Alleviating post-injection seismic hazard in enhanced geothermal systems: Insights from a multi-scale study

Induced earthquakes after the termination of fluid injection are a major obstacle to the success of Enhanced Geothermal Systems (EGS) and other contemporary subsurface injection activities. Recent advances reveal that changes in pore pressure, poroelastic stress, Coulomb stress and several time-dependent factors could lead to post-injection seismicity. However, recommendations for well shut-in strategies to mitigate post-injection seismicity are currently fragmented. To explore the optimal shut-in strategy in typical EGS, we performed laboratory-scale fluid depressurization experiments and field-scale modeling of the Pohang EGS in South Korea. We find that immediate fluid extraction after injection prevents the fault from high-velocity dynamic slip and subsequent sustained slow slip in the laboratory, and is expected to reduce the likelihood of the M_W 3.2 and M_W 5.5 earthquakes occurred at the field-scale Pohang EGS in 2017, which can be explained by the mitigated pore pressure buildup and reduced total Coulomb stress change. We propose immediate fluid extraction as the preferable shut-in strategy for EGS developed in low-permeability crystalline reservoirs, particularly if earthquakes are expected to occur on faults in close proximity and with sufficient hydraulic connection to injection wells. Nevertheless, we also contend that if poroelastic effects are pronounced in highly fractured EGS reservoirs, the effectiveness of fluid extraction as a shut-in strategy should be evaluated based on the locations and orientations of faults.