigkeiten zu erzeugen. Aufgrund der extremen Seltenheit ihrer Erstarrungsgesteine wurden karbonatische Schmelzen zur Erklärung von Leitfähigkeitsanomalien im Mantel bisher meist nicht berücksichtigt. Die Anisotropie der Leitfähigkeit muss prinzipiell nicht von einem intrinsisch anisotropen homogenen Mantel herrühren, sondern kann auch durch makroskopische laterale Leitfähig- keitskontraste verursacht sein. Es läge dann ein heterogener Mantel vor. Die Unterschei- dung zwischen einer „echten“ Anisotropie (homogener Mantel) und einer „scheinbaren“ Anisotropie (heterogener Mantel) geschieht mittels der Methode der geomagnetischen Tie- fensondierung (Schmucker, 1970), mit der laterale Leitfähigkeitskontraste aufgedeckt wer- den können. In dieser Arbeit wird ein 3D-Modell vorgestellt, das eine sehr gute Datenanpassung auf- weist und völlig auf das Eingliedern anisotroper Schichten verzichtet. Stattdessen wird ein heterogener Mantel postuliert. Es wird gezeigt, dass die elektrische Anisotropie in einer hochleitfähigen makroskopischen Struktur im Mantel begründet sein kann, die mit der Methode der geomagnetischen Tiefensondierung nicht aufgelöst wird. Die hochleitfähige Struktur wird durch das Vorhandensein karbonatischer Schmelzen unter den Vulkanen der känozoischen Vulkanprovinz erklärt. Die Existenz der karbonatischen Schmelzen wird ab- schließend auf der Grundlage geochemischer Analysen von Magmen diskutiert. Ferner wird gezeigt, dass die bisher oft vernachlässigten hochleitfähigen Sedimente im Norden Deutsch- lands einen erheblichen Effekt haben und eine wichtigen Beitrag zur Erklärung der Daten leisten.

Ebigbo, A. Modelling of biofilm growth and its influence on CO2 and water (two-phase) flow in porous media, Dissertation, University of Stuttgart, 131 pp., 2009. [Download PDF] [View Abstract]Bacterial biofilms are groups of microbial cells attached to surfaces and to each other. Cells in a biofilm are protected from adverse external conditions. In natural environments, this attached mode of growth is more successful than the suspended mode, and a major portion of microbial activity takes place at surfaces. In porous media, biofilms are used as bioreactors (e.g, in wastewater treatment) and as biobarriers (e.g., in enhanced oil recovery). They are also used in the containment and degradation of contaminants in groundwater aquifers. It has been proposed that biofilms be used as biobarriers for the mitigation of carbon dioxide (CO2) leakage from a geological storage reservoir. The concentration of greenhouse gases -- particularly carbon dioxide (CO2) -- in the atmosphere has been on the rise in the past decades. One of the methods which have been proposed to help reduce anthropogenic CO2 emissions is the capture of CO2 from large, stationary point sources and storage in deep geological formations. The caprock is an impermeable geological layer which prevents the leakage of stored CO2, and its integrity is of utmost importance for storage security. As mentioned above, biofilms could be used as biobarriers which help prevent the leakage of CO2 through the caprock in injection well vicinity. Due to the high pressure build-up during injection, the caprock in the vicinity of the well is particularly at risk of fracturing. The biofilm could also protect well cement from corrosion by CO2-rich brine. The goal of this work is to develop and test a numerical model which is capable of simulating the development of a biofilm in a CO2 storage reservoir. This involves the description of the growth of the biofilm, flow and transport in the geological formation, and the interaction between the biofilm and the flow processes. Important processes which are accounted for in the model include the effect of biofilm growth on the permeability of the formation, the hazardous effect of supercritical CO2 on suspended and attached bacteria, attachment and detachment of biomass, and two-phase fluid flow processes. The partial differential equations which describe the system are discretised in space with a vertex-centered finite volume method, and an implicit Euler scheme is used for time discretisation. The model is tested by comparing simulation results to experimental data. In a test case simulation, the model predicts the extent of biomass accumulation near an injection well and its effect on the permeability of the formation. The simulations show that the biobarrier is only effective for a limited amount of time. Regular injection of nutrients would be necessary to sustain the biofilm. In future work, the model could be extended to account for the active precipitation of minerals by the biofilm which would lead to a more enduring barrier. The model also needs to be extended to account for more than one growth-limiting factor. This would allow for the simulation of injection strategies which aim at growing a biofilm at some distance from the injection well. Biofilme, die in einem porösen Medium wachsen, blockieren Poren und verändern dabei die Eigenschaften des porösen Mediums. Diese veränderten Eigenschaften werden bei der biologischen Filtration (z. B. bei der Abwasserbehandlung), bei der biologischen Altlastensanierung (z. B. für die Erstellung hydraulischer Barrieren) und bei anderen Fragestellungen auf diesem Gebiet genutzt. Eine hydraulische Barriere biologischen Ursprungs könnte z. B. auch in einer geologischen Kohlendioxid-Lagerstätte eingesetzt werden, um das Entweichen von CO2 zu verhindern. CO2 ist das derzeit für am Wichtigsten erachtete anthropogene Treibhausgas. Die globale Erderwärmung wird demnach sehr stark durch die in den letzten Jahrzehnten stattfindende Anreicherung von anthropogenen Treibhausgasen in der Atmosphäre mitverursacht. Die Freisetzung von CO2 kann mit Hilfe effizienterer Technologien und alternativer Energiequellen reduziert werden. CO2-Emissionen können aber auch reduziert werden, indem man CO2 aus Kraftwerksabgasen abscheidet und in tiefen geologischen Formationen speichert. Bei den physikalischen Bedingungen, die in diesen unterirdischen Lagerstätten herrschen, liegt CO2 im überkritischen Zustand vor, gekennzeichnet durch eine hohe Dichte und geringe Viskosität. Diese Lagerstätten enthalten oft salzhaltiges Wasser, das dichter ist als CO2. Eine möglichst undurchlässige geologische Deckschicht verhindert das Aufsteigen des leichteren CO2 an die Erdoberfläche. Jedoch müssen, z. B. im Rahmen von Risikostudien, mögliche Störungen oder Risse in dieser Deckschicht betrachtet werden, die zu einem Entweichen des CO2 führen könnten. Die Deckschicht in der Nähe eines CO2-Injektionsbrunnens ist besonders gefährdet. Der hohe Druckanstieg während der ersten Injektionsphase, Zementkorrosion am Brunnen aufgrund des CO2-reichen Formationswassers und eventuelle Beschädigungen der Deckschicht während der Erstellung des Bohrlochs sind als mögliche Ursachen für gestörte Deckschichten zu nennen. Biobarrieren könnten verwendet werden, um solche Risiken zu minimieren, z. B. indem sie Risse in der Deckschicht abdichten oder den Bohrlochzement vor Korrosion schützen. Eine Biobarriere kann aus einem Biofilm selbst bestehen, aber auch aus vom Biofilm begüngstigten mineralischen Ablagerungen. Die vorliegende Arbeit behandelt im Wesentlichen die Entwicklung eines numerischen Modells, um die Anreicherung von mikrobieller Biomasse im Untergrund simulieren zu können. Das entwickelte Modell soll in der Lage sein, das Abdichten der beschädigten geologischen Deckschicht einer unterirdischen Kohlendioxid-Lagerstätte mit Hilfe von Biofilmen zu simulieren. Dafür müssen einerseits Strömungsprozesse und andererseits auch die mikrobielle Aktivität sowie die Interaktion dieser Vorgänge in porösen Medien richtig beschrieben werden. Die Anreicherung von Bakterien in einem porösen Medium beeinflusst die hydraulischen Eigenschaften des Mediums und als Folge davon auch die darin stattfindende Strömung. Im Gegenzug bestimmt die Strömung den Transport der mikrobiellen Nährstoffe und damit auch die Verteilung mikrobieller Wachstumsraten. Dementsprechend ist die richtige Beschreibung der Wechselwirkung zwischen Strömung und mikrobiellen Prozessen eine wesentliche Herausforderung in der Modellbildung.

Kong, X.-Z. Investigation on some dynamical characteristics of granular material, MSc Thesis, University of Science and Technology of China, 60 pp., 2006. [View Abstract]This paper presents experimental and theoretical studies on segregation of granular media subjected to external vertical sinusoidal vibration, vibration energy absorption properties of granular materials, dynamic behaviors of sandpile formation, and the stop-and-go motion in vertical pipe flow. Results of our studies not only extend the knowledge of non-linear properties of granular media under some dynamical conditions, such as vibrating, shearing, and flowing etc, but also is a theoretical instruction in the processing, storing and transporting of granular media in applied engineering. In the experiments with vertical sinusoidal vibration of granular material in containers with bottleneck(s), new segregation patterns and convection properties were found: (1)Under the condition of two-dimensional containers with one bottleneck, with the variation of the width of bottleneck, big and small particles show ``Two Side segregation Pattern", ``Left Side segregation Pattern" and a pattern that big particles segregate to the upper-left part of the container. Furthermore, the angle of segregation interface and the angle of free-surface of granular bed follow a determinate rule, and the convection rolls of granular bed turn relatively and regularly. (2)Under the condition of three-dimensional container with one bottleneck, big particles aggregate and form a ``Ring-like Segregation" pattern, and the position and length of big particle cluster can be adjusted by changing the vibration frequency. And mechanism of segregation is proposed. (3)Under the condition of three-dimensional container with a series of bottlenecks along its vertical axis, according to the differences of ``Mobility" of each particle, particles will segregate alternantly. The simulation results of discrete element method (DEM) are in agreement with the experimental observations. The discovering of these new segregation patterns are of theoretical significance to studying the complex dynamical properties of granular materials, which also would be the engineering instruction for avoiding or enhancing segregation in the processing and transporting of granular materials. A DEM simulation of particle segregation patterns controlled by vibration frequency is carried out. It was found that: at low vibration frequency, large particles clustered on the top of the granular bed; as the frequency raised, large particles sinked into the granular assembly; at very high vibration frequency, large particles went up again. The mechanism of the transition of large particles clustering on the top---sinking---rising is studied in detail. The relation within inputted energy, void fraction and the effective restitution coefficient are found out. A Cellular Automata (CA) simulation of dynamic of sandpile formation is performed. A kinetic energy sandpile model, taking into account of grain inertia and the moving directions of the toppling grain, is developed and used to study the behaviour of sandpiles. In this model, the inertial effects are based on the toppling kinetic energy. The CA model reproduces the phenomenon of sandpile formation by revolving rivers which was found in previous experimental study in literature, and the relation between anglar speed and the height of sandpile is obtained. The simulation resluts reveal the non-symmetry of the sandpile formation and the selectivity of motion path of sand. The concepts and measuring method of internal friction are successfully applied to the measurement of low frequency vibration energy absorption properties of granular materials. An Invert Torsion Pendulum is designed to measure the energy absorption properties of granular materials under low frequency vibration using the free-attenuation mode. The vibration energy absorption of granular material decreases non-linearly with the increment of vibration amplitude under low frequency vibration. These results here can provide evidence for the understanding of some granular collective behaviors. When particles discharge from an open-top capillary pipe, it was found that, with particles of a particular size range, the outflux fluctuates greatly with time and the bulk dense granular flow in the pipe shows stop-and-go motion when the filling height is much larger than a threshold. When the filling height reduces towards the threshold, undergoing a transitional stage, the outflux and the bulk movement become much more stable. A heuristic theory taking into account of the granular compaction and interstitial air pressure effect is proposed to explain the appearing and disappearing of the stop-and-go motion. Finally, the conclusion of this paper and the prospect of further work are presented.

Ebigbo, A. Thermal Effects of Carbon Dioxide Sequestration in the Subsurface. Diplomarbeit, Institut für Wasserbau, MSc Thesis, University of Stuttgart, 57 pp., 2005. [View Abstract]In order to secure long-term storage of CO 2 in the subsurface, one has to be able to model the mass transport of CO2. The physical properties of CO2 have a very strong influence on the mass transport. These physical properties, in turn are dependent on temperature (and pressure). Hence, the modelling of heat transport in the subsurface during CO2 injection should be part of mass transport. In the simple set-up examined in this thesis, the Joule-Thompson cooling (as a result of the pressure drop) plays an important role near the injection point. The amount of cooling at the injection point depends on the temperature of the injected CO2 (and of course on the pressure difference that brought about the cooling). The temperature at that region achieves a stable value with time. The impermeable layer should be deeper than the depth at which the CO2 becomes gaseous as it is easier to trap less mobile supercritical or liquid CO2.

Saar, M.O. Geological Fluid Mechanics Models at Various Scales, Dissertation, University of California, Berkeley, 153 pp., 2003. [View Abstract]In this dissertation, I employ concepts from fluid mechanics to quantitatively investigate geological processes in hydrogeology and volcanology. These research topics are addressed by utilizing numerical and analytical models but also by conducting field and lab work. Percolation theory is of interest to a wide range of physical sciences and thus warrants research in itself. Therefore, I developed a computer code to study percolation thresholds of soft-core polyhedra. Results from this research are applied to study the onset of yield strength in crystal-melt suspensions such as magmas. Implications of yield strength development in suspensions, marking the transition from Newtonian to Bingham fluid, include the pahoehoe-'a'a transition and the occurrence of effusive versus explosive eruptions. I also study interactions between volcanic processes and groundwater as well as between groundwater and seismicity (hydroseismicity). In the former case, I develop numerical and analytical models of coupled groundwater and heat transfer. Here, perturbations from a linear temperature-depth profile are used to determine groundwater flow patterns and rates. For the hydroseismicity project I investigate if seasonal elevated levels of seismicity at Mt. Hood, Oregon, are triggered by groundwater recharge. Both hydroseismicity and hydrothermal springs occur on the southern flanks of Mt. Hood. This suggests that both phenomena are related while also providing a connection between the research projects involving groundwater, heat flow, and seismicity. Indeed, seismicity may be necessary to keep faults from clogging thus allowing for sustained activity of hydrothermal springs. Finally, I present research on hydrologically induced volcanism, where a process similar to the one suggested for hydroseismicity is invoked. Here, melting of glaciers, or draining of lakes, during interglacial periods reduce the confining pressure in the subsurface which may promote dike formation and result in increased rates of volcanism. In general, problems discussed in this dissertation involve interactions among processes that are traditionally associated with separate research disciplines. However, numerous problems in the geosciences require a multidisciplinary approach, as demonstrated here. In addition, employing several analytical and numerical methods, such as signal processing, inverse theory, computer modeling, and percolation theory, allows me to study such diverse processes in a quantitative way.

Saar, M.O. The Relationship Between Permeability, Porosity, and Microstucture in Vesicular Basalts, MSc Thesis, University of Oregon, 101 pp., 1998. [View Abstract]This thesis presents measurements of permeability, \(k\), porosity, \(\Phi\), and microstructural parameters of vesicular basalts. Measurements are compared with theoretical models. A percolation theory and a Kozeny-Carman model are used to interpret the measurements and to investigate relationships between porosity, microstructure, and permeability. Typical permeabilities for vesicular basalts are in the range of \(10^{-14}\) < \(k\) < \(10^{-10} m^2\). Best permeability estimates, following power laws predicted by percolation theory, are obtained when samples are used that show 'impeded aperture widening' due to rapid cooling and no bubble collapse (scoria and some flow samples). However, slowly cooled diktytaxitic samples contain elongated, 'collapsed' bubbles. Measurements indicate that the vesicle pathway network remains connected and preserves high permeabilities. Image-analysis techniques are unsuccessful if used for Kozeny-Carman equation parameter determination for vesicular materials, probably because the average interbubble aperture size that determines \(k\) is not resolved with such a technique.