Reconsidering the Causes of the Post-Injection Induced Seismicity at the Enhanced Geothermal System of Basel (Switzerland)

Induced seismicity is responsible of the mistrust in the deployment of geothermal power-plants in urban areas. Multiple cases of Enhanced Geothermal Systems (EGS), such as Basel (Switzerland) and more recently Pohang (South Korea), were shut down after the occurrence of high-magnitude induced seismicity. The most striking feature of the majority of these induced seismicity cases was that they occurred after shut-in. Induced seismicity can be triggered by different mechanisms. The most common one is pore pressure diffusion in the reservoir, which reduces the effective normal stress acting across pre-existing fault surfaces, weakening the shear resistance and promoting slip of faults. Poromechanical, thermal and chemical perturbations, and tectonic settings of the area, influence induced seismicity as well. Yet, the causes of post-injection seismicity are not well understood. Understanding the triggering mechanisms of the post-injection induced seismicity might allow to mitigate the occurrence of earthquakes with high magnitude. To this end, we investigate the seismicity induced at Basel EGS. After analyzing the seismic cluster, we have derived a simplified equivalent model in which each cluster is represented by a slipping fault. We use this model as the basis for hydro- mechanical numerical simulations of fluid injection and consequent fault reactivation, which we perform by means of finite element method with zero thickness elements to represent faults. We have also studied the static stress changes associated with each fault rupture based on Okada’s solution. By coupling these two analyses, we are able to represent the induced seismicity at Basel and understand the respective impacts of pore pressure buildup and static stress transfer. After shut-in, pore pressure diffuses away from the wellhead, but it cannot trigger the post-injection seismicity alone. However, pore pressure changes combined with static stress transfer explain the highest magnitude events at BaselPeer reviewe