Coupled Thermo-Hydro-Mechanical Simulations of A Supercritical Geothermal System
Understanding coupled processes is essential to properly assess the performance and risks of supercritical geothermal systems. These systems are found in especially hot reservoirs, at the deep root of volcanic areas, in which water is encountered in supercritical conditions because the pressure (p) and temperature (T) are above their critical values, i.e., p > 22.06 MPa and T > 373.95 ºC. Thus, supercritical geothermal systems experience strong induced deformations and stress redistributions induced by pore pressure variations and cooling during cold water reinjection performed to balance fluid depletion. While pressure changes are balanced in the injection and production well doublet, strong cooling occurs asymmetrically around the injection well, which significantly contracts the rock and causes a thermal stress reduction. The effective stress changes may lead to fault reactivation and induced seismicity. In order to investigate the potential of seismicity induced by cold water injection in deep supercritical geothermal reservoirs, we have performed coupled thermo-hydro-mechanical (THM) simulations of a doublet system with a steeply dipping conductive fault between the two wells. The fault is modeled as a continuum with higher permeability and compliance than the surrounding rock. We have performed THM finite element analyses with the object-oriented, C++ based and open-source finite element solver OpenGeoSys. We have implemented advanced features such as: i) porosity-dependent permeability, ii) full poro-mechanical coupling and iii) IAPWS-97 equations of state for water present in various phases. The porosity-dependent permeability evolves along with porosity changes, which in turn are a function of the changes of temperature, pore pressure and stress. The injected cold water absorbs heat from the rock mass on its way toward the production well, crossing the fault. Under conditions relevant for supercritical reservoirs, deformation is mainly controlled the cooling front and is less affected by pressure changes, which reach a pseudo-steady state shortly after the beginning of fluid circulation. Fluid velocity is highest in the fault, whereas major mechanical effects are a consequence of thermal deformation. We compare cold water re-injection with a case of isothermal injection, showing that the rate of seismicity production strongly decreases when thermal effects are neglected. This difference highlights the importance of controlling injection conditions not only in terms of pore pressure, but temperature as well. Possible risk mitigation strategies could include limiting cold-water injection by either increasing inflow temperature or by decreasing the flow rate.
| Main Authors: | , , , , |
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| Other Authors: | |
| Format: | comunicación de congreso biblioteca |
| Language: | English |
| Published: |
2019-11-04
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| Online Access: | http://hdl.handle.net/10261/194701 |
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| Summary: | Understanding coupled processes is essential to properly assess the performance and risks of supercritical geothermal systems. These systems are found in especially hot reservoirs, at the deep root of volcanic areas, in which water is encountered in supercritical conditions because the pressure (p) and temperature (T) are above their critical values, i.e., p > 22.06 MPa and T > 373.95 ºC. Thus, supercritical geothermal systems experience strong induced deformations and stress redistributions induced by pore pressure variations and cooling during cold water reinjection performed to balance fluid depletion. While pressure changes are balanced in the injection and production well doublet, strong cooling occurs asymmetrically around the injection well, which significantly contracts the rock and causes a thermal stress reduction. The effective stress changes may lead to fault reactivation and induced seismicity. In order to investigate the potential of seismicity induced by cold water injection in deep supercritical geothermal reservoirs, we have performed coupled thermo-hydro-mechanical (THM) simulations of a doublet system with a steeply dipping conductive fault between the two wells. The fault is modeled as a continuum with higher permeability and compliance than the surrounding rock. We have performed THM finite element analyses with the object-oriented, C++ based and open-source finite element solver OpenGeoSys. We have implemented advanced features such as: i) porosity-dependent permeability, ii) full poro-mechanical coupling and iii) IAPWS-97 equations of state for water present in various phases. The porosity-dependent permeability evolves along with porosity changes, which in turn are a function of the changes of temperature, pore pressure and stress.
The injected cold water absorbs heat from the rock mass on its way toward the production well, crossing the fault. Under conditions relevant for supercritical reservoirs, deformation is mainly controlled the cooling front and is less affected by pressure changes, which reach a pseudo-steady state shortly after the beginning of fluid circulation. Fluid velocity is highest in the fault, whereas major mechanical effects are a consequence of thermal deformation. We compare cold water re-injection with a case of isothermal injection, showing that the rate of seismicity production strongly decreases when thermal effects are neglected. This difference highlights the importance of controlling injection conditions not only in terms of pore pressure, but temperature as well. Possible risk mitigation strategies could include limiting cold-water injection by either increasing inflow temperature or by decreasing the flow rate. |
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