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

Abstract

Deep seated fault systems play an important role for deep geothermal applications, particularly in volcanically inactive regions with low natural matrix permeability. Faults could be significant for convective heat transport. Therefore, the permeability within a fault zone is the 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, there is a significant variance of the permeability analyzing the cross section through a fault. A selection of typically aquifer rocks of the Upper Rhine Graben (URG) were chemically-mineralogically investigated and characterized. Additionally, we examined the hydraulic properties (porosity and permeability) of undeformed rock material. Rock mechanical investigations were carried out for indication of physical rock features and generation of fresh, artificial faults. Chemical and mineralogical investigations are made, 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. Fluid-rock 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 for the laboratory experiments will be compared with observations at natural fault systems in cores of deep boreholes and in abandoned mines of the Black Forest. Further experiments with samples from the fault systems will be conducted in the laboratory and in situ. We present here the first results of an ongoing project.