In-Situ Stress State of the Ruhr Region (Germany) and its Implications for Geothermal Energy Utilization

In this study, we carry out a reactivation potential analysis of discontinuities revealed from heavily faulted and folded Upper Carboniferous rock strata of the Ruhr region. We based this study on the notion that slip is controlled by the ratio of shear to normal effective stresses acting on a pre-existing plane of weakness in the prevailing stress field configuration. The results of this analysis were supported by indicators of localized fluid flow, both on micro and macro scales, which confirm the relationship between secondary permeability and in-situ stress state in the Ruhr region. Findings from this study, in conjunction with results of destructive laboratory testing, indicate that the steep NW-SE- and NNE-SSW-striking planar discontinuities are likely to be either close to the critical state or critically stressed in the in-situ stress configuration in the Ruhr region. These planar structures, as evidenced by indicators of localised permeability, are the main fluid pathways in the studied region. The NE-SW-striking discontinuities, on the other hand, are most likely to be closed and hydraulically inactive. Based on results gained from this study, implications for the utilization of deep geothermal energy in the region were discussed.