Modeling induced seismicity due to fluid injection and withdrawal in deep boreholes: a Coulomb stress approach.

Abstract

Fluid injection and withdrawal in deep wells is a basic procedure in mining activities and deep resources exploitation, i.e. oil and gas extraction, geothermal exploitation, geothermal permeability enhancement and waste fluid disposal. All these activities have the potential to induce seismicity, as demonstrated by the 2006 Basel earthquake of magnitude ML=3.4. Despite several decades of experience, the mechanism of induced seismicity is not known in detail, preventing an effective risk assessment and/or mitigation. In this work, we give an interpretation of induced seismicity based on the computation of Coulomb stress changes resulting from fluid injection/withdrawal at depth, mainly focused to interpret induced seismicity due to Enhanced Geothermal System (EGS) reservoir stimulation. Seismicity is in fact, theoretically, more likely where Coulomb stress changes are larger. For modeling purposes, here we simulate the thermodynamic evolution of the system after fluids injected/withdrawn. The retrieved changes of pressure and temperature are subsequently considered as sources of incremental stress changes, which are then converted to Coulomb stress changes on favored faults, taking into account also the background regional stress. Numerical results are then applied to simulate the water injection used to create the fractured reservoir at the Soultz-sous-Forets (France) EGS site. For such simulation, we use both isotropic and non-isotropic permeability models, the last ones based on previous inference of this kind found in literature. The obtained results show that our approach provides a very good description of induced seismicity, and gives a natural explanation to the different impact, in terms of induced seismicity, respectively of fluid injection and fluid withdrawal. In particular, it accurately reproduces the location and mechanisms of induced seismicity at this and likely at the other EGS sites, thus representing a powerful tool for its interpretation and mitigation.