The role of temperature-enhanced fault closure in promoting postinjection pressure diffusion and seismicity in enhanced geothermal systems

Abstract

Post shut-in seismic events in enhanced geothermal systems (EGSs) occur predominantly at the outer rim of the co-injection seismic cloud. The concept of postinjection fracture and fault closure near the injection well has been proposed and validated as a mechanism for enhancing post shut-in pressure diffusion that promotes seismic hazard. This phenomenon is primarily attributed to the poro-elastic closure of fractures resulting from the reduction of wellbore pressure after injection termination. However, the thermal effects in EGSs, mainly including heat transfer and thermal stress, may not be trivial and their role in postinjection fault closure and pressure evolution needs to be explored. In this study, we performed numerical simulations to analyze the relative importance of poro-elasticity, heat transfer, and thermo-elasticity in promoting postinjection fault closure and pressure diffusion. The numerical model was first validated against analytical solutions in terms of fluid pressure diffusion and against heated flow-through experiments in terms of thermal processes. We then quantified and distinguished the contribution of each individual mechanism by comparing four different shut-in scenarios simulated under different coupled conditions. Our results highlight the importance of poro-elastic fault closure in promoting postinjection pressure buildup and seismicity, and suggest that heat transfer can further augment the fault closure-induced pressure increase and thus potentially intensify the postinjection seismic hazard, with minimal contribution from thermo-elasticity.