Chemical interactions of a geothermal battery in the Malm reservoir of Munich

Abstract The Malm reservoir in the South German Molasse Basin, characterized by its highly heterogeneous karstified carbonate structure, offers significant potential for geothermal energy production and seasonal heat storage. This study investigates the chemical interactions and risks associated with the operation of a geothermal battery, a subsurface heat storage system, in this reservoir. Using reactive transport simulations, we model the effects of injecting CO₂-inhibited thermal water at elevated temperatures (135 °C) during thermal charging and cooled thermal water (60 °C) during thermal discharging and compared the results to conventional geothermal operation. The study highlights the influence of heterogeneity, reactive surface area-to-volume ratios, and dolomitization on chemical interactions in the reservoir. Our results reveal that CO₂ inhibition effectively mitigates scaling risks and prevents formation damage near the storage well, while driving modest porosity increases through calcite dissolution near both storage and injection wells. Conversely, in fully dolomitic zones, minor porosity reductions are observed during thermal charging. Significant chemical changes are confined to the near-wellbore region. The chemical de-risking conducted in this study contribute to the feasibility of integrating geothermal batteries into renewable energy systems, potentially providing an economically viable solution to seasonal energy storage while supporting the decarbonization of district heating networks.