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Ahmad Ghassemi - One of the best experts on this subject based on the ideXlab platform.
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a reactive thermo poroelastic analysis of Water Injection into an enhanced geothermal reservoir
Geothermics, 2014Co-Authors: Chakra Rawal, Ahmad GhassemiAbstract:Abstract Coupled thermo-poro-chemo-mechanical processes in geothermal systems impact the reservoir response during Injection and production procedures by affecting fracture permeability. A three-dimensional numerical model is presented to analyze these processes during fluid Injection into geothermal reservoirs. The solid mechanics aspect of the problem is computed using the displacement discontinuity boundary element method (BEM) while transport processes within the facture are modeled using the finite element method (FEM). The FEM and BEM formulations are integrated to set up a system of equations for unknown temperature, pressure, concentration, and fracture aperture. The fluid diffusion, heat conduction and solute diffusion in the reservoir are treated using BEM so that the need of infinite reservoir domain discretization is eliminated. The numerical model is used to analyze the fracture response to non-isothermal reactive flow in EGS. Numerical examples of SiO 2 undersaturated-Cold Water Injection into the geothermal reservoir show that silica dissolves from the rock matrix, increasing the fracture aperture. The zone of silica dissolution spreads into the fracture with continuous fluid Injection. At large Injection times, thermoelastic stress has a greater impact on fracture aperture compared to poroelastic stress. Simulations that consider natural fracture stiffness heterogeneity show the development of a non-uniform flow path within the crack, with lower rock matrix cooling and thus enhanced silica reactivity in the high stiffness regions. As a result, areas of higher joint normal stiffness show lower aperture increases in response to the thermo-poroelastic processes, but a higher aperture expansion due to silica dissolution. Depending on the injectate saturation state with respect to quartz, silica is added or removed from the rock matrix. This process is likely to impact the rock matrix properties and its mechanical response to stress perturbations associated with fluid circulation.
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a three dimensional integral equation model for calculating poro and thermoelastic stresses induced by Cold Water Injection into a geothermal reservoir
International Journal for Numerical and Analytical Methods in Geomechanics, 2009Co-Authors: X X Zhou, Ahmad Ghassemi, A H D ChengAbstract:Poro-mechanical and thermo-mechanical processes change the fracture aperture and thus affect the Water flow pattern in the fracture during the Cold Water Injection into enhanced geothermal systems (EGS). In addition, the stresses generated by these processes contribute to the phenomenon of reservoir seismicity. In this paper, we present a three-dimensional (3D) partially coupled poro-thermoelastic model to investigate the poroelastic and thermoelastic effects of Cold Water Injection in EGS. In the model, the lubrication fluid flow and the convective heat transfer in the fracture are modeled by the finite element method, while the pore fluid diffusion and heat conductive transfer in the reservoir matrix are assumed to be 3D and modeled by the boundary integral equation method without the need to discretize the reservoir. The stresses at the fracture surface and in the reservoir matrix are obtained from the numerical model and can be used to assess the variation of in situ stress and induced seismicty with Injection/extraction. Application of the model shows that rock cooling induces large tensile stresses and increases fracture conductivity, whereas the rock dilation caused by fluid leakoff decreases fracture aperture and increases compressive total stresses around the Injection zone. However, increases in pore pressure reduce the effective stresses and can contribute to rock failure, fracture slip, and microseismic activity. Copyright © 2009 John Wiley & Sons, Ltd.
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effects of heat extraction on fracture aperture a poro thermoelastic analysis
Geothermics, 2008Co-Authors: Ahmad Ghassemi, Andrew Nygren, Alexander H D ChengAbstract:Poroelastic and thermoelastic effects of Cold-Water Injection in an enhanced (or engineered) geothermal system (EGS) are investigated by considering flow in a pre-existing fracture in a hot, rock matrix that could be permeable or impermeable. Assuming plane fracture geometry, expressions are derived for changes in fracture aperture caused by cooling and fluid leak-off into the matrix. The corresponding induced pressure profile is also calculated. The problem is analytically solved for the cases pertaining to a constant fluid Injection rate with a constant leak-off rate. Results show that although fluid loss from the fracture into the matrix reduces the pressure in the crack, the poroelastic stress associated with fluid leak-off tends to reduce the aperture and increase the pressure in the fracture. High rock stiffness and low fluid diffusivity cause the poroelastic contraction of the fracture opening to slowly develop in time. The maximum reduction of aperture occurs at the Injection point and become negligible near the extraction point. The solution also shows that thermally induced stress increases the fracture aperture near the Injection point and, as a result, the fluid pressure at this point is greatly reduced. The thermoelastic effects are particularly dominant near the inlet compared to those of poroelasticity, but are pronounced everywhere along the fracture for large times. Although poroelasticity associated with leak-off does not change the fracture aperture significantly for low permeability rocks, it can lead to pore pressure increase and cause nearby fractures to slip.
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a 3 d study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs
International Journal of Rock Mechanics and Mining Sciences, 2007Co-Authors: Ahmad Ghassemi, S Tarasovs, Alexander H D ChengAbstract:Heat extraction from enhanced geothermal systems (EGS) can greatly affect the behavior of joints and other discontinuities in the reservoir. Fracture permeability can change in response to fluid Injection/extraction, rock cooling, variations of stress field, and mineral dissolution/precipitation. The reduction in effective stress caused by pore pressure increase can cause the slippage of discontinuities, thus inducing seismicity. Studies have shown that thermal stresses generated by Cold Water Injection have a similar effect. In order to assess the influence of thermal stresses on fracture opening and slippage, a 3-D coupled heat extraction/thermal stress/elastic displacement discontinuity model is used in this study. The effects of each mechanism on fracture slip is estimated with particular reference to the Coso geothermal field. The results indicate that under typical field conditions, a substantial increase in fracture slip is observed when thermal stresses are taken into account. The temporal evolution of the thermal stresses suggests that the rock mass deformation will not cease upon stoppage of Water Injection, which can be a cause of delayed seismic activity.
Patrick F Dobson - One of the best experts on this subject based on the ideXlab platform.
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influence of Injection induced cooling on deviatoric stress and shear reactivation of preexisting fractures in enhanced geothermal systems
Geothermics, 2017Co-Authors: Pierre Jeanne, Jonny Rutqvist, Patrick F DobsonAbstract:Abstract Cold Water Injection into a hot, fractured, geothermal reservoir may trigger shear activation of pre-existing fractures that can help to enhance reservoir permeability, but may also result in unwanted seismicity. In this paper, we investigate through numerical modeling of a hypothetical geothermal reservoir how Injection-induced cooling may influence the potential for shear activation, paying special attention to the evolution of deviatoric stress under various stress regimes. In each case, we consider either a reservoir with homogeneous hydraulic properties or the presence of a more permeable fracture zone intersecting the Injection well. This fracture zone is either oriented in the maximum (SHmax) or minimum (Shmin) horizontal stress direction. Our main finding is that depending on the configuration, Injection-induced cooling stresses can favor or prevent shear reactivation of the preexisting fracture, and this effect can vary temporally and spatially.
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the northwest geysers egs demonstration project california part 2 modeling and interpretation
Geothermics, 2016Co-Authors: Jonny Rutqvist, Pierre Jeanne, Craig Hartline, Patrick F Dobson, Donald W. Vasco, Julio García, Lawrence Hutchings, Ankit Singh, Mark WaltersAbstract:Abstract In this paper, we summarize the results of coupled thermal, hydraulic, and mechanical (THM) modeling in support of the Northwest Geysers EGS Demonstration Project, which aims at enhancing production from a known High Temperature Reservoir (HTR) (280–400 °C) located under the conventional (240 °C) geothermal steam reservoir. The THM modeling was conducted to investigate geomechanical effects of Cold-Water Injection during the stimulation of the EGS, first to predict the extent of the stimulation zone for a given Injection schedule, and then to conduct interpretive analyses of the actual stimulation. By using a calibrated THM model based on historic Injection and microseismic data at a nearby well, we could reasonably predict the extent of the stimulation zone around the Injection well, at least for the first few months of Injection. However, observed microseismic evolution and pressure responses over the one-year stimulation-Injection revealed more heterogeneous behavior as a result of more complex geology, including a network of shear zones. Therefore, for an interpretive analysis of the one-year stimulation campaign, we included two sets of vertical shear zones within the model; a set of more permeable NW-striking shear zones and a set of less permeable NE-striking shear zones. Our modeling indicates that the microseismic events in this system are related to shear reactivation of pre-existing fractures, triggered by the combined effects of Injection-induced cooling around the Injection well and rapid (but small) changes in steam pressure as far as a kilometer from the Injection well. Overall, the integrated monitoring and modeling of microseismicity, ground surface deformations, reservoir pressure, fluid chemical composition, and seismic tomography depict an EGS system hydraulically bounded by some of the NE-striking low permeability shear zones, with the more permeable NW-striking shear zone providing liquid flow paths for stimulation deep (several kilometers) down into the HTR. The modeling indicates that a significant mechanical degradation (damage) inferred from seismic tomography, and potential changes in fracture porosity inferred from cross-well pressure responses, are related to shear rupture in the stimulation zone driven by both pressure and cooling effects.
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The Northwest Geysers EGS Demonstration Project, California: Pre-stimulation Modeling and Interpretation of the Stimulation
Mathematical Geosciences, 2015Co-Authors: Jonny Rutqvist, Pierre Jeanne, Craig Hartline, Patrick F Dobson, Donald W. Vasco, Julio García, Curtis M. Oldenburg, Mark WaltersAbstract:The Northwest Geysers Enhanced Geothermal System (EGS) demonstration project aims to create an EGS by directly and systematically injecting cool Water at relatively low pressure into a known High Temperature (280–400 °C) Zone (HTZ) located under the conventional (240 °C) geothermal steam reservoir at The Geysers geothermal field in California. In this paper, the results of coupled thermal, hydraulic, and mechanical (THM) analyses made using a model developed as part of the pre-stimulation phase of the EGS demonstration project is presented. The model simulations were conducted in order to investigate Injection strategies and the resulting effects of Cold-Water Injection upon the EGS system; in particular to predict the extent of the stimulation zone for a given Injection schedule. The actual Injection began on October 6, 2011, and in this paper a comparison of pre-stimulation model predictions with micro-earthquake (MEQ) monitoring data over the first few months of a one-year Injection program is presented. The results show that, by using a calibrated THM model based on historic Injection and MEQ data at a nearby well, the predicted extent of the stimulation zone (defined as a zone of high MEQ density around the Injection well) compares well with observed seismicity. The modeling indicates that the MEQ events are related to shear reactivation of preexisting fractures, which is triggered by the combined effects of Injection-induced cooling around the Injection well and small changes in steam pressure as far as half a kilometer away from the Injection well. Pressure-monitoring data at adjacent wells and satellite-based ground-surface deformation data were also used to validate and further calibrate reservoir-scale hydraulic and mechanical model properties. The pressure signature monitored from the start of the Injection was particularly useful for a precise back-calculation of reservoir porosity. The first few months of reservoir pressure and surface deformation data were useful for estimating the reservoir-rock permeability and elastic modulus. Finally, although the extent of the calculated stimulation zone matches the field observations over the first few months of Injection, the observed surface deformations and MEQ evolution showed more heterogeneous behavior as a result of more complex geology, including minor faults and fracture zones that are important for consideration in the analysis of energy production and the long-term evolution of the EGS system.
Sharad Kelkar - One of the best experts on this subject based on the ideXlab platform.
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a coupled thermo hydro mechanical modeling of fracture aperture alteration and reservoir deformation during heat extraction from a geothermal reservoir
Geothermics, 2017Co-Authors: S. N. Pandey, A. Chaudhuri, Sharad KelkarAbstract:Abstract Hot Water extraction and Cold Water Injection into an underground geothermal reservoir cause mechanical deformation of rock matrix and rock joints/fractures. That leads to alteration of hydraulic transmissivity. To study the evolution of reservoir transmissivity we performed coupled Thermo-Hydro-Mechanical (THM) simulations using a robust code called Finite Element for Heat and Mass Transfer (FEHM) for a 3-D domain with a single fracture connecting the Injection and production wells. Rock fracture was modeled as a thin equivalent porous medium. We established dynamic relations between the properties of the equivalent porous medium and fracture aperture. In this paper we discuss the alteration of fracture aperture due to heat extraction. The channeling of flow between Injection and production wells by THM effects causes faster temperature drawdown and reduces energy production. The model also predicted fracture opening near Injection well and closure at far field locations. We also simulated the aperture alteration for different joint stiffness, thermal expansion coefficients and rock matrix permeabilities. Increase in rock matrix permeability not only causes the leakage of injected Water but also increases matrix contraction due to cooling and therefore the aperture growth. Additionally we reported the effect of thermo-poro-elastic deformation on the expansion and contraction of the formation for different reservoir properties. We established that in the early-stages the compaction/expansion of the formation was controlled by pore pressure change but in the late-stage it was controlled by thermal contraction.
Alexander H D Cheng - One of the best experts on this subject based on the ideXlab platform.
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effects of heat extraction on fracture aperture a poro thermoelastic analysis
Geothermics, 2008Co-Authors: Ahmad Ghassemi, Andrew Nygren, Alexander H D ChengAbstract:Poroelastic and thermoelastic effects of Cold-Water Injection in an enhanced (or engineered) geothermal system (EGS) are investigated by considering flow in a pre-existing fracture in a hot, rock matrix that could be permeable or impermeable. Assuming plane fracture geometry, expressions are derived for changes in fracture aperture caused by cooling and fluid leak-off into the matrix. The corresponding induced pressure profile is also calculated. The problem is analytically solved for the cases pertaining to a constant fluid Injection rate with a constant leak-off rate. Results show that although fluid loss from the fracture into the matrix reduces the pressure in the crack, the poroelastic stress associated with fluid leak-off tends to reduce the aperture and increase the pressure in the fracture. High rock stiffness and low fluid diffusivity cause the poroelastic contraction of the fracture opening to slowly develop in time. The maximum reduction of aperture occurs at the Injection point and become negligible near the extraction point. The solution also shows that thermally induced stress increases the fracture aperture near the Injection point and, as a result, the fluid pressure at this point is greatly reduced. The thermoelastic effects are particularly dominant near the inlet compared to those of poroelasticity, but are pronounced everywhere along the fracture for large times. Although poroelasticity associated with leak-off does not change the fracture aperture significantly for low permeability rocks, it can lead to pore pressure increase and cause nearby fractures to slip.
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a 3 d study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs
International Journal of Rock Mechanics and Mining Sciences, 2007Co-Authors: Ahmad Ghassemi, S Tarasovs, Alexander H D ChengAbstract:Heat extraction from enhanced geothermal systems (EGS) can greatly affect the behavior of joints and other discontinuities in the reservoir. Fracture permeability can change in response to fluid Injection/extraction, rock cooling, variations of stress field, and mineral dissolution/precipitation. The reduction in effective stress caused by pore pressure increase can cause the slippage of discontinuities, thus inducing seismicity. Studies have shown that thermal stresses generated by Cold Water Injection have a similar effect. In order to assess the influence of thermal stresses on fracture opening and slippage, a 3-D coupled heat extraction/thermal stress/elastic displacement discontinuity model is used in this study. The effects of each mechanism on fracture slip is estimated with particular reference to the Coso geothermal field. The results indicate that under typical field conditions, a substantial increase in fracture slip is observed when thermal stresses are taken into account. The temporal evolution of the thermal stresses suggests that the rock mass deformation will not cease upon stoppage of Water Injection, which can be a cause of delayed seismic activity.
Jonny Rutqvist - One of the best experts on this subject based on the ideXlab platform.
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influence of Injection induced cooling on deviatoric stress and shear reactivation of preexisting fractures in enhanced geothermal systems
Geothermics, 2017Co-Authors: Pierre Jeanne, Jonny Rutqvist, Patrick F DobsonAbstract:Abstract Cold Water Injection into a hot, fractured, geothermal reservoir may trigger shear activation of pre-existing fractures that can help to enhance reservoir permeability, but may also result in unwanted seismicity. In this paper, we investigate through numerical modeling of a hypothetical geothermal reservoir how Injection-induced cooling may influence the potential for shear activation, paying special attention to the evolution of deviatoric stress under various stress regimes. In each case, we consider either a reservoir with homogeneous hydraulic properties or the presence of a more permeable fracture zone intersecting the Injection well. This fracture zone is either oriented in the maximum (SHmax) or minimum (Shmin) horizontal stress direction. Our main finding is that depending on the configuration, Injection-induced cooling stresses can favor or prevent shear reactivation of the preexisting fracture, and this effect can vary temporally and spatially.
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the northwest geysers egs demonstration project california part 2 modeling and interpretation
Geothermics, 2016Co-Authors: Jonny Rutqvist, Pierre Jeanne, Craig Hartline, Patrick F Dobson, Donald W. Vasco, Julio García, Lawrence Hutchings, Ankit Singh, Mark WaltersAbstract:Abstract In this paper, we summarize the results of coupled thermal, hydraulic, and mechanical (THM) modeling in support of the Northwest Geysers EGS Demonstration Project, which aims at enhancing production from a known High Temperature Reservoir (HTR) (280–400 °C) located under the conventional (240 °C) geothermal steam reservoir. The THM modeling was conducted to investigate geomechanical effects of Cold-Water Injection during the stimulation of the EGS, first to predict the extent of the stimulation zone for a given Injection schedule, and then to conduct interpretive analyses of the actual stimulation. By using a calibrated THM model based on historic Injection and microseismic data at a nearby well, we could reasonably predict the extent of the stimulation zone around the Injection well, at least for the first few months of Injection. However, observed microseismic evolution and pressure responses over the one-year stimulation-Injection revealed more heterogeneous behavior as a result of more complex geology, including a network of shear zones. Therefore, for an interpretive analysis of the one-year stimulation campaign, we included two sets of vertical shear zones within the model; a set of more permeable NW-striking shear zones and a set of less permeable NE-striking shear zones. Our modeling indicates that the microseismic events in this system are related to shear reactivation of pre-existing fractures, triggered by the combined effects of Injection-induced cooling around the Injection well and rapid (but small) changes in steam pressure as far as a kilometer from the Injection well. Overall, the integrated monitoring and modeling of microseismicity, ground surface deformations, reservoir pressure, fluid chemical composition, and seismic tomography depict an EGS system hydraulically bounded by some of the NE-striking low permeability shear zones, with the more permeable NW-striking shear zone providing liquid flow paths for stimulation deep (several kilometers) down into the HTR. The modeling indicates that a significant mechanical degradation (damage) inferred from seismic tomography, and potential changes in fracture porosity inferred from cross-well pressure responses, are related to shear rupture in the stimulation zone driven by both pressure and cooling effects.
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The Northwest Geysers EGS Demonstration Project, California: Pre-stimulation Modeling and Interpretation of the Stimulation
Mathematical Geosciences, 2015Co-Authors: Jonny Rutqvist, Pierre Jeanne, Craig Hartline, Patrick F Dobson, Donald W. Vasco, Julio García, Curtis M. Oldenburg, Mark WaltersAbstract:The Northwest Geysers Enhanced Geothermal System (EGS) demonstration project aims to create an EGS by directly and systematically injecting cool Water at relatively low pressure into a known High Temperature (280–400 °C) Zone (HTZ) located under the conventional (240 °C) geothermal steam reservoir at The Geysers geothermal field in California. In this paper, the results of coupled thermal, hydraulic, and mechanical (THM) analyses made using a model developed as part of the pre-stimulation phase of the EGS demonstration project is presented. The model simulations were conducted in order to investigate Injection strategies and the resulting effects of Cold-Water Injection upon the EGS system; in particular to predict the extent of the stimulation zone for a given Injection schedule. The actual Injection began on October 6, 2011, and in this paper a comparison of pre-stimulation model predictions with micro-earthquake (MEQ) monitoring data over the first few months of a one-year Injection program is presented. The results show that, by using a calibrated THM model based on historic Injection and MEQ data at a nearby well, the predicted extent of the stimulation zone (defined as a zone of high MEQ density around the Injection well) compares well with observed seismicity. The modeling indicates that the MEQ events are related to shear reactivation of preexisting fractures, which is triggered by the combined effects of Injection-induced cooling around the Injection well and small changes in steam pressure as far as half a kilometer away from the Injection well. Pressure-monitoring data at adjacent wells and satellite-based ground-surface deformation data were also used to validate and further calibrate reservoir-scale hydraulic and mechanical model properties. The pressure signature monitored from the start of the Injection was particularly useful for a precise back-calculation of reservoir porosity. The first few months of reservoir pressure and surface deformation data were useful for estimating the reservoir-rock permeability and elastic modulus. Finally, although the extent of the calculated stimulation zone matches the field observations over the first few months of Injection, the observed surface deformations and MEQ evolution showed more heterogeneous behavior as a result of more complex geology, including minor faults and fracture zones that are important for consideration in the analysis of energy production and the long-term evolution of the EGS system.
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analysis of Injection induced micro earthquakes in a geothermal steam reservoir the geysers geothermal field california
Lawrence Berkeley National Laboratory, 2008Co-Authors: Jonny Rutqvist, J Rutqvist, C M OldenburgAbstract:In this study we analyze relative contributions to the cause and mechanism of Injection-induced micro-earthquakes (MEQs) at The Geysers geothermal field, California. We estimated the potential for inducing seismicity by coupled thermal-hydrological-mechanical analysis of the geothermal steam production and Cold Water Injection to calculate changes in stress (in time and space) and investigated if those changes could induce a rock mechanical failure and associated MEQs. An important aspect of the analysis is the concept of a rock mass that is critically stressed for shear failure. This means that shear stress in the region is near the rock-mass frictional strength, and therefore very small perturbations of the stress field can trigger an MEQ. Our analysis shows that the most important cause for Injection-induced MEQs at The Geysers is cooling and associated thermal-elastic shrinkage of the rock around the injected fluid that changes the stress state in such a way that mechanical failure and seismicity can be induced. Specifically, the cooling shrinkage results in unloading and associated loss of shear strength in critically shear-stressed fractures, which are then reactivated. Thus, our analysis shows that cooling-induced shear slip along fractures is the dominant mechanism of Injection-induced MEQs at The Geysers.