Using 3D Thermo-Hydro-Mechanical Modeling to Estimate Seismic and Aseismic Fault Reactivation Potential Due to Deep Geothermal Energy Exploitation in Western Germany

Deep geothermal energy (DGE) systems exploit heat at depths greater than 400 m. While energy production using DGE in southern Germany has been successful on a limited scale, associated earthquakes have (temporarily and permanently) halted operations in the Netherlands and Belgium. Seismic hazard assessment studies incorporating reservoir rocks' physical and hydraulic properties are necessary to predict pore pressure and stress changes which could lead to fault reactivation. To date, many modeling scenarios have focused on mechanical stress changes not yet coupled to the thermal stress response, which may be critical to discern when deformation changes from aseismic (stable) to seismic (unstable). Devonian carbonate rocks in western Germany represent a potential reservoir for DGE exploitation. Representative outcropping analogues exposed in several regional quarries provide an opportunity to measure and quantify structural characteristics that can be incorporated into models to predict the mechanical and thermal response to geothermal stimulation. In this work, we use discrete fracture network analysis results of outcropping Devonian carbonate rocks as input for 3D thermo-hydro-mechanical (THM) models. We explore different injection/extraction scenarios with a particular focus on the pore pressure and stress changes expected on faults optimally-oriented for slip. Our results show significant (> 1 MPa) pore pressure changes are expected ~10s of meters from the injection well, while thermal stress changes dominate at distances of ~100s m. Regional stress orientation and magnitude in the Ruhr region suggest that NNW-SSE-trending strike-slip faults may be prone to reactivation when affected by pore pressure and/or stress changes larger than ~1 MPa. As the next step, we will introduce the stress and pore pressure changes calculated on optimally-oriented faults as stress perturbations in a rate-state (R-S) friction model. Due to the nonlinear nature of the R-S friction system, small stress changes can generate markedly different responses with variable stability (i.e., aseismic or seismic slip). It is, therefore, crucial to identify the necessary conditions to maintain aseismic rupture speed to minimize the seismic hazard connected with DGE operations.