Local Stress Anomaly, their Interplay to Deep Seated Fault Structures and Geomechanical Characterization of Geothermal Reservoirs in southern Germany

Due to an increased heat flow and the presence of several aquifers, the Upper Rhine Graben (URG) offers favorable conditions for geothermal utilization and several geothermal projects have emerged in the last decade. Previous research in this tectonic setting proved that the hydraulic and geomechanical behavior of the geothermal reservoirs is mainly governed by the existence of mesoscale fractures and deep large-scale fault systems. The occurrence of induced seismicity as a result of fluid injection, however, clearly demonstrates that a better understanding on their role is still required. The local stress field can be significantly influenced by the existence of fault zones. A good example of this behavior can be observed from a case study in the Molasse Basin. There, measured image logs in three geothermal wells demonstrate that the stress field in the vicinity of these reservoir structures can be perturbed. Fracture data as well as stress indicators were interpreted using these logs and analyzed to determine the local stress field. Two of the wells exhibited E-W and N-S fracture sets that correspond to the regional N-S oriented strike slip stress field of the Mollase Basin. The third well exhibited a markedly different fracture set that indicated a N-S oriented normal faulting regime. This secondary stress field is rotated 40° counter clockwise from the strike slip stress regime. The secondary fracture system and the stress regime are interpreted as a result of local stress changes influenced by existing fault structures. This research aims to develop a better approach for characterization and parameterization of deep reaching faults and is intended to establish an improved understanding of their impact on reservoir mechanics. We perform geomechanical experiments (triaxial tests) with different rock types (granite and sedimentary rocks) to gain insight into fault parameterization and behavior under varying P/T conditions. Furthermore, geological field work on existing faults accessible in underground mines will be carried out to characterize these structures in-situ. Besides this lab and field approach, another focus lies on the development of site-specific numerical models to predict the stress distribution and the permeability evolution of large-scale fault systems over geologic time scales.