Department Logo for Earth, Planetary, and Space Sciences

On the role of thermal stress and fluid pressure in deformation and seismicity of geothermal reservoir


Oct. 6, 2021, noon - 1 p.m.
Slichter 3853

Presented By:
Kyungjae (KJ) Im
Caltech

See Event on Google. Subscribe to Calendar

Geothermal areas are particularly prone to induced seismicity and earthquake triggering. In this work, we numerically investigate two interesting observations related to seismicity at Coso and Brawley geothermal field in California. The Coso geothermal field lies just north of the surface ruptures driven by the 2019 Ridgecrest M7.1 earthquake in an area where coseismic stress changes should have triggered aftershocks, but a gap of aftershocks is observed there. Brawley swarm occurred 2 years after the onset of geothermal activity. Intriguingly the swarm, including M5.4 event occurred at a >5km depth, much larger than the ~1km reach of the geothermal wells. Both area experiences strong surface subsidence during geothermal operation, implying significant stress change and deformation in the reservoir. To model accurate field scale thermo-hydro-mechanical response, we set a permeable reservoir embedded in a large hosting domain using a Tough-FLAC coupled simulator. The reservoir is assumed to fail according to the Mohr-Coulomb criteria. Additionally, for the Brawley field, a critically stressed normal fault is embedded through the reservoir and host rock, as observed in a previous study. All simulation parameters are determined to reproduce the observed production, pressure and surface deformation of each field. Both simulations result in comparably rapid poro-elastic stress change over a broad area of reservoir and host, followed by slow but strongly accumulating thermal stress change within localized areas near injection and production zones. The simulation successfully reproduces the flow rates and surface deformation of the fields. In Coso simulation, we found that thermal contraction of the reservoir induces significant stress depletion and contributed to impede Ridgecrest aftershock triggering. In Brawley simulation, deep earthquake is triggered by stress transition from normal fault reactivation driven by thermo-poro-elastic stress. Our study shows how a geothermal operation can, in principle, contribute to seismic hazard mitigation through the aseismic release of tectonic stresses within a geothermal field but points to the difficulty of mitigating the hazard posed by stress transfers in the surrounding area.