Fluid-Rock Interaction in Deep Fault Systems and the Influence on Permeability in Typical Rocks of the Upper Rhine Graben, Southwest Germany

Deep-seated fault systems play an important role for deep geothermal applications, particularly in volcanically inactive regions with low matrix permeability. Faults can be significant for convective heat transport. Therefore, the permeability within a fault zone is a crucial parameter for an efficient geothermal usage, if the matrix permeability of the aquifer is low. Its magnitude varies substantially with depth and pressure, rock composition, fluid chemistry, and thermal properties. Additionally, permeability can vary significantly through a cross section of a fault. A selection of typical aquifer rocks of the Upper Rhine Graben (URG) were chemically-mineralogically investigated and characterized. Additionally, we examined the hydraulic properties (i.e., porosity and permeability) of undeformed rock material. Mechanical rock investigations were carried out to determine the physical rock features and delineate fresh and/or artificial faults. Chemical and mineralogical investigations are performed, since mineral precipitation and roughness have a significant influence on the geomechanical model, the permeability of the fault zone and consequently on the efficiency of geothermal applications. Fluidrock interaction experiments were conducted with an autoclave on fresh fracture surfaces. We used synthetic fluids similar to fluids of the Upper Rhine Graben in greater depth. The results gathered from laboratory experiments will be compared with the observations of natural fault systems by cores from deep boreholes and abandoned mines of the Black Forest. Further experiments on the samples of the fault systems will be conducted in the laboratory and in-situ. We present here the first results of an ongoing