Slip versus dilation tendency parameter space as a tool for prospecting structurally controlled geothermal systems in greenfield: insights from the greater Ruhr region

Abstract

Faults play a vital role in the Earth's hydraulic system by facilitating fluid flow when dilating but are simultaneously known to enhance the hazard of the system once slipping in shear mode. As dilation of a fault enhances its permeability, shear failure can lead to loss of boreholes or seismic events of societal concern. This study investigates the interrelationships and temporal evolution of slip and dilation tendencies of mapped faults in the greater Ruhr region (NW Germany) for prospecting structurally controlled geothermal energy resources. In light of limited geological data from deep geothermal reservoirs, we use an analytical–probabilistic model to estimate fault reactivation potential, accounting for uncertainties in in situ stress conditions, fault geometries, and frictional properties. The cumulative distribution functions of dilation tendency, slip tendency, and fracture susceptibility indicate that more than half of the combined length of NW-SE faults have a high reactivation probability, whereas the NE-SW faults remain not optimally oriented for shear failure in the current stress field. Utilizing the slip versus dilation tendency parameter space, we prove that the NW-SE faults experience mostly hybrid failure modes and positive dilation, whereas NE-SW faults experience exclusively compactive failure modes and negative dilation. Introducing the reduced-risk dilation tendency, we identify fault segments that are most suitable for geothermal development, which show high hydraulic conductivity and low shear failure potential. With a temporal evolution of the slip and dilation tendencies computed using a fully coupled thermo–hydro–mechanical model, we show the fault reactivation potential and its failure mode evolution in time and space during geothermal fluid production. We conclude that the combined static and time-dependent slip versus dilation tendency parameter space offers a promising framework for prospecting structurally controlled deep geothermal energy systems in greenfield areas.