The risk of fluid-injection-induced fault reactivation in carbonate reservoirs: an investigation of a geothermal system in the Ruhr region (Germany)

In this study, we investigate the potential for fluid-injection-induced fault reactivation and induced seismicity risk during simultaneous injection-extraction operation in a theoretical geothermal doublet system in a carbonate reservoir in the Ruhr region. Using a coupled three-dimensional thermo-hydro-mechanical approach, we investigate the probability of injection-induced fault rupture. We perform a sensitivity study assuming variability of the fault and matrix permeability, injection/production flow rates, well placement options, rock thermal properties, and evaluate the influence of thermally induced stresses. The ruptured fault areas were calculated based on a Coulomb friction law and a notion that the shear slip is controlled by the ratio of shear to effective normal stresses acting on a pre-existing plane of weakness in the in situ stress field configuration. Ruptured fault areas in the intrinsically not critically-stressed environment, using location-specific empirical correlations, were used to compute local moment magnitudes of potential earthquakes. Based on this study, we conclude that, in the long-term, thermally-induced stresses play a dominant role during fault reactivation and greatly increase the likelihood for induced seismicity. We, therefore, propose that a minimum safe distance between an injection well and a fault should be based primarily on the radius of a thermal plume generated during the expected lifetime of a geothermal system. Results from this study provide valuable insights for the development of future deep geothermal systems in the Ruhr region and other geothermal reservoirs worldwide.

Automated summary

In this study, the potential for fluid-injection-induced fault reactivation and induced seismicity risk during simultaneous injection-extraction operation in a geothermal doublet system in the Ruhr region was investigated. A thermo-hydro-mechanical approach was used to investigate the probability of injection-induced fault rupture. Sensitivity study was performed to evaluate the influence of various factors and it was concluded that thermally-induced stresses play a dominant role during fault reactivation and greatly increase the likelihood for induced seismicity. The results of this study provide valuable insights for the development of future deep geothermal systems worldwide.