Insights on post-injection seismicity through analysis of the Enhanced Geothermal System at Basel (Switzerland)

AbstractInduced seismicity is a limiting factor for the development of Enhanced Geothermal Systems (EGS) and has led to the cancellation of a few projects. Its causal mechanisms are not fully understood, especially those of post-injection seismicity. Here, we revisit the controversial case of the Basel EGS (Switzerland) to better understand the mechanisms that induced seismicity by simulating the hydro-mechanical response to hydraulic stimulation of a pre-existing fault network built on the basis of the monitored seismicity. Simulation results show that the faults located in the vicinity of the injection well fail during injection, triggered by pore pressure buildup coupled with poroelastic stressing, whereas distant faults are stabilized by poroelastic effects depending on the orientation. After injection stops, poroelastic stress relaxation leads to the immediate rupture of these previously stabilized faults. Shear-slip stress transfer, which also contributes to post-injection reactivation of distant faults, is enhanced in faults with slip-induced friction weakening. This work presents a modeling approach to understand the multiple processes leading to the rupture of pre-existent fractures in EGS reservoirs, which is key to improve our induced seismicity forecasting and mitigating capability.