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Publications of GEG Members


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Publications of GEG Members are listed below

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Underlined names are links to recent or past GEG members

REFEREED PUBLICATIONS IN JOURNALS

101. 
Adams, B.M., D. Vogler, T.H. Kuehn, J.M. Bielicki, N. Garapati, and M.O. Saar Heat Depletion in Sedimentary Basins and its Effect on the Design and Electric Power Output of CO2 Plume Geothermal (CPG) Systems Renewable Energy, (in press). [View Abstract]CO2 Plume Geothermal (CPG) energy systems circulate geologically stored CO2 to extract geothermal heat from naturally permeable sedimentary basins. CPG systems can generate more electricity than brine systems in geologic reservoirs with moderate temperature and permeability. Here, we numerically simulate the temperature depletion of a sedimentary basin and find the corresponding CPG electricity generation variation over time. We find that for a given reservoir depth, temperature, thickness, permeability, and well configuration, an optimal well spacing provides the largest average electric generation over the reservoir lifetime. If wells are spaced closer than optimal, higher peak electricity is generated, but the reservoir heat depletes more quickly. If wells are spaced greater than optimal, reservoirs maintain heat longer but have higher resistance to flow and thus lower peak electricity is generated. Additionally, spacing the wells 10% greater than optimal affects electricity generation less than spacing wells 10% closer than optimal. Our simulations also show that for a 300 m thick reservoir, a 707 m well spacing provides consistent electricity over 50 years, whereas a 300 m well spacing yields large heat and electricity reductions over time. Finally, increasing injection or production well pipe diameters does not necessarily increase average electric generation.

100. 
Garapati, N., B.M. Adams, M.R. Fleming, T.H. Kuehn, and M.O. Saar Combining brine or CO2 geothermal preheating with low-temperature waste heat: A higher-efficiency hybrid geothermal power system Journal of CO2 Utilization, 42, 2020. [Download PDF] [View Abstract]Hybrid geothermal power plants operate by using geothermal fluid to preheat the working fluid of a higher temperature power cycle for electricity generation. This has been shown to yield higher electricity generation than the combination of a stand-alone geothermal power plant and the higher-temperature power cycle. Here, we test both a direct CO2 hybrid geothermal system and an indirect brine hybrid geothermal system. The direct CO2 hybrid geothermal system is a CO2 Plume Geothermal (CPG) system, which uses CO2 as the subsurface working fluid, but with auxiliary heat addition to the geologically produced CO2 at the surface. The indirect brine geothermal system uses the hot geologically produced brine to preheat the secondary working fluid (CO2) within a secondary power cycle. We find that the direct CPG-hybrid system and the indirect brine-hybrid system both can generate 20 % more electric power than the summed power of individual geothermal and auxiliary systems in some cases. Each hybrid system has an optimum turbine inlet temperature which maximizes the electric power generated, and is typically between 100 ◦C and 200 ◦C in the systems examined. The optimum turbine inlet temperature tends to occur where the geothermal heat contribution is between 50 % and 70 % of the total heat addition to the hybrid system. Lastly, the CO2 direct system has lower wellhead temperatures than indirect brine and therefore can utilize lower temperature resources.

99. 
Lima, M., P. Schädle, C. Green, D. Vogler, M.O. Saar, and X.-Z. Kong Permeability Impairment and Salt Precipitation Patterns during CO2 Injection into Single Natural Brine-filled Fractures Water Resources Research, 56/8, pp. e2020WR027213, 2020. [Download PDF] [View Abstract]Formation dry-out in fracture-dominated geological reservoirs may alter the fracture space, impair rock absolute permeability and cause a significant decrease in well injectivity. In this study, we numerically model the dry-out processes occurring during supercritical CO2 (scCO2) injection into single brine-filled fractures and evaluate the potential for salt precipitation under increasing effective normal stresses in the evaporative regime. We use an open-source, parallel finite-element framework to numerically model two-phase flow through 2-Dimensional fracture planes with aperture fields taken from naturally fractured granite cores at the Grimsel Test Site in Switzerland. Our results reveal a displacement front and a subsequent dry-out front in all simulated scenarios, where higher effective stresses caused more flow channeling, higher rates of water evaporation and larger volumes of salt precipitates. However, despite the larger salt volumes, the permeability impairment was lower at higher effective normal stresses. We conclude that the spatial distribution of the salt, precipitated in fractures with heterogeneous aperture fields, strongly affects the absolute permeability impairment caused by formation dry-out. The numerical simulations assist in understanding the behavior of the injectivity in fractures and fracture networks during subsurface applications that involve scCO2 injection into brine.

98. 
Middleton, R, J Ogland-Hand, B Chen, J Bielicki, K Ellet, D Harp, and R Kammer Identifying Geologic Characteristics and Operational Decisions to Meet Global Carbon Sequestration Goals Energy and Environmental Science, 2020. [Download PDF]

97. 
Hefny, M., C.-Z. Qin, M.O. Saar, and A. Ebigbo Synchrotron-based pore-network modeling of two-phase flow in Nubian Sandstone and implications for capillary trapping of carbon dioxide International Journal of Greenhouse Gas Control, 103/1031642, 2020. [Download PDF] [View Abstract]Depleted oil fields in the Gulf of Suez (Egypt) can serve as geothermal reservoirs for power production using a CO2-Plume Geothermal (CPG) system, while geologically sequestering CO2. This entails the injection of a substantial amount of CO2 into the highly permeable brine-saturated Nubian Sandstone. Numerical models of two-phase flow processes are indispensable for predicting the CO2-plume migration at a representative geological scale. Such models require reliable constitutive relationships, including relative permeability and capillary pressure curves. In this study, 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 0.65 μm3 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 sequential primary drainage–main imbibition cycle of quasi-static invasion in order to quantify (1) the CO2 and brine relative permeability curves, (2) the effect of initial wetting-phase saturation (i.e. the saturation at the point of reversal from drainage to imbibition) on the residual–trapping potential, and (3) study the relative permeability–saturation hysteresis. The results illustrate the sensitivity of the pore-scale fluid-displacement and trapping processes on some key parameters (i.e. advancing contact angle, pore-body-to-throat aspect ratio, and initial wetting-phase saturation) and improve our understanding of the potential magnitude of capillary trapping in Nubian Sandstone.

96. 
Zhang, S., and X. Ma Global Frictional Equilibrium via Stochastic, Local Coulomb Frictional Slips ESSOAr, 2020. [Download PDF]

95. 
Krietsch, H., V.S. Gischig, J. Doetsch, K.F. Evans, L. Villliger, M. Jalali, B. Valley, S. Löw, and F. Amman Hydromechanical processes and their influence on the stimulation effected volume: observations from a decameter-scale hydraulic stimulation project Solid Earth, 11/5, pp. 1699-1729, 2020. [Download PDF] [View Abstract]Six hydraulic shearing experiments have been conducted in the framework of the In-situ Stimulation and Circulation experiment within a decameter-scale crystalline rock volume at the Grimsel Test Site, Switzerland. During each experiment fractures associated with one out of two shear zone types were hydraulically reactivated. The two shear zone types differ in terms of tectonic genesis and architecture. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in 11 boreholes around the injection locations. As a result of the stimulation, the near wellbore transmissivity increased up to 3 orders in magnitude.With one exception, jacking pressures were unchanged by the stimulations. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone architectures govern the seismo-hydromechanical response. The elevated fracture fluid pressures associated with the stimulations propagated mostly along the stimulated shear zones. The absence of high-pressure signals away from the injection point for most experiments (except two out of six experiments) is interpreted as channelized flow within the shear zones. The observed deformation field within 15–20m from the injection point is characterized by variable extensional and compressive strain produced by fracture normal opening and/or slip dislocation, as well as stress redistribution related to these processes. At greater distance from the injection location, strain measurements indicate a volumetric compressive zone, in which strain magnitudes decrease with increasing distance. These compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. Our hydromechanical data reveal that the overall stimulation effected volume is significantly larger than implied by the seismicity cloud and can be subdivided into a primary stimulated and secondary effected zone.

94. 
Gischig, V.S., D. Giardini, F. Amann, "et al.", Keith F. Evans, "et al.", A. Kittilä, X. Ma, "et al.", M.O. Saar, and "et al." 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.

93. 
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, 12/4, pp. 793-801, 2020. [Download PDF] [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.

92. 
Leal, A. M. M., S. Kyas, D. Kulik, and M. O. Saar Accelerating Reactive Transport Modeling: On‑Demand Machine Learning Algorithm for Chemical Equilibrium Calculations Transport in Porous Media, 133, pp. 161-204, 2020. [Download PDF] [View Abstract]During reactive transport modeling, the computing cost associated with chemical equilibrium calculations can be 10 to 10,000 times higher than that of fluid flow, heat transfer, and species transport computations. These calculations are performed at least once per mesh cell and once per time step, amounting to billions of them throughout the simulation employing high-resolution meshes. To radically reduce the computing cost of chemical equilibrium calculations (each requiring an iterative solution of a system of nonlinear equa-tions), we consider an on-demand machine learning algorithm that enables quick and accu-rate prediction of new chemical equilibrium states using the results of previously solved chemical equilibrium problems within the same reactive transport simulation. The training operations occur on-demand, rather than before the start of the simulation when it is not clear how many training points are needed to accurately and reliably predict all possible chemical conditions that may occur during the simulation. Each on-demand training opera-tion consists of fully solving the equilibrium problem and storing some key information about the just computed chemical equilibrium state (which is used subsequently to rap-idly predict similar states whenever possible). We study the performance of the on-demand learning algorithm, which is mass conservative by construction, by applying it to a reactive transport modeling example and achieve a speed-up of one or two orders of magnitude (depending on the activity model used). The implementation and numerical tests are car-ried out in Reaktoro (reakt oro.org), a unified open-source framework for modeling chemi-cally reactive systems.

91. 
Ma, X., M.O. Saar, and L.-S. Fan Coulomb Criterion - Bounding Crustal Stress Limit and Intact Rock Failure: Perspectives Powder Technology, 374, pp. 106-110, 2020. [Download PDF] [View Abstract]In this perspective article, we illustrate the importance and versatility of the Coulomb criterion that serves as a bridge between the fields of powder technology and rock mechanics/geomechanics. We first describe the essence of the Coulomb criterion and its physical meaning, revealing surprising similarities regarding its applica- tions between both fields. We then discuss the rock mechanics applications and limitations at two extreme scales, the Earth's crust (tens of kilometers) and intact rocks (meters). We finally offer thoughts on bridging these scales. The context of the article is essential not only to the rock mechanics/geomechanics community but also to a broader powder technology community.

90. 
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, 15/8, pp. 2327-2340, 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.

89. 
von Planta, C., D. Vogler, P. Zulian, M.O. Saar, and R. Krause Contact between rough rock surfaces using a dual mortar method International Journal of Rock Mechanics and Mining Sciences (IJRMMS), 133, pp. 104414, 2020. [Download PDF] [View Abstract]The mechanical behavior of fractures in rocks has strong implications for reser- voir engineering applications. Deformations, and the corresponding change in contact area and aperture field, impact rock fracture stiffness and permeability, thus altering the reservoir properties significantly. Simulating contact between fractures is numerically difficult as the non-penetration constraints lead to a nonlinear problem and the surface meshes of the solid bodies on the opposing fracture sides may be non-matching. Furthermore, due to the complex geome- try, the non-penetration constraints must be updated throughout the solution procedure. Here we present a novel implementation of a dual mortar method for contact. It uses a non-smooth sequential quadratic programming method as solver, and is suitable for parallel computing. We apply it to a two body con- tact problem consisting of realistic rock fracture geometries from the Grimsel underground laboratory in Switzerland. The contributions of this article are: 1) a novel, parallel implementation of a dual mortar and non-smooth sequential quadratic programming method, 2) realistic rock geometries with rough sur- faces, and 3) numerical examples, which prove that the dual mortar method is capable of replicating the nonlinear closure behavior of fractures observed in laboratory experiments.

88. 
Ma, J., L. Querci, B. Hattendorf, M.O. Saar, and X.-Z. Kong The effect of mineral dissolution on the effective stress law for permeability in a tight sandstone Geophysical Research Letters, 2020. [Download PDF] [View Abstract]We present flow-through experiments to delineate the processes involved in permeability changes driven by effective stress variations and mineral cement dissolution in porous rocks. CO2-enriched brine is injected continuously into a tight sandstone under in-situ reservoir conditions for 455 hours. Due to the dolomite cement dissolution, the bulk permeability of the sandstone specimen significantly increases, and two dissolution passages are identified near the fluid inlet by X-ray CT imaging. Pre- and post-reaction examinations of the effective stress law for permeability suggest that after reaction the bulk permeability is more sensitive to pore pressure changes and less sensitive to effective stress changes. These observations are corroborated by Scanning Electron Microscopy and X-ray CT observations. This study deepens our understanding of the effect of mineral dissolution on the effective stress law for permeability, with implications for characterizing subsurface mass and energy transport, particularly during fluid injection/production into/from geologic reservoirs.

87. 
Hefny, M., A. Zappone, Y. Makhloufi, A. de Haller, and A. Moscariello A laboratory approach for the calibration of seismic data in the western part of the Swiss Molasse Basin: the case history of well Humilly-2 (France) in the Geneva area Swiss Journal of Geosciences , 113/11, 2020. [Download PDF] [View Abstract]A collection of 81 plugs were obtained from the Humilly-2 borehole (France), that reached the Permo-Carboniferous sediments at a depth of 3051 m. Experimental measurements of physical parameters and mineralogical analysis were performed to explore the links between sedimentary facies and seismic characteristics and provide a key tool in the interpretation of seismic field data in terms of geological formations. The plugs, cylinders of 22.5 mm in diameter and ~30 mm in length were collected parallel and perpendicular to the bedding in order to explore their anisotropy. Ultrasound wave propagation was measured at increasing confining pressure conditions up to 260 MPa, a pressure where all micro-fractures are considered closed. The derivatives of velocities with pressure were established, allowing the simulation of lithological transitions at in-situ conditions. At room conditions, measured grain densities [kg/m3] range from 2630 to 2948 and velocities vary from 4339 to 6771 m/s and 2460 to 3975m/s for P- and S-waves propagation modes, respectively. The largest seismic-reflections coefficients were calculated for the interface between the evaporitic facies of the Keuper (Lettenkohle) and the underlying Muschelkalk carbonates (Rc= 0.3). The effective porosity has a range of 0.23% to 16.65%, while the maximum fluid permeability [m2] is 9.1e-16. A positive correlation between porosity and ultrasound velocity has been observed for P- and S-waves. The link between velocities and modal content of quartz, dolomite, calcite, and micas has been explored. This paper presents a unique set of seismic parameters potentially useful for the calibration of seismic data in the Geneva Molasse Basin.

86. 
Fleming, M.R., B.M. Adams, T.H. Kuehn, J.M. Bielicki, and M.O. Saar Increased Power Generation due to Exothermic Water Exsolution in CO2 Plume Geothermal (CPG) Power Plants Geothermics, 88/101865, 2020. [Download PDF] [View Abstract]A direct CO2-Plume Geothermal (CPG) system is a novel technology that uses captured and geologically stored CO2 as the subsurface working uid in sedimentary basin reservoirs to extract geothermal energy. In such a CPG system, the CO2 that enters the production well is likely saturated with H2O from the geothermal reser- voir. However, direct CPG models thus far have only considered energy production via pure (i.e. dry) CO2 in the production well and its direct conversion in power generation equipment. Therefore, we analyze here, how the wellhead uid pressure, temperature, liquid water fraction, and the resultant CPG turbine power output are impacted by the production of CO2 saturated with H2O for reservoir depths ranging from 2.5 km to 5.0 km and geothermal temperature gradients between 20 °C/km and 50 °C/km. We demonstrate that the H2O in solution is exothermically exsolved in the vertical well, increasing the uid temperature relative to dry CO2, resulting in the production of liquid H2O at the wellhead. The increased wellhead uid temperature increases the turbine power output on average by 15% to 25% and up to a maximum of 41%, when the water enthalpy of exsolution is considered and the water is (conservatively) removed before the turbine, which decreases the uid mass ow rate through the turbine and thus power output. We show that the enthalpy of exsolution and the CO2-H2O so- lution density are fundamental components in the calculation of CPG power generation and thus should not be neglected or substituted with the properties of dry CO2.

85. 
Tutolo, B., A. Luhmann, X.-Z. Kong, B. Bagley, D. Alba-Venero, N. Mitchell, M.O. Saar, and W.E. Seyfried Contributions of visible and invisible pores to reactive transport in dolomite Geochemical Perspectives Letters, I4, pp. 42-46, 2020. [Download PDF] [View Abstract]Recent technical advances have demonstrated the importance of pore-scale geochemical processes for governing Earth's evolution. However, the contribution of pores at different scales to overall geochemical reactions remains poorly understood. Here, we integrate multiscale characterization and reactive transport modeling to study the contribution of pore-scale geochemical proceses to the hydrogeochemical evolution of dolomite rock samples during CO2-driven dissolution experiments. Our results demonstrate that approximately half of the total pore volume is invisible at the scale of commonly used imaging techniques. Comparison of pre- and post-experiment analyses demonstrate that porosity-increasing, CO2-driven dissolution processes preferentially occur in pores 600 nm – 5 μm in size, but pores <600 nm in size show no change during experimental alteration. This latter observation, combined with the anomalously high rates of trace element release during the experiments, suggests that nanoscale pores are accessible to through-flowing fluids. A three-dimensional simulation performed directly on one of the samples shows that steady-state pore-scale trace element reaction rates must be ~10× faster than that of dolomite in order to match measured effluent concentrations, consistent with the large surface area-to-volume ratio in these pores. Together, these results yield a new conceptual model of pore-scale processes, and urge caution when interpreting the trace element concentrations of ancient carbonate rocks.

84. 
Kyas, S., S. Repin, J. M. Nordbotten, and K. Kumar Guaranteed and computable error bounds for approximations constructed by an iterative decoupling of the Biot problem Computers & Mathematics with Applications, pp. 1-28, 2020. [Download PDF] [View Abstract]The paper is concerned with guaranteed a posteriori error estimates for a class of evolutionary problems related to poroelastic media governed by the quasi-static linear Biot equations. The system is decoupled by employing the fixed-stress split scheme, which leads to an iteratively solved semi-discrete system. The error bounds are derived by combining a posteriori estimates for contractive mappings with the functional type error control for elliptic partial differential equations. The estimates are applicable to any approximation in the admissible functional space and are independent of the discretization method. They are fully computable, do not contain mesh-dependent constants, and provide reliable global estimates of the error measured in the energy norm. Moreover, they suggest efficient error indicators for the distribution of local errors and can be used in adaptive procedures.

83. 
Kittilä, A., M.R. Jalali, M.O. Saar, and X.-Z. Kong Solute tracer test quantification of the effects of hot water injection into hydraulically stimulated crystalline rock Geothermal Energy, 8/17, 2020. [Download PDF] [View Abstract]When water is injected into a fracture-dominated reservoir that is cooler or hotter than the injected water, the reservoir permeability is expected to be altered by the injection-induced thermo-mechanical effects, resulting in the redistribution of fluid flow in the reservoir. These effects are important to be taken into account when evaluating the performance and lifetime particularly of Enhanced Geothermal Systems (EGS). In this paper, we compare the results from two dye tracer tests, conducted before (at ambient temperature of 13 °C) and during the injection of 45 °C hot water into a fractured crystalline rock at the Grimsel Test Site in Switzerland. Conducting a moment analysis on the recovered tracer residence time distribution (RTD) curves, we observe, after hot water injection, a significant decrease in the total tracer recovery. This recovery decrease strongly suggests that fluid flow was redistributed in the studied rock volume and that the majority of the injected water was lost to the far-field. Furthermore, by using temperature measurements, obtained from the same locations as the tracer RTD curves, we conceptualize an approach to estimate the fracture surface area contributing to the heat exchange between the host rock and the circulating fluid. Our moment analysis and simplified estimation of fracture surface area provide insights into the hydraulic properties of the hydraulically active fracture system and the changes in fluid flow. Such insights are important to assess the heat exchange performance of a geothermal formation during fluid circulation and to estimate the lifetime of the geothermal formation, particularly in EGS.

82. 
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. [Download PDF] [View Abstract]Petrothermal systems seek to extract energy from hot, low-permeability rocks that in northern Switzerland lie at 4–6 km depth. Permeability is enhanced by performing large ‘hydraulic stimulation’ injections, such as occurred in the 5 km deep well at Basel, Switzerland. Knowledge of the discontinuity properties and distribution in target reservoirs can help design such stimulations, but characterisation is challenging because of limited rock mass exposure. Rock mass fractures below Basel were investigated in previous studies by analyses of acoustic tele- viewer data of the Basel-1 well and by fault plane solutions. In addition, seismic events with high similarity of seismic waveforms were grouped into clusters and interpreted to have originated from slip of patches of a common fault or fault zone. In this study we investigate the orientations of fault patches from seismic clusters, considering relative location errors of cluster events with Monte-Carlo simulations, and use common events in clusters ranging in size from few 10s to many 100s of metres to explore the internal architecture of larger fault zones at Basel. We nd that the orientations of microseismically-inferred faults and borehole fractures are similar and that fracture zones consist of sub-parallel or ‘anastomosing’ fractures.

81. 
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] [View Abstract]We monitored the seismohydromechanical rock mass response to high‐pressure fluid injection during a decameter‐scale hydraulic stimulation experiment in crystalline rock at the Grimsel Test Site, Switzerland. Time series recorded at two pressure monitoring locations show abrupt pressure increases that change in amplitude and appearance between subsequent stimulation cycles. Induced seismicity correlates with the propagation of one of these pressure fronts. Deformation data along the same shear zone shows permanent fracture dislocation preceded by strong transient fracture opening. We interpret these observations as nonlinear pressure diffusion along flow channels that reorganize in response to hydromechanical effects during stimulation. Combining these observations with the in situ stress field estimated before stimulation, we argue that the underlying hydromechanical processes involve mixed‐mode stimulation with both Mode I and II/III fracture dislocation.

80. 
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, 269/115012, 2020. [Download PDF] [View Abstract]We investigate the potential for extracting heat from produced natural gas and utilizing supercritical carbon dioxide (CO2) as a working uid for the dual purpose of enhancing gas recovery (EGR) and extracting geo- thermal energy (CO2-Plume Geothermal – CPG) from deep natural gas reservoirs for electric power generation, while ultimately storing all of the subsurface-injected CO2. Thus, the approach constitutes a CO2 capture double- utilization and storage (CCUUS) system. The synergies achieved by the above combinations include shared infrastructure and subsurface working uid. We integrate the reservoir processes with the wellbore and surface power-generation systems such that the combined system’s power output can be optimized. Using the subsurface uid ow and heat transport simulation code TOUGH2, coupled to a wellbore heat-transfer model, we set up an anticlinal natural gas reservoir model and assess the technical feasibility of the proposed system. The simulations show that the injection of CO2 for natural gas recovery and for the establishment of a CO2 plume (necessary for CPG) can be conveniently combined. During the CPG stage, following EGR, a CO2-circulation mass owrate of 110 kg/s results in a maximum net power output of 2 MWe for this initial, conceptual, small system, which is scalable. After a decade, the net power decreases when thermal breakthrough occurs at the production wells. The results con rm that the combined system can improve the gas eld’s overall energy production, enable CO2 sequestration, and extend the useful lifetime of the gas eld. Hence, deep (partially depleted) natural gas re- servoirs appear to constitute ideal sites for the deployment of not only geologic CO2 storage but also CPG.

79. 
Rossi, E. , M.O. Saar, and Ph. Rudolf von Rohr The influence of thermal treatment on rock-bit interaction: a study of a combined thermo-mechanical drilling (CTMD) concept Geothermal Energy, 8/16, 2020. [Download PDF] [View Abstract]To improve the economics and viability of accessing deep georesources, we propose a combined thermo–mechanical drilling (CTMD) method, employing a heat source to facilitate the mechanical removal of rock, with the aim of increasing drilling performance and thereby reducing the overall costs, especially for deep wells in hard rocks. In this work, we employ a novel experiment setup to investigate the main parameters of interest during the interaction of a cutter with the rock material, and we test untreated and thermally treated sandstone and granite, to understand the underlying rock removal mechanism and the resulting drilling performance improvements achievable with the new approach. We find that the rock removal process can be divided into three main regimes: first, a wear-dominated regime, followed by a compression-based progression of the tool at large penetrations, and a final tool fall-back regime for increasing scratch distances. We calculate the compressive rock strengths from our tests to validate the above regime hypothesis, and they are in good agreement with literature data, explaining the strength reduction after treatment of the material by extensive induced thermal cracking of the rock. We evaluate the new method’s drilling performance and confirm that thermal cracks in the rock can considerably enhance subsequent mechanical rock removal rates and related drilling performance by one order of magnitude in granite, while mainly reducing the wear rates of the cutting tools in sandstone.

78. 
Jia, Y., W. Wu, and X.-Z. Kong Injection-induced slip heterogeneity on faults in shale reservoirs International Journal of Rock Mechanics and Mining Sciences, 131, pp. 1-6, 2020. [Download PDF] [View Abstract]Managing fluid stimulation protocols is an effective means to mitigate the risk of injection-induced earthquakes during shale gas development. The success of these protocols is dependent on our understanding of fluid pressure heterogeneity and the associated inhomogeneous slip on critically stressed faults. Here we show the evolution of velocity-weakening zone on a simulated fault, derived from fluid injection and velocity stepped experiments, and the corresponding non-uniform fluid pressure distribution, recovered from coupled hydro-mechanical simulations. Our results indicate that the sharp extension of velocity-weakening zone occurs before the nucleation of fault rupture, which could be an indicator to avoid the reactivation of other fault patches beyond the stimulated zone. The dynamic rupture is estimated to extend much faster than the maximum speed of the velocity-weakening zone front. We infer that the velocity-weakening zone may further expand and fully control the fault behavior after multiple slip events.

77. 
Kaufl, S., V. Grayver, J. Comeau, V. Kuvshinov, M. Becken, J. Kamm, B. Erdenechimeg, and D. Sodnomsambuu Magnetotelluric multiscale 3-D inversion reveals crustal and upper mantle structure beneath the Hangai and Gobi-Altai region in Mongolia Geophysical Journal International, 221/1002-1028, 2020. [Download PDF] [View Abstract]Central Mongolia is a prominent region of intracontinental surface deformation and intraplate volcanism. To study these processes, which are poorly understood, we collected magnetotelluric (MT) data in the Hangai and Gobi-Altai region in central Mongolia and derived the first 3-D resistivity model of the crustal and upper mantle structure in this region. The geological and tectonic history of this region is complex, resulting in features over a wide range of spatial scales, which that are coupled through a variety of geodynamic processes. Many Earth properties that are critical for the understanding of these processes, such as temperature as well as fluid and melt properties, affect the electrical conductivity in the subsurface. 3-D imaging using MT can resolve the distribution of electrical conductivity within the Earth at scales ranging from tens of metres to hundreds of kilometres, thereby providing constraints on possible geodynamic scenarios. We present an approach to survey design, data acquisition, and inversion that aims to bridge various spatial scales while keeping the required field work and computational cost of the subsequent 3-D inversion feasible. MT transfer functions were estimated for a 650 × 400 km2 grid, which included measurements on an array with regular 50 × 50 km2 spacing and along several profiles with a denser 5–15 km spacing. The use of telluric-only data loggers on these profiles allowed for an efficient data acquisition with a high spatial resolution. A 3-D finite element forward modelling and inversion code was used to obtain the resistivity model. Locally refined unstructured hexahedral meshes allow for a multiscale model parametrization and accurate topography representation. The inversion process was carried out over four stages, whereby the result from each stage was used as input for the following stage that included a finer model parametrization and/or additional data (i.e. more stations, wider frequency range). The final model reveals a detailed resistivity structure and fits the observed data well, across all periods and site locations, offering new insights into the subsurface structure of central Mongolia. A prominent feature is a large low-resistivity zone detected in the upper mantle. This feature suggests a non-uniform lithosphere-asthenosphere boundary that contains localized upwellings that shallow to a depth of 70 km, consistent with previous studies. The 3-D model reveals the complex geometry of the feature, which appears rooted below the Eastern Hangai Dome with a second smaller feature slightly south of the Hangai Dome. Within the highly resistive upper crust, several conductive anomalies are observed. These may be explained by late Cenozoic volcanic zones and modern geothermal areas, which appear linked to mantle structures, as well as by major fault systems, which mark terrane boundaries and mineralized zones. Well resolved, heterogeneous low-resistivity zones that permeate the lower crust may be explained by fluid-rich domains.

76. 
Comeau, J., M. Becken, S. Kaufl, V. Grayver, V. Kuvshinov, Ts. Shoovdor, B. Erdenechimeg, and D. Sodnomsambuu Evidence for terrane boundaries and suture zones across Southern Mongolia detected with a 2-dimensional magnetotelluric transect Earth, Planets and Space, 72/1--13, pp. 1-13, 2020. [Download PDF] [View Abstract]Southern Mongolia is part of the Central Asian Orogenic Belt, the origin and evolution of which is not fully known and is often debated. It is composed of several east–west trending lithostratigraphic domains that are attributed to an assemblage of accreted terranes or tectonic zones. This is in contrast to Central Mongolia, which is dominated by a cratonic block in the Hangai region. Terranes are typically bounded by suture zones that are expected to be deep-reaching, but may be difficult to identify based on observable surface fault traces alone. Thus, attempts to match lithostratigraphic domains to surface faulting have revealed some disagreements in the positions of suspected terranes. Furthermore, the subsurface structure of this region remains relatively unknown. Therefore, high-resolution geophysical data are required to determine the locations of terrane boundaries. Magnetotelluric data and telluric-only data were acquired across Southern Mongolia on a profile along a longitude of approximately 100.5° E. The profile extends ~ 350 km from the Hangai Mountains, across the Gobi–Altai Mountains, to the China–Mongolia border. The data were used to generate an electrical resistivity model of the crust and upper mantle, presented here, that can contribute to the understanding of the structure of this region, and of the evolution of the Central Asian Orogenic Belt. The resistivity model shows a generally resistive upper crust (0–20 km) with several anomalously conductive features that are believed to indicate suture zones and the boundaries of tectonic zones. Moreover, their spatial distribution is coincident with known surface fault segments and active seismicity. The lower crust (30–45 km) becomes generally less resistive, but contains an anomalously conductive feature below the Gobi–Altai zone. This potentially agrees with studies that have argued for an allochthonous lower crust below this region that has been relaminated and metamorphosed. Furthermore, there is a large contrast in the electrical properties between identified tectonic zones, due to their unique tectonic histories. Although penetration to greater depths is limited, the magnetotelluric data indicate a thick lithosphere below Southern Mongolia, in contrast to the previously reported thin lithosphere below Central Mongolia.

75. 
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.

74. 
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.

73. 
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.

72. 
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.

71. 
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.

70. 
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.

69. 
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.

68. 
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.

67. 
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. 141-184, 2019. [Download PDF] [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.

66. 
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 (PAMM), 22, 2019. [Download PDF] [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.

65. 
Nejati, M., M.L.T. Dambly, and M.O. Saar A methodology to determine the elastic properties of anisotropic rocks from a single uniaxial compression test Journal of Rock Mechanics and Geotechnical Engineering, 11/6, pp. 1166-1183, 2019. [Download PDF] [View Abstract]This paper introduces a new methodology to measure the elastic constants of transversely isotropic rocks from a single uniaxial compression test. We first give the mathematical proof that a uniaxial compression test provides only four independent strain equations. As a result, the exact determination of all five independent elastic constants from only one test is not possible. An approximate determination of the Young's moduli and the Poisson's ratios is however practical and efficient when adding the Saint–Venant relation as the fifth equation. Explicit formulae are then developed to calculate both secant and tangent definitions of the five elastic constants from a minimum of four strain measurements. The results of this new methodology applied on three granitic samples demonstrate a significant stress-induced nonlinear behavior, where the tangent moduli increase by a factor of three to four when the rock is loaded up to 20 MPa. The static elastic constants obtained from the uniaxial compression test are also found to be significantly smaller than the dynamic ones obtained from the ultrasonic measurements.

64. 
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.

63. 
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.

62. 
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.

61. 
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.

60. 
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.

59. 
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.

58. 
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.

57. 
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]

56. 
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.

55. 
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.

54. 
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]

53. 
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.

52. 
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.

51. 
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.

50. 
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.

49. 
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.

48. 
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.

47. 
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.

46. 
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.

45. 
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]

44. 
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.

43. 
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.

42. 
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.

41. 
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.

40. 
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.

39. 
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.

38. 
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.

37. 
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.

36. 
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.

35. 
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.

34. 
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.

33. 
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.

32. 
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.

31. 
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.

30. 
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.

29. 
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.

28. 
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.

27. 
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.

26. 
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.

25. 
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.

24. 
Rudolf von Rohr, Ph., M. Kant, and E. Rossi An apparatus for thermal spallation of a borehole Patent EP3450675A1, 2017.

23. 
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.

22. 
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 (reaktoro.org), 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.

21. 
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.

20. 
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.

19. 
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.

18. 
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]

17. 
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.

16. 
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.

15. 
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.

14. 
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.

13. 
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.

12. 
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.

11. 
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.

10. 
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.

9. 
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.

8. 
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.

7. 
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.

6. 
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.

5. 
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.

4. 
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.

3. 
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.

2. 
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.

1. 
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.


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PROCEEDINGS REFEREED

43. 
Rossi, E., B. Adams, D. Vogler, Ph. Rudolf von Rohr, B. Kammermann, and M.O. Saar Advanced drilling technologies to improve the economics of deep geo-resource utilization , Applied Energy Symposium: MIT A+B, pp. 1-6, (in press). [View Abstract]Access to deep energy resources (geothermal energy, hydrocarbons) from deep reservoirs will play a fundamental role over the next decades. However, drilling of deep wells to extract deep geo-resources is extremely expensive. As a fact, drilling deep wells into hard, crystalline rocks represents a major challenge for conventional rotary drilling systems, featuring high rates of drill bit wear and requiring frequent drill bit replacements, low penetration rates and poor process efficiency. Therefore, with the aim of improving the overall economics to access deep geo-resources in hard rocks, in this work, we focus on two novel drilling methods, namely: the Combined Thermo-Mechanical Drilling (CTMD) and the Plasma-Pulse Geo-Drilling (PPGD) technologies. The goal of this research and development project is the effective reduction of the costs of drilling in general and particularly regarding accessing and using deep geothermal energy, oil or gas resources. In this work, we present these two novel drilling technologies and focus on evaluating the process efficiency and the drilling performance of these methods, compared to conventional rotary drilling.

42. 
Birdsell, D., and M. Saar Modeling Ground Surface Deformation at the Swiss HEATSTORE Underground Thermal Energy Storage Sites , Proceedings World Geothermal Congress, 2020. [Download PDF] [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.

41. 
Dorj, P., F. Samrock, and B. Erdenechimeg Update of Geothermal Development of Mongolia , Proceedings World Geothermal Congress, 2020. [View Abstract]A first large scale detailed geophysical exploration work in Arkhangai province (a largest geothermal active zone) is done between 2019 and 2020. Based on the result of this geophysical exploration work a combined geothermal district heating and power production plant will be built in Arkhangai province in the coming few years. Ground source heat pump application is broadly introduced in the country using ground water and soil heating system.

40. 
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. [Download PDF] [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.

39. 
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.

38. 
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.

37. 
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.

36. 
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.

35. 
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.

34. 
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.

33. 
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.

32. 
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.

31. 
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.

30. 
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.

29. 
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.

28. 
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. [Download PDF] [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.

27. 
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, Jun 2019, pp. 1-5, 2019. [Download PDF] [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. In this work, we investigate a novel drilling method combining thermal spallation and conventional drilling. This method aims to reduce the rock removal efforts of conventional drilling by thermally assisting the drilling process by flame jets. Laboratory experiments are conducted on the combined drilling concept by studying the effects of flame treatments on the mechanical strength of hard and soft rocks. In addition, investigation on the interaction between the rock and a cutting tool, permits to show that the combined method can drastically improve the drilling performance in terms of rate of penetration, bit wearing and the required mechanical energy to remove the material. As a proof-of-concept of the method, a field demonstration is presented, where the technology is implemented in a conventional drill rig in order to show the process feasibility as well as to quantify its performance under realistic conditions.

26. 
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.

25. 
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.

24. 
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.

23. 
von 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.

22. 
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.

21. 
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]

20. 
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.

19. 
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.

18. 
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.

17. 
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.

16. 
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.

15. 
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.

14. 
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.

13. 
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.

12. 
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.

11. 
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]

10. 
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]

9. 
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.

8. 
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.

7. 
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.

6. 
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

5. 
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: https://www.researchgate.net/publication/274138343_Numerical_Study_of_Multi-Fluid_and_Multi-Level_Geothermal_System_Performance [accessed Jun 12, 2017].

4. 
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.

3. 
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: https://www.researchgate.net/publication/271702360_Superheating_of_Low-Temperature_Geothermal_Working_Fluids_to_Boost_Electricity_Production_Comparison_between_Water_and_CO2_Systems [accessed Jun 12, 2017].

2. 
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.

1. 
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: https://www.researchgate.net/publication/273193986_Geothermal_Energy_Production_at_Geologic_CO2_Sequestration_sites_Impact_of_Thermal_Drawdown_on_Reservoir_Pressure [accessed Jun 12, 2017].


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THESES

10. 
Hefny, M. Rock Physics and Heterogeneities Characterization Controlling Fluid flow in Reservoir Rocks, Dissertation, 2020. [Download PDF] [View Abstract]Integrating geothermal energy production with CO2 capture and sequestration (CCS) in deep saline aquifers or oil/gas reservoirs is a promising approach in order to stabilize atmospheric CO2 concentrations while producing a reliable net-zero energy supply. The underlying base energy system is a so-called CO2-Plume Geothermal (CPG) power plant, where the captured CO2 is circulated in geological reservoirs. Within these reservoirs, the CO2 is naturally geothermally heated, produced to the surface, where it is expanded in a turbine for generating electricity. It is then cooled and finally combined with any CO2 additional stream, from any CO2 emitter, before being re-injected into the subsurface reservoir. The CO2 re-injection along with the continued supply of captured CO2 results in the continued growth of the subsurface CO2 plume. To ensure that 100% of the subsurface-injected CO2 is eventually permanently stored underground, it is essential to predict the migration and distribution of the CO2 in the subsurface reservoir. In this way, injection and flow can be maximized through the reservoir while keeping the risk of leakage through the sealing caprock at a minimum. In this study, we combine laboratory experiments and numerical techniques to understand characteristic features of subsurface fluid migration/entrapment of CO2 occurring within deep geological formations. The results cover three main topics at different scales: Calibration of seismic data Seismic reflection imaging within the earth's upper crust may be greatly distorted due to the attenuation in seismic waves, particularly the high-frequency waves, passing through fluid-bearing rocks. Our approach is to understand the origin of seismic reflectors at the microscopic scale. Experimental measurements at ultrasonic frequencies and under high confining pressures were performed to explore the link between the intrinsic rock properties (i.e. mineralogy, porosity, grain density, permeability) and the characteristics seismic response. We provide a unique set of seismic parameters necessary to calibrate seismic surveys in the Swiss Molasse Basin. This calibration of the seismic data provides a starting point for generating a synthetic seismic trace, based on the calculated reflection coefficients. The synthetic trace then correlates with both the seismic field data and the well logs in order to dynamically simulate wave propagation in the porous (and saturated) media. Our results improve the seismic interpretations of the geological reservoir and caprock geometries. CPG reservoir flow impedance Petrophysical properties of subsurface reservoirs are generally poorly understood, which increases the uncertainty related to the CO2 potential and storage security as well as the potential to use the CO2, stored in the reservoir, to produce geothermal energy. To this end, we investigate whether the Nubian Sandstone (a common reservoir rock found in the Gulf of Suez at depths of 2.5 to 4.4 km, with a geothermal gradient of 35.7 oC/km, confined by multiple aquitards) can serve as a CCS/CPG subsurface target. We combine field permeability estimates with laboratory measurements to provide constraints for reservoir modelling to estimate the power generation potential of the Nubian Sandstone reservoir. The reservoir geometry is constrained by seismic surveys, which show that the region of interest has several extensional faults with an accumulative dip-slip displacement of 810 m. We estimate the reservoir flow impedance [kPa.s/kg] for each compartmentalized block using a 1D analytical Darcy solution, developed for a single-phase fluid in an inverted 5-spot well-pattern configuration. With this flow impedance, we determine a potential net electric power of 1137 kWe for the deepest fault block (depth: 4.0 km, surface area: 1.0 km2, pressure: 40 MPa, well diameter: 0.41 m). This potential net power decreases by 47% for a smaller well diameter of 0.17 m, and 88% for over-pressurized zones (pressure: 62 MPa). Overall, we estimate a potential net electric power generation capacity for the entire field (with a reservoir footprint of 15 km2) of 12 MWe and a Levelized Cost Of Electricity (LCOE) of less than 150 $/MWh (0.15 $/kWh), depending on the availability of infrastructure and other resources (i.e. CO2 sources, geophysical exploration, and the existence of a well network). These results substantially reduce uncertainties in assessing the geothermal prospect in the Hammam Faraun hot spring region, Sinai Peninsula, Egypt. Capillary trapping: Implication for CCS This study describes how digital rock physics investigations compare with laboratory experiments to quantify two-phase fluid flow properties. Three-dimensional images, with a voxel size of 0.65 m3 of a Nubian Sandstone 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 perform sequential primary drainage--main imbibition cycle of quasi-static invasion to quantify: (1) the CO2 and brine relative permeability curves, (2) the effect of initial wetting-phase saturation (i.e. the saturation at the point of reversal from drainage to imbibition) on the residual–trapping potential, and (3) study the relative permeability–saturation hysteresis. The results illustrate the sensitivity of the pore-scale fluid-displacement and trapping processes on some key parameters (i.e. advancing contact angle, pore-body-to-throat aspect ratio, and initial wetting-phase saturation) and improve our understanding of the potential magnitude of capillary trapping in Nubian Sandstone. Finally, we are developing a numerical model in MOOSE (Multiphysics Object Oriented Simulation Environment) to (1) quantify the CO2 saturation profiles at the reservoir-scale and (2) simulate the reservoir behaviour under different physical processes and different well pattern configurations. We base our reservoir simulations on a well-established static geological model from an offshore oilfield in the Gulf of Suez (the model that has been developed under CPG reservoir flow impedance part; Chapter 3), utilizing the physical properties and two-phase fluid flow behavior properties that we have obtained from the Capillary trapping part (Chapter 4). The first results of the reservoir simulations show how the CO2-plume evolves over time and predict a heat extraction potential for the heterogeneous reservoirs.

9. 
Lima, M. Evolution of permeability of natural fractures due to THMC processes in the context of CO2-based reservoir applications, Dissertation, 242 pp., 2020. [View Abstract]As reserves decline and development costs increase, especially in frontier regions, understanding and preventing formation damage is crucial to guarantee the economic feasibility of exploitation of deep subsurface geological reservoirs. Formation damage is characterized by a zone of reduced permeability, potentially caused by coupled thermo-hydraulic-mechanical-chemical (THMC) processes that arise during typical operations, such as injection of fluids that are not preexisting in the reservoir, e.g., dry carbon dioxide. In this context, carbon dioxide injection can be considered as a perturbation to the presupposed equilibrated system. Injection of supercritical carbon dioxide (scCO2) into geological reservoirs is involved in several geoengineering activities, such as geothermal energy extraction, hydrocarbon production, and geological CO2 storage. In fractured-dominated reservoirs, THMC effects arising from scCO2 injection into brine-filled formations have a high potential of impairing the reservoir permeability due to the particularities of flow through fractures. These THMC effects act in a coupled and complex manner. Despite wide research on different effects impacting reservoir permeability, there is a need to conduct further studies, especially through laboratory experiments, to determine the hydraulic conductivity of fractures under prescribed combinations of confining stress, temperature and fluid chemical imbalance. This work focuses on fracture permeability behavior subjected to coupled THMC processes, by investigating: (1) the impact of effective normal stress on fracture absolute permeability in numerical simulations, supported by laboratory experiments, and on relative permeability curves in laboratory experiments; (2) the impact of temperature on fracture absolute permeability in laboratory experiments; and (3) the impact of mineral precipitation of fracture absolute permeability in numerical simulations. All rock specimens in this work are granodiorite specimens obtained from the Grimsel Test Site (GTS), Switzerland. Therefore, the originality of this work lies in providing results on natural fractures submitted to coupled THMC effects, which are challenging to predict but, if the morphology of the fracture is available, the investigations potentially give valuable insights. First, to evaluate the impact of effective normal stress, sigma', on fracture permeability, laboratory and numerical studies were performed. For the numerical studies, aperture fields of six naturally fractured specimens were first obtained under zero-stress conditions, via photogrammetric scans, and afterward under effective normal stress conditions of 2–30 MPa and temperatures of 25–400 oC, by means of a contact mechanics model. Both, mechanical and hydraulic apertures decreased with an increasing \sigma' and temperature, and the results show that the distributions of fracture apertures have a direct impact on the fracture permeability response. Laboratory single-phase flow-through tests conducted in a GTS fractured specimen under effective normal stress conditions of 2–15 MPa also showed decreases of fracture permeability due to increasing \sigma'. Additionally, laboratory tests were conducted to investigate the drying process of brine by scCO2 injection into another GTS fractured specimen, under conditions of 5-10 MPa effective normal stresses. A novel approach was developed to delineate the evolution of brine saturation and relative permeability from measurements of fluid production and pressure gradient across the specimen. Results from the laboratory experiments revealed lower mobility of brine and higher mobility of the scCO2 phase under higher effective normal compressive stresses. The analysis of relative permeabilities and fractional flow also suggested that higher effective normal compressive stresses increased channelling and decreased brine displacement efficiencies. Finally, lowering effective normal compressive stresses seems to hinder water evaporation. Secondly, laboratory flow-through tests on naturally-fractured granodiorite specimens were performed to investigate the impact of temperature on fracture absolute permeability. Two specimens were subjected to a constant flow rate of deionized water and pressure gradient across the fracture was measured, while step-wise and constant-rate changes in temperature were applied to the pressure cell. For different levels of confining stresses (20-40 MPa), the temperature varied from 25 oC to 140 oC, for 2 to 3 cycles, yielding decreases in absolute permeability of 20-75%. This decrease was more pronounced during the cycles subjected to higher confining stresses. The tests show hysteretic behavior during individual load cycles, indicating temperature path-dependent behavior of permeability. Chemical analysis of the effluent samples suggests that the decrease in fracture absolute permeability is caused by THMC processes such as thermal dilation, mechanical creep, and pressure dissolution, triggered by high temperatures. Thirdly, to investigate the impact of mineral precipitation on fracture permeability, a novel numerical model was implemented into the MOOSE framework to simulate the injection of scCO2 into a brine-filled heterogeneous single fracture. The numerical model captures the changes in the fracture aperture distribution due to the volume of precipitated salt, which arises from the supersaturation of brine after water evaporation into the scCO2 stream. The simulations were carried out with aperture fields under effective normal stresses of 2--10 MPa. The results indicate impairments of fracture permeability due to mineral precipitation up to 21.98%, and larger impairments were observed for the cases of lower effective normal stresses. Interestingly, despite the experienced larger permeability reductions, lower effective normal stresses promoted lower volumes of precipitated salt, relative to the initial volume, when compared to the volumes of salt observed for higher sigma'. For cases of higher effective normal stress, the precipitated salt in regions that did not impair the permeability as much as it did for the cases of lower effective normal stresses. The simulation results demonstrate the importance of considering not only the overall reduction of the fracture volume when studying formation damage in heterogeneous fractures, but also the spatial distribution of the precipitate throughout the aperture field under the considered effective normal stress state. To sum up, this thesis highlights the importance of fracture closure driven by increasing temperature and effective normal stress, as well as clogging by mineral precipitation, as these effects can compromise the long-term operation of enhanced geothermal systems or the operation of any reservoir where the operation performance depends strongly on the transmissivity of natural or stimulated fractures.

8. 
Rossi, E. Combined Thermo-Mechanical Drilling Technology to Enhance Access to Deep Geo-Resources, Dissertation, ETH Zürich, 2020. [Download PDF]

7. 
Naets, I. Visualizing contact areas in roughfractures using 3D printing, MSc Thesis, ETH Zurich, 2018.

6. 
Li, S Inverse Modeling of Discrete Fracture Network Using Trans-Dimensional MCMC Method, MSc Thesis, ETH Zürich, 2018.

5. 
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.

4. 
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.

3. 
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.

2. 
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.

1. 
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.