Numerical Assessment of Aquifer Thermal Energy Storage (ATES) in Deeper Siliciclastic Aquifers in Berlin from a Geochemical Perspective

Germany's aquifers hold significant potential for geothermal applications at both shallow and greater depths. For the North German Basin, siliciclastic aquifers in the Lower Jurassic (Hettangian-Pliensbachian) and Upper Triassic (Rhaetian) formations are promising targets for deeper ATES systems. However, high-temperature ATES applications at greater depths remain rare, primarily due to high investment costs, operational uncertainties, and associated risks.

This study investigates fluid-mineral interactions in an anoxic siliciclastic aquifer (carbonate content <1 wt.%) of Hettangian age in Berlin, representative of the North German Basin. Numerical modelling was conducted, using the geochemical code PHREEQC, to study the impact of gas pressure and temperature on fluid-mineral equilibria. Specifically, the effects of groundwater temperature at constant partial gas pressures were analysed. Furthermore, the tendency for mineral precipitation/dissolution was evaluated under varying partial pressures of carbon dioxide and oxygen at selected temperatures (5°C–120°C), simulating the effects of carbon dioxide degassing and oxygen intrusion on the reservoir.

Using site specific mineralogical and geochemical composition, contour plots were generated to visualize precipitation and dissolution trends as functions of temperature and carbon dioxide or oxygen concentrations in the studied system. Results indicate (1) negligible pressure-dependent changes in the stability of iron (hydr-)oxide and carbonate mineral phases; (2) calcite precipitation at elevated temperatures due to changes in species activity and the temperature-dependent solubility constant of calcite, (3) Correcting hydrochemical field data is essential to match thermodynamic equilibrium conditions in the aquifer and significantly affects modelling results, especially with regard to iron speciation and iron mineral phase stability.