Impact of borehole path deviations on the efficiency of a medium-deep geothermal storage system: Case study of the SKEWS MD-BTES Demosite

Underground heat storage applications such as Medium Deep Borehole Thermal Energy Storage (MD-BTES) systems are a promising approach for reducing greenhouse gas emissions. In MD-BTES, approx. 20 to 40 Borehole Heat Exchangers (BHE) are strategically arranged at 5-m intervals, reaching depths of 400–1000 m. The first MD-BTES was constructed on the Lichtwiese campus of the Technical University of Darmstadt in 2022 to validate its technological feasibility and evaluate its economic and environmental impacts through scientific test cycles. A major challenge during construction was maintaining borehole verticality without the use of expensive directional drilling technology, to prevent borehole collisions and efficiency losses. However, gyroscopic measurements post-drilling indicated average deviations from verticality ranging between 3° and 5°. Numerical simulations using Feflow 8.0 with favourable and unfavourable scenarios and different storage configurations demonstrated that borehole path deviations significantly affect outlet temperatures during heat discharge, the amount of heat stored and extracted, and overall storage efficiency. After 30 years of operation with inlet temperatures of 90 °C/30 °C (charging/discharging), deviations of 0–12° in the unfavourable scenario could result in 3–10 °C lower outlet temperatures during discharge and efficiency losses of 5–38 %. Conversely, in the favourable scenario, borehole path deviations had no significant impact on storage performance if all BHEs deviated in the same direction. These results enable the estimation of potential performance losses based on borehole path deviations, facilitating an economic comparison between the expected performance losses and the costs of employing directional drilling technology. This study underscores the importance of developing cost-effective vertical drilling techniques for MD-BTES construction, which can play an important role in meeting climate protection targets. • In 2022, a MD-BTES Demonstrator was built on the Lichtwiese campus of TU Darmstadt. • Measurements after drilling indicated deviation from the vertical of 3° to 5°. • Different deviation scenarios based on the real BHE trajectories. • The thermal influence of different deviations was calculated numerically. • Storage performances losse due to borehole path deviatons can now be estimated.