Reservoir Induced Seismicity: Where, When, Why and How Strong?

Abstract

With the increasing number of (deep) geothermal projects in industrialized regions, the potential risk associated with reservoir induced seismicity has become a critical point of discussion. A detailed understanding of the physical processes associated with reservoir induced seismicity is required for assessing and (eventually) mitigating the seismic risk.

We present a hydro-mechanical model describing fluid injection induced seismicity. The model is based on the physical processes of fluid pressure and stress diffusion with triggering of the induced seismicity being controlled by Coulomb friction. Earthquake magnitudes are primarily determined by the rupture area, i.e. the spatial extend over which fracture critically is exceeded coseismically.

We use a three-dimensional finite element implementation of the model to simulate in situ hydraulic overpressures and induced seismicity for a typical hydraulic stimulation scenario. The model seismicity exhibits similar features as observed at various geothermal sites. These include the spatio-temporal seismicity distribution with a strong manifestation of the Kaiser Effect, event magnitude distributions, and the occurrence of the largest magnitude events after shut-in of the injection well.

Our results demonstrate that characteristic observations of the induced seismicity can be explained already with a relatively simple physical model with almost no a priori assumptions. In particular, the increase of the event magnitudes with stimulation duration can be attributed to a geometrical effect, where stress criticality is approached over a larger reservoir area. The numerical implementation of our hydro-mechanical model provides a powerful tool for addressing the seismic risk associated with reservoir stimulations.