Geomechanical modelling of tectonic stresses in deep geothermal reservoirs of the Upper Rhine Graben, Germany

The demand for energy transition is growing rapidly as climate change accelerates, and concerns about energy security have also increased due to heightened geopolitical tensions. Geothermal energy is a reliable and sustainable solution that produces both heat and electricity with low greenhouse gas emissions and reduces dependence on fossil fuels. The Upper Rhine Graben (URG) is widely recognised as one of Europe’s most promising regions for geothermal development. However, the risk of induced seismicity associated with fluid injection and production processes poses a significant challenge to public acceptance and project viability. Therefore, understanding the crustal stress state is crucial to ensuring a safe and efficient operation of a geothermal plant.

This study employed a three-dimensional (3D) geomechanical-numerical modelling approach to predict the local in situ stress distribution and fracture networks in a faulted reservoir located near Karlsruhe, Baden-Württemberg, Germany. The structural model showing the subsurface geometry was built using a horizon and fault interpretation derived from 3D seismic data provided by an industry partner. This structural model was discretised and parameterised utilizing Visage (Petrel Reservoir Geomechanics software from SLB). Rock mechanical properties, including modulus of elasticity, Poisson’s ratio, bulk density, Biot coefficient, tensile strength, unconfined compressive strength, cohesion, friction angle, and hydraulic properties, were assigned to each formation. These properties were derived from samples taken in outcrops with Muschelkalk, Buntsandstein, Rotliegend, and crystalline basement rocks for laboratory testing.

The boundary conditions of the Finite Element model were calibrated using the minimum horizontal stress magnitudes measured in a nearby well, and the orientation of the maximum horizontal stress obtained from the World Stress Map database. After validation, modelling results provide a prognosis of the complete 3D stress tensor in the entire model domain. Among others, this can be used for well path planning and optimal well placement. To evaluate the probability of reactivation of the faults under the modelled stress conditions, a slip tendency analysis was performed. In particular, faults within the Muschelkalk formation exhibited a higher slip tendency compared to other target units, indicating zones of elevated seismic risk. These findings provide critical insights for geothermal reservoir development and contribute to risk mitigation strategies aimed at minimising induced seismicity.