Deep geothermal power plants are baseload-capable and can serve as substitutes for fossil fuel power plants. In the Ruhr region of Germany, a densely populated area with high energy demand, potential deep geothermal reservoirs have been limited to a few sedimentary layers, specifically Carboniferous and Devonian strata. These reservoirs have not been explored with hydraulic field studies for geothermal purposes so far. Deep boreholes for geothermal research are non-existent, so shallow boreholes provide an analogue for evaluating the potential of deep reservoirs.
For this study, a naturally-fractured Carboniferous sandstone and a karstic dolomitic limestone reservoir of the Devonian were investigated using geophysical borehole measurements, hydraulic and hydrochemical experiments, and acoustic/optical televiewer recordings.
The sandstone formation (investigated depth: 20 – 305 m) showed a low hydraulic conductivity, and it is probably necessary to drill into fault zones or to enhance the permeability via stimulation in future geothermal projects. Chemical enhancement of permeability was attempted using CO2 to dissolve carbonate fillings in fractures, but further research is needed to evaluate the effectiveness of this method.
The dolomitic limestone formation (investigated depth: 10 – 224 m) exhibits exceptional permeability due to enlarged fractures, cavities and vuggy porosity resulting from karstic processes, making it a promising reservoir overall. However, further confirmation is required at reservoir depths (e.g. 2500 – 4000 m).
In summary, deeper geothermal reservoirs in the Ruhr region have not been explored enough so far, and further field studies are necessary to evaluate their potential for geothermal energy production.