# Prof. Dr. Martin O. Saar

###### Chair for Geothermal Energy and Geofluids

Prof. Dr. Martin O. Saar
Geothermal Energy & Geofluids
Institute of Geophysics
NO F 51.2
Sonneggstrasse 5
CH-8092 Zurich Switzerland

Contact
 Phone +41 44 632 3465 Email saarm(at)ethz.ch

 Dominique Ballarin Dolfin Phone +41 44 632 3465 Email ballarin(at)ethz.ch

My research interests are in geophysical fluid dynamics of subsurface multiscale, multiphase, multicomponent, reactive fluid (groundwater, hydrocarbon, CO2) and energy (heat, pressure) transport, such as water- and CO2-based geothermal energy utilization, geologic CO2 storage, grid-scale energy storage, enhanced oil recovery, and groundwater flow. Methods include computer simulations, laboratory experiments, and field analyses.

This professorship and the associated Geothermal Energy and Geofluids (GEG) Group is generously endowed by the Werner Siemens Foundation, which is hereby gratefully acknowledged.

## Publications

### PROCEEDINGS REFEREED

 8 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, Proceedings ARMA 2017, (in press). AbstractThe 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. / no_download 7 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, Proceedings of the 42nd Workshop on Geothermal Reservoir Engineering Stanford University, 2017. 6 Garapati, N., J. Randolph, S. Finsterle, and M.O. Saar Simulating Reinjection of Produced Fluids Into the Reservoir, Proceedings of 41st Workshop on Geothermal Reservoir Engineering, 2016. AbstractABSTRACT 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. / 5 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, 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. AbstractRecent 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]. / 4 Garapati, N., J.B. Randolph, and M.O. Saar Superheating Low-Temperature Geothermal Resources to Boost Electricity Production, Proceedings of the 40th Workshop on Geothermal Reservoir Engineering 2015, 2, pp. 1210-1221, 2015. AbstractLow-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]. / 3 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, Proceedings World Geothermal Congress 2015, 2015. AbstractWe 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]. / 2 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, Proceedings to the World Geothermal Congress, 2015. AbstractSedimentary 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 / 1 Bailey, P., J. Myre, S.C.D. Walsh, D.J. Lilja, and M.O. Saar Accelerating Lattice Boltzmann Fluid Flow Simulations Using Graphics Processors, IEEE, pp. 550-557, 2009. AbstractLattice Boltzmann Methods (LBM) are used for the computational simulation of Newtonian fluid dynamics. LBM-based simulations are readily parallelizable; they have been implemented on general-purpose processors, field-programmable gate arrays (FPGAs), and graphics processing units (GPUs). Of the three methods, the GPU implementations achieved the highest simulation performance per chip. With memory bandwidth of up to 141 GB/s and a theoretical maximum floating point performance of over 600 GFLOPS, CUDA-ready GPUs from NVIDIA provide an attractive platform for a wide range of scientific simulations, including LBM. This paper improves upon prior single-precision GPU LBM results for the D3Q19 model by increasing GPU multiprocessor occupancy, resulting in an increase in maximum performance by 20%, and by introducing a space-efficient storage method which reduces GPU RAM requirements by 50% at a slight detriment to performance. Both GPU implementations are over 28 times faster than a single-precision quad-core CPU version utilizing OpenMP. /

### THESES

 2 Saar, M.O. Geological Fluid Mechanics Models at Various Scales, Dissertation University of California, Berkeley, 153 pp., 2003. AbstractIn this dissertation, I employ concepts from fluid mechanics to quantitatively investigate geological processes in hydrogeology and volcanology. These research topics are addressed by utilizing numerical and analytical models but also by conducting field and lab work. Percolation theory is of interest to a wide range of physical sciences and thus warrants research in itself. Therefore, I developed a computer code to study percolation thresholds of soft-core polyhedra. Results from this research are applied to study the onset of yield strength in crystal-melt suspensions such as magmas. Implications of yield strength development in suspensions, marking the transition from Newtonian to Bingham fluid, include the pahoehoe-‘a’a transition and the occurrence of effusive versus explosive eruptions. I also study interactions between volcanic processes and groundwater as well as between groundwater and seismicity (hydroseismicity). In the former case, I develop numerical and analytical models of coupled groundwater and heat transfer. Here, perturbations from a linear temperature-depth profile are used to determine groundwater flow patterns and rates. For the hydroseismicity project I investigate if seasonal elevated levels of seismicity at Mt. Hood, Oregon, are triggered by groundwater recharge. Both hydroseismicity and hydrothermal springs occur on the southern flanks of Mt. Hood. This suggests that both phenomena are related while also providing a connection between the research projects involving groundwater, heat flow, and seismicity. Indeed, seismicity may be necessary to keep faults from clogging thus allowing for sustained activity of hydrothermal springs. Finally, I present research on hydrologically induced volcanism, where a process similar to the one suggested for hydroseismicity is invoked. Here, melting of glaciers, or draining of lakes, during interglacial periods reduce the confining pressure in the subsurface which may promote dike formation and result in increased rates of volcanism. In general, problems discussed in this dissertation involve interactions among processes that are traditionally associated with separate research disciplines. However, numerous problems in the geosciences require a multidisciplinary approach, as demonstrated here. In addition, employing several analytical and numerical methods, such as signal processing, inverse theory, computer modeling, and percolation theory, allows me to study such diverse processes in a quantitative way. / 1 Saar, M.O. The Relationship Between Permeability, Porosity, and Microstucture in Vesicular Basalts, MSc Thesis University of Oregon, 101 pp., 1998. AbstractThis thesis presents measurements of permeability, $$k$$, porosity, $$\Phi$$, and microstructural parameters of vesicular basalts. Measurements are compared with theoretical models. A percolation theory and a Kozeny-Carman model are used to interpret the measurements and to investigate relationships between porosity, microstructure, and permeability. Typical permeabilities for vesicular basalts are in the range of $$10^{-14}$$ < $$k$$ < $$10^{-10} m^2$$. Best permeability estimates, following power laws predicted by percolation theory, are obtained when samples are used that show 'impeded aperture widening' due to rapid cooling and no bubble collapse (scoria and some flow samples). However, slowly cooled diktytaxitic samples contain elongated, 'collapsed' bubbles. Measurements indicate that the vesicle pathway network remains connected and preserves high permeabilities. Image-analysis techniques are unsuccessful if used for Kozeny-Carman equation parameter determination for vesicular materials, probably because the average interbubble aperture size that determines $$k$$ is not resolved with such a technique. /

show/hide list of publications

### REFEREED PUBLICATIONS IN JOURNALS

 8 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, Proceedings ARMA 2017, (in press). AbstractThe 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. / no_download 7 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, Proceedings of the 42nd Workshop on Geothermal Reservoir Engineering Stanford University, 2017. 6 Garapati, N., J. Randolph, S. Finsterle, and M.O. Saar Simulating Reinjection of Produced Fluids Into the Reservoir, Proceedings of 41st Workshop on Geothermal Reservoir Engineering, 2016. AbstractABSTRACT 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. / 5 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, 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. AbstractRecent 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]. / 4 Garapati, N., J.B. Randolph, and M.O. Saar Superheating Low-Temperature Geothermal Resources to Boost Electricity Production, Proceedings of the 40th Workshop on Geothermal Reservoir Engineering 2015, 2, pp. 1210-1221, 2015. AbstractLow-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]. / 3 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, Proceedings World Geothermal Congress 2015, 2015. AbstractWe 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]. / 2 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, Proceedings to the World Geothermal Congress, 2015. AbstractSedimentary 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 / 1 Bailey, P., J. Myre, S.C.D. Walsh, D.J. Lilja, and M.O. Saar Accelerating Lattice Boltzmann Fluid Flow Simulations Using Graphics Processors, IEEE, pp. 550-557, 2009. AbstractLattice Boltzmann Methods (LBM) are used for the computational simulation of Newtonian fluid dynamics. LBM-based simulations are readily parallelizable; they have been implemented on general-purpose processors, field-programmable gate arrays (FPGAs), and graphics processing units (GPUs). Of the three methods, the GPU implementations achieved the highest simulation performance per chip. With memory bandwidth of up to 141 GB/s and a theoretical maximum floating point performance of over 600 GFLOPS, CUDA-ready GPUs from NVIDIA provide an attractive platform for a wide range of scientific simulations, including LBM. This paper improves upon prior single-precision GPU LBM results for the D3Q19 model by increasing GPU multiprocessor occupancy, resulting in an increase in maximum performance by 20%, and by introducing a space-efficient storage method which reduces GPU RAM requirements by 50% at a slight detriment to performance. Both GPU implementations are over 28 times faster than a single-precision quad-core CPU version utilizing OpenMP. /
 2 Saar, M.O. Geological Fluid Mechanics Models at Various Scales, Dissertation University of California, Berkeley, 153 pp., 2003. AbstractIn this dissertation, I employ concepts from fluid mechanics to quantitatively investigate geological processes in hydrogeology and volcanology. These research topics are addressed by utilizing numerical and analytical models but also by conducting field and lab work. Percolation theory is of interest to a wide range of physical sciences and thus warrants research in itself. Therefore, I developed a computer code to study percolation thresholds of soft-core polyhedra. Results from this research are applied to study the onset of yield strength in crystal-melt suspensions such as magmas. Implications of yield strength development in suspensions, marking the transition from Newtonian to Bingham fluid, include the pahoehoe-‘a’a transition and the occurrence of effusive versus explosive eruptions. I also study interactions between volcanic processes and groundwater as well as between groundwater and seismicity (hydroseismicity). In the former case, I develop numerical and analytical models of coupled groundwater and heat transfer. Here, perturbations from a linear temperature-depth profile are used to determine groundwater flow patterns and rates. For the hydroseismicity project I investigate if seasonal elevated levels of seismicity at Mt. Hood, Oregon, are triggered by groundwater recharge. Both hydroseismicity and hydrothermal springs occur on the southern flanks of Mt. Hood. This suggests that both phenomena are related while also providing a connection between the research projects involving groundwater, heat flow, and seismicity. Indeed, seismicity may be necessary to keep faults from clogging thus allowing for sustained activity of hydrothermal springs. Finally, I present research on hydrologically induced volcanism, where a process similar to the one suggested for hydroseismicity is invoked. Here, melting of glaciers, or draining of lakes, during interglacial periods reduce the confining pressure in the subsurface which may promote dike formation and result in increased rates of volcanism. In general, problems discussed in this dissertation involve interactions among processes that are traditionally associated with separate research disciplines. However, numerous problems in the geosciences require a multidisciplinary approach, as demonstrated here. In addition, employing several analytical and numerical methods, such as signal processing, inverse theory, computer modeling, and percolation theory, allows me to study such diverse processes in a quantitative way. / 1 Saar, M.O. The Relationship Between Permeability, Porosity, and Microstucture in Vesicular Basalts, MSc Thesis University of Oregon, 101 pp., 1998. AbstractThis thesis presents measurements of permeability, $$k$$, porosity, $$\Phi$$, and microstructural parameters of vesicular basalts. Measurements are compared with theoretical models. A percolation theory and a Kozeny-Carman model are used to interpret the measurements and to investigate relationships between porosity, microstructure, and permeability. Typical permeabilities for vesicular basalts are in the range of $$10^{-14}$$ < $$k$$ < $$10^{-10} m^2$$. Best permeability estimates, following power laws predicted by percolation theory, are obtained when samples are used that show 'impeded aperture widening' due to rapid cooling and no bubble collapse (scoria and some flow samples). However, slowly cooled diktytaxitic samples contain elongated, 'collapsed' bubbles. Measurements indicate that the vesicle pathway network remains connected and preserves high permeabilities. Image-analysis techniques are unsuccessful if used for Kozeny-Carman equation parameter determination for vesicular materials, probably because the average interbubble aperture size that determines $$k$$ is not resolved with such a technique. /