| Popis: |
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 depth of 4000 to 6000 m. High permeability rates combined with sufficient thermal conductivities were 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), which quantification of the geothermal potential is still lacking. Compared to Bavaria, a supraregional carbonate mountain belt is exposed at the Remscheid-Altena anitcline (NRW) from late Devonian and early 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 at various spacial scales were observed by scanline surveys. These discontinuities were characterized by trace length, true spacing, roughness, aperture, and filling materials. Joint 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 300 m thick and located at 4000 to 6000 m depth, dipping northwards at a shallow dip angle of about 30 to 40°. An extrapolation of the measured layer orientation and dip suggests that the carbonate reservoir extends 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 fracture network in the reservoir and its orientation with respect to the prevailing maximum horizontal stress before concluding with implications for fluid flow: We proposed to focus on discontinuities that are approximately N–S oriented for upcoming geothermal applications, because the geothermal potential of the characterized reservoir matrix is insufficient for deep geothermal applications. Our results indicate that even higher permeability can be expected for karstified formations related to the reef facies. Our compiled data set consisting of laboratory and field measurements may provide a good basis for 3D subsurface modeling and numerical prediction of fluid flow in the naturally fractured carbonate reservoir. Further studies have to be elaborated to verify, if the fractured reservoir could possibly be reactivated by, for instance, hydraulic stimulation and thus enable geothermal applications. |
