Assessment of a medium-deep borehole thermal energy storage site in the crystalline basement: A case study of the demo site Lichtwiese Campus, Darmstadt

The use of intermittent renewable energy sources is of great importance for reducing greenhouse gas emissions. Medium-deep borehole thermal energy storage systems (MD-BTES) represent an economic solution. At the Technical University of Darmstadt, Germany, an MD-BTES consisting of three 750 m deep borehole heat exchangers was constructed as a demonstrator. Before construction, a comprehensive dataset consisting of electrical conductivity tomography profiles, gravity measurements, 2D seismic profiles, and petrophysical data of nearby outcrops was obtained. In the seismic data, multiple geological facies and faults could be identified and correlated with lithologies based on the gravimetric dataset and drilling data. The data facilitated optimization of the drilling work, modeling, and planning of the system. In this project, the combination of 2D-seismic with well data proved to be especially useful for detailed site characterization, while electrical resistivity tomography and gravimetry provided additional constraints on the subsurface and were used for data regionalization. The results emphasize that at least one exploratory well should be included when planning medium-deep reservoirs in potentially heterogeneous crystalline settings with insufficient outcrop data. The experiences made during this work greatly benefit the planning and execution of future MD-BTES projects by facilitating the development of a cost-efficient site characterization strategy.