Investigating the role of thermo-poro-elastic stresses and deformation on induced seismicity

Kyungjae Im, Jean-Philippe Avouac, Elias R. Heimisson, & Derek Elsworth

Submitted August 11, 2020, SCEC Contribution #10361, 2020 SCEC Annual Meeting Poster #051

We investigate the influence of thermo- and poro-elastic stresses on induced seismicity in the context of geothermal energy production at Coso and Brawley in California. Coso geothermal field is one of the largest geothermal fields operated for ~30 years, and the Brawley geothermal field is a smaller and shallower geothermal system operated for ~10 years. 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 Coso field simulation predicts a cumulative surface subsidence of ~70cm over 30 years and the Brawley simulation predicts surface subsidence of ~20 cm over 10 years across the area generally consistent with the subsidence measured from InSAR. The surface deformations are influenced by both thermo- and poro-elastic stress changes. For the Brawley simulation, surface rupture is observed, which additionally contributes to the surface deformation. Both Brawley and Coso cases show strong Coulomb stress increase within the reservoirs consistent with the dense cloud of observed induced seismicity and a strong Coulomb stress drop in a halo surrounding the depleted reservoir due to the development of compressive circumferential stress. The strongly accumulated Coulomb stress change and subsequent failure in the Coso area lead to shear stress depletion, which can explain the lack of aftershocks following the 2019 Ridgecrest earthquakes. In the case of Brawley, a zone of strong positive Coulomb stress change is predicted below the reservoir, due to the stress transfer via aseismic fault reactivation. The model explains the triggering of a Mw5.4 earthquake in 2012 at ~7km depth in that zone.

Im, K., Avouac, J., Heimisson, E. R., & Elsworth, D. (2020, 08). Investigating the role of thermo-poro-elastic stresses and deformation on induced seismicity . Poster Presentation at 2020 SCEC Annual Meeting.

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