Evolution of fault reactivation potential in deep geothermal systems. Insights from the greater Ruhr region, Germany

Abstract. The success of deep geothermal systems depends on the presence of fault zones in the subsurface. Faults play a vital role in the Earth's plumbing system by facilitating fluid flow when they dilate, but are simultaneously known to enhance the hazard of the system once slipping in shear mode. As dilation of a fault enhances its permeability significantly, shear failure can lead to loss of boreholes or seismic events of economic concern. In this study, we present the evolution of reactivation potential of major faults during 25-year production period in deep generic geothermal systems in the greater Ruhr region in western Germany. To determine the pre-operational in situ stress state we use a recently published comprehensive dataset of stress magnitude data from the greater Ruhr region in an analytical-probabilistic model accounting for uncertainties of in situ stress, fault geometry, and frictional properties for a prospective reservoir in the Devonian Massenkalk formations. The resulting cumulative distribution functions of dilation and slip tendency of given fault sets suggests that more than half of the combined length of NW-SE-striking faults have a high reactivation probability, whereas the NE-SW-striking faults remain not optimally-oriented in the regional stress field. Using the relationship between dilation and slip tendency, we propose fault segments suitable for geothermal development that exhibit high hydraulic conductivity, i.e. high dilation tendency, and lower potential for shear failure, i.e. low slip tendency. In the second step, we employ generic thermo-hydro-mechanical models to quantify induced spatio-temporal stress changes on selected fault planes due to long-term geothermal production. We find that after 25 years thermal stress changes contribute significantly to the change of the reactivation potential which should be accounted for while planning deep geothermal systems.