Exploration, Development and Characterisation of a Medium-Deep Borehole Thermal Energy Storage

This work presents the assessment of a medium-deep borehole thermal energy storage system (MD-BTES) constructed in a complex crystalline environment in Darmstadt, Germany. In addition to thermal performance analysis, the work also summarizes key findings from the drilling and exploration phases of the demonstrator. The assessment integrates geological field studies, geophysical logging and surveys, laboratory analyses, thermal experiments, and numerical modelling to develop a more comprehensive understanding of both the storage system and its complex geological setting. This study was conducted within the Seasonal Crystalline Borehole Thermal Energy Storage (SKEWS) demonstrator project with meth-odological links to the Horizon Europe project PUSH-IT. Parts of the methodology were adapted for application in the GeoLaB project, and the geological datasets archived were integrated into the AMPEDEK 2 petrothermal property reference database. Initial geological investigations revealed that the MD-BTES site is located near a geological unconformity and potentially within a fault zone. A multidisciplinary exploration strategy was applied to further characterize subsurface conditions. Seismic and gravimetric surveys, electrical resistivity tomography (ERT), and petrophysical analysis of outcrop analogues were conducted. The investigations revealed basaltic and granitic lithologies, separated by an east-dipping fault structure. Electrical resistivity tomography results indicated low resistivities linked to weathering and high groundwater levels. Petrophysical measurements confirmed less favourable storage conditions in the Permian Basalts due to high porosity and low thermal conductivity. At the same time, the underlying Granite showed strong potential with lower porosities and higher conductivities. Geophysical well logs and drill cuttings further improved the geological model, identifying altered and fractured zones through gamma ray, sonic, resistivity, and magnetic susceptibility profiles. Distributed thermal response testing (dGRT) confirmed thermal conductivity variability along depth (2.0 – 2.8 W m⁻¹K⁻¹), correlating with changes in lithology and degree of hydrothermal alteration. Despite insulation, heat losses were observed due to ther-mal short-circuiting in the borehole, especially in the upper sections. Design improvements, such as increasing flow rates or optimizing pipe arrangements, are suggested to mitigate these losses. Thermal testing revealed higher borehole thermal resistance in the upper section (0 – 180 m), where low-conductivity grout and lithology variations coincide, complicating interpretation. Nonetheless, the combination of dGRT and conventional GRT yielded high-quality thermal characterisation data, which were used to calibrate a detailed COMSOL model. This model, incorporating fibre optic temperature profiles, enables close agreement between measured and simulated temperatures and serves as a powerful tool for future design iterations. Based on the COMSOL model, it could be inferred that the impact of the thermal short circuit on storage operation for a one BHE system is rather low on a low single-digit level. In conclusion, this study demonstrates the critical role of integrating geological, geophysical, and modelling approaches in planning and optimizing MD-BTES systems. The results provide a strong foundation for technical refinement, cost reduction, and strategic deployment of medium-deep geothermal heat storage within sustainable district heating infrastructure.