Characterization of discontinuities in potential reservoir rocks for geothermal applications in the Rhine-Ruhr metropolitan area (Germany)

Abstract

The importance of research into clean and renewable energy solutions has increased over the last decade. Geothermal energy provision is proven to meet both conditions. Therefore, conceptual models for deep geothermal applications were developed for different field sites regarding different local conditions. In Bavaria, Germany, geothermal applications were successfully carried out in carbonate horizons at depths of 4000 to 6000 m. Matrix permeability and thermal conductivity was mainly studied in karstified carbonates from the Late Jurassic reef facies. Similar to Bavaria, carbonates are located in the east of the Rhenohercynian Massif, in North Rhine-Westphalia (NRW), for which quantification of the geothermal potential is still lacking. Compared to Bavaria, a supraregional carbonate mountain belt is exposed at the Remscheid-Altena anticline (in NRW) from the Upper Devonian and Lower Carboniferous times. The aim of our study was to examine the potential geothermal reservoir by field and laboratory investigations. Therefore, three representative outcrops in Wuppertal, Hagen-Hohenlimburg, and Hönnetal were studied. During field surveys, 1068 discontinuities (139 open fractures without any filling, 213 joints, 413 veins filled with calcite, and 303 fractures filled with debris deposits) at various spatial scales were observed by scanline surveys. These discontinuities were characterized by trace length, true spacing, roughness, aperture, and filling materials. Discontinuity orientation analysis indicated three dominant strike orientations in NNW–SSE, NW–SE, and NE–SW directions within the target horizon of interest. This compacted limestone layer (Massenkalk) is approximately 150 m thick and located at 4000 to 6000 m depth, dipping northwards at a dip angle of about 30 to 40∘. An extrapolation of the measured layer orientation and dip suggests that the carbonate reservoir could hypothetically extend below Essen, Bochum, and Dortmund. Our combined analysis of the field and laboratory results has shown that it could be a naturally fractured carbonate reservoir. We evaluated the potential discontinuity network in the reservoir and its orientation with respect to the prevailing maximum horizontal stress before concluding with implications for fluid flow: we proposed focusing on prominent discontinuities striking NNW–SSE for upcoming geothermal applications, as these (1) are the most common, (2) strike in the direction of the main horizontal stress, (3) have a discontinuity permeability that significantly exceeds that of the reservoir rock matrix, and (4) only about 38 % of these discontinuities were observed with a calcite filling. The remaining discontinuities either showed no filling material or showed debris deposits, which we interpret as open at reservoir depth. Our results indicate that even higher permeability can be expected for karstified formations related to the reef facies and hydrothermal processes. Our compiled data set, consisting of laboratory and field measurements, may provide a good basis for 3D subsurface modelling and numerical prediction of fluid flow in the naturally fractured carbonate reservoir.