Mission Statement
The Geothermal Energy & Geofluids group is endowed by the Werner Siemens Foundation and investigates reactive fluid (water, CO2, CxHy, N2) and (geothermal) energy (heat, pressure) transfer in the Earth’s crust employing computer simulations, laboratory experiments and field analyses to gain fundamental insights and to address a wide range of societal goals and concerns.
GEG News
27.03.2020
Doctoral Examination Mahmoud Hefny
On Thursday, March 26th, Mahmoud Hefny has successfully defended his PhD thesis, entitled: “Rock physics and heterogeneities characterization controlling fluid flow in reservoir rocks”.
25.03.2020
Doctoral Examination Philipp Schädle
On Tuesday, March 24th, Philipp Schädle has successfully defended his PhD thesis, entitled: “Flow and transport through fractured rock – numerical approaches to account for fracture heterogeneity”.
11.03.2020
Up-to date news from the CORONAVIRUS RESOURCE CENTER at Johns Hopkins University (USA)
Corona Virus map from John Hopkins University
For information on ETH’s rules and recommendations concerning the Coronavirus situation
read here
03.02.2020
New photo gallery of geothermal exploration for green energy in Mongolia. Friedemann Samrock
The burning of coal for heating is a major cause of harmful air pollution and massive greenhouse gas emissions in Mongolia’s cities. This photo gallery provides authentic insights into the progress of introducing a scientific framework for geothermal exploration.
read more
08.01.2020
Doctoral Examination Edoardo Rossi
On Tuesday, January 7th, Edoardo Rossi has successfully passed the Doctoral Examination. Edoardo’s PhD thesis title: “Combined Thermo-Mechanical Drilling technology to enhance access to deep geo-resources”.
01.12.2019
Prix d’Excellence by the Enovo Foundation
Marthe Faber has been awarded the ‘Prix d’Excelence’ by the Enovo Fondation and its partners, the Association da Vinci and l’ANEIL (National Association of Luxembourg Engineering Students). The prize is awarded to students with the best thesis grade in engineering. Marthe conducted the research for her MSc thesis ‘Usage Of Inter-Site Electromagnetic Transfer Functions In Exploration For Geothermal Resources’ at GEG in 2018.
read more – link to the Foundation de Luxembourg
read more – link to the Foundation de Luxembourg
Marthe Faber
20.12.2018
Top Ranking for ETH Zurich’s Earth Sciences in Nature Index 2018
ETH is listed number one among the 200 best academic institutions in Earth & Environmental Sciences in the Nature Index 2018. This index is based on publications in the 82 journals selected as the most important journals by two panels of scientists, which are independent of the Nature Publishing Group.
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25.10.2018
SNF Ambizione Grant
Dr. Allan Leal has been awarded an SNF Ambizione grant to develop innovative computational methods for ultra-fast simulations of coupled physical and chemical processes using machine learning and GPU parallel computing, starting on January 1, 2019.
Link to personal homepage of Allan Leal
Link to personal homepage of Allan Leal
01.10.2018
SNF R4D Grant
Dr. Friedemann Samrock has been awarded an SNF R4D grant to develop a geoscientific framework for geothermal exploration and energy utilization in Mongolia, started on September 1, 2018.
Link to Project’s page of SNF
Link to Project’s page of SNF
Inauguration of commemorative plaque for the Werner Siemens Foundation
12.09.2017
© Felix Wey
Werner Siemens Foundation visits the Geothermal Energy & Geofluids Group and has a guided tour at the GEG Laboratories.
read more on ETH Foundation webpage
read more on News of GEG webpage
GEG Videos
Inexhaustible resource of clean, renewable Geothermal Energy.
© ETH Zurich
Grimsel rock lab, feasibility of geothermal power plants.
© ETH Zurich
Grimsel rock laboratory, safer drilling methods.
© 3sat nano
© 2018 ETH Zurich
GEG Events
20.05.2020
A Novel Stochastic Coulomb Friction Model: Implications for the State of Crustal StressShihuai Zhang (GEG group presentation)
GEG Meetings, ETH Zurich read more
24.05.2020
Laser-Induced Fluorescence (LIF) study of solute transport in 3D-printed fractured porous mediaMehrdad Ahkami (contributed talk)
InterPore 2020, Qingdao read more
NEXT EVENT
27.05.2020
Current development of a micro-scale reactive transport solverPowei Huang (GEG group presentation)
GEG Meetings, ETH Zurich read more
12.08.2020
Advanced drilling technologies to improve the economics of deep geo-resource utilizationEdoardo Rossi (talk)
MITAB 2020, Massachusetts Institute of Technology (MIT), Boston, USA read more
GEG Open Positions
No open positions available at the moment
GEG Paper Alerts
Injection-induced slip heterogeneity on faults in shale reservoirs
Jia, Y., W. Wu, and X.-Z. Kong 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. Solute tracer test quantification of the effects of hot water injection into hydraulically stimulated crystalline rock
Kittilä, A., M.R. Jalali, M.O. Saar, and X.-Z. Kong Geothermal Energy, (in press). [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. Magnetotelluric multiscale 3-D inversion reveals crustal and upper mantle structure beneath the Hangai and Gobi-Altai region in Mongolia
Kaufl, S., V. Grayver, J. Comeau, V. Kuvshinov, M. Becken, J. Kamm, B. Erdenechimeg, and D. Sodnomsambuu 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. Evidence for terrane boundaries and suture zones across Southern Mongolia detected with a 2-dimensional magnetotelluric transect
Comeau, J., M. Becken, S. Kaufl, V. Grayver, V. Kuvshinov, Ts. Shoovdor, B. Erdenechimeg, and D. Sodnomsambuu 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. A Numerical Investigation into Key Factors Controlling Hard Rock Excavation via Electropulse Stimulation
Vogler, D., S.D.C. Walsh, and M.O. Saar Journal of Rock Mechanics and Geotechnical Engineering, (in press). [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.
Increased Power Generation due to Exothermic Water Exsolution in CO2 Plume Geothermal (CPG) Power Plants
Fleming, M.R., B.M. Adams, T.H. Kuehn, J.M. Bielicki, and M.O. Saar Geothermics, (in press). [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. The influence of thermal treatment on rock-bit interaction: A study of a combined thermo-mechanical drilling (CTMD) concept
Rossi, E. , M.O. Saar, and Ph. Rudolf von Rohr Geothermal Energy, (in press). [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 experimental 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. Guaranteed and computable error bounds for approximations constructed by an iterative decoupling of the Biot problem
Kyas, S., S. Repin, J. M. Nordbotten, and K. Kumar Computers & Mathematics with Applications, (in press). [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. Accelerating Reactive Transport Modeling: On‑Demand Machine Learning Algorithm for Chemical Equilibrium Calculations
Leal, A. M. M., S. Kyas, D. Kulik, and M. O. Saar Transport in Porous Media, pp. 1-44, 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. Measurement of the Natural Convection Heat Transfer in a Magnesium Oxide Electrolytic Cell Concept
Venstrom, L, J Yager, T Vervynckt, J Ogland-Hand, and S Nudehi Thermal Science and Engineering Applications, (in press). [Download PDF] Combining natural gas recovery and CO2-based geothermal energy
extraction for electric power generation
Ezekiel, J., A. Ebigbo, B. M. Adams, and M. O. Saar Applied Energy, 269C/115012, 2020. [Download PDF] Scaling of L-mode heat flux for ITER and COMPASS-U divertors, based on five tokamaks
Horacek, J., et al., M. Ezzat, et al., EUROfusion MST1 Team, JET Contributors, and MAST-U Team Nuclear Fusion, (in press). [Download PDF] [View Abstract]This contribution aims to improve existing scalings of the L-mode power decay length, especially for plasma configurations with strike points at the ITER-relevant location - closed vertical divertor targets. We propose 13 new scalings based on data from the tokamaks JET, EAST, MAST, Alcator C-mod and COMPASS, and validate them against the output of the 2D turbulence code HESEL. The analysis covers 500 divertor heat flux profiles (obtained by probes or IR cameras), measured in L-mode discharges with varying 12 global plasma parameters (all well predictable). We find that two previously published scalings [Eich, J.Nucl.Mat. 438 (2013) S72; Scarabosio, J.Nucl.Mat. 438 (2013) S426] (based on outer targets of AUG and JET) describe well all the JET, C-mod and COMPASS profiles, not only at outer horizontal and vertical targets, but surprisingly also at the inner vertical targets. In contrast, EAST, HESEL and especially MAST data are poorly described by these scalings. We therefore derive 13 new scalings describing 85-92 % of the measured decay lengths variability. The reader is suggested to use as many as possible scalings from here, depending on which parameters have available. Despite the fact that the scaling candidates are based on different parameters, predictions for the highest current L-modes in ITER are all very similar. Just prior to the L-H transition, in ITER baseline scenario, all the scalings predict values 2.5-3.5 mm (mapped to outer midplane), shorter for a single scaling based on predicted stored plasma energy. 1.6-2.6 mm is predicted for the COMPASS-Upgrade tokamak. In attached L-mode plasma, our results imply (using significant assumptions) steady-state surface-perpendicular heat flux around 10 MW/m2 for ITER, and 20 MW/m2 for COMPASS-Upgrade. Simulation of rock failure modes in thermal spallation drilling
Vogler, D., S.D.C. Walsh, Ph. Rudolf von Rohr, and M.O. Saar Acta Geotechnica, 2020. [Download PDF] [View Abstract]Thermal spallation drilling is a contact-less means of borehole excavation that works by exposing a rock surface to a high-temperature jet flame. In this study, we investigate crucial factors for the success of such thermal drilling operations using numerical simulations of the thermomechanical processes leading to rock failure at the borehole surface. To that end, we integrate a model developed for spalling failure with our thermomechanical simulations. In particular, we consider the role of material heterogeneities, maximum jet-flame temperature and maximum jet-flame temperature rise time on the onset of inelastic deformation and subsequent damage. We further investigate differences in energy consumption for the studied system configurations. The simulations highlight the importance of material composition, as thermal spallation is favored in fine-grained material with strong material heterogeneity. The model is used to test the relationship between the jet-flame temperature and the onset of thermal spallation. A combined thermo-mechanical drilling technology for deep geothermal and hard rock reservoirs
Rossi, E., S. Jamali, V. Wittig, M.O. Saar, and Ph. Rudolf von Rohr 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. Field test of a Combined Thermo-Mechanical Drilling technology. Mode I: Thermal spallation drilling
Rossi, E., S. Jamali, M.O. Saar, and Ph. Rudolf von Rohr 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. Field test of a Combined Thermo-Mechanical Drilling technology. Mode II: Flame-assisted rotary drilling
Rossi, E., S. Jamali, D. Schwarz, M.O. Saar, and Ph. Rudolf von Rohr 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. Characterization of the effects of hydraulic stimulation with tracer-based temporal moment analysis and tomographic inversion
Kittilä, A., M.R. Jalali, M. Somogyvári, K.F. Evans, M.O. Saar, and X.-Z. Kong 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. On the directional dependency of Mode I fracture toughness in anisotropic rocks
Nejati, M., A. Aminzadeh, T. Driesner, and M.O. Saar 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. Changing Flow Paths Caused by Simultaneous Shearing and Fracturing Observed During Hydraulic Stimulation
Krietsch, H., L. Villiger, J. Doetsch, V. Gischig, K.F. Evans, B. Brixel, M.R. Jalali, S. Loew, D. Giardini, and F. Amann Geophysical Research Letters, 47, 2020. [Download PDF] Modelling of hydro-mechanical processes in heterogeneous fracture intersections using a fictitious domain method with variational transfer operators
von Planta, C., D. Vogler, X. Chen, M.G.C. Nestola, M.O. Saar, and R. Krause 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. Comparative study of Basel EGS reservoir faults inferred from analysis of microseismic cluster datasets with fracture zones obtained from well log analysis
Ziegler, M., and K.F. Evans Journal of Structural Geology, 130, 2020. A lattice-Boltzmann study of permeability-porosity relationships and mineral precipitation patterns in fractured porous media
Ahkami, M., A. Parmigiani, P.R. Di Palma, M.O. Saar, and X.-Z. Kong 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. Simulating Electropulse Fracture of Granitic Rock
Walsh, S.D.C., and D. Vogler 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.