Quantification of pre-operational seismic hazard of deep geothermal systems exemplified in the greater Ruhr region (Germany)

In 2018, after more than 700 years, the last black coal mine ceased operation in the greater Ruhr region in western Germany. In addition to the repurposing of the unused subsurface infrastructure for heat storage projects, the utilization of deep geothermal resources, located below mining levels in the Devonian Massenkalk formations, is the most promising way of facilitating the green energy transition in this highly populated region. The operation of fluid injection or withdrawal during geothermal production alters subsurface stress and poses questions about the possible reactivation of faults in the greater Ruhr region, with strong implications for seismic hazard potential. Deep geothermal resources depend on permeable pathways (i.e., fault zones and fracture networks) in the subsurface, which are known to have certain intrinsic pre-operational seismic hazard. In this study, we evaluate the probability of reactivation (using the so-called slip tendency and fracture susceptibility indicators) and dilation tendency of major faults accounting for uncertainties of in situ stress conditions, fault geometries, and frictional properties. Furthermore, we investigate the spatio-temporal evolution of the slip tendency of a major fault in the region during geothermal operations utilizing coupled thermo-hydro-mechanical numerical models. This study benefits from a recently published in situ stress database of the greater Ruhr region derived from 429 hydrofracturing tests performed across several coal mines. The assessment of fault reactivation utilizing probabilistic and coupled numerical modelling approaches, as presented in this study, aims at de-risking the exploration of deep geothermal systems in the greater Ruhr region and can hopefully serve as an example for quantification of pre-operational seismic hazards in other regions. Based on preliminary results, we find that the NW-SE striking faults are much more prone to reactivation, in comparison to the NE-SW ones, which are oriented unfavorably under prevailing stress conditions. We also find that thermal stress contributes significantly to fault stability, especially during long-term geothermal production.