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

Geothermal energy is widely recognized as a key contributor to decarbonization. Nevertheless, as of 2023, 81% of Germany's heating and cooling energy demand was still met by fossil fuels, with only 13% supplied by geothermal systems. Aquifer Thermal Energy Storage (ATES) offers a solution to bridge the gap between seasonal thermal energy supply and demand by storing excess heat in summer and cold in winter.

Germany's aquifers, particularly in the North German Basin (NGB), Upper Rhine Graben (URG), and German Molasse Basin (GMB), hold significant potential for geothermal applications at both shallow and greater depths. For the NGB, 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.

A key requirement for sustainable ATES operation is to maintain the mechanical, physical, and chemical stability of the subsurface. Changes in physico-chemical boundary conditions, such as temperature fluctuations, oxygen intrusion, and carbon dioxide outgassing, can adversely impact groundwater quality, hydraulic permeability, and well integrity. Key processes causing porosity alterations include ochre formation and incrustation from iron and manganese precipitation, aluminium precipitation, sintering and silicification from lime, silicate, sulfate, or sulfide deposits, sanding and colmation, and biofilm formation through microbial activity.

This study investigates fluid-mineral interactions in a siliciclastic aquifer of Hettangian age in Berlin, representative of the North German Basin. Numerical modelling was conducted, using the geochemical code PHREEQC version 3.7.3, 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, 20°C, 40°C, 60°C, 80°C), simulating the effects of carbon dioxide outgassing and oxygen intrusion on the reservoir. Using the site specific mineralogical and geochemical composition, we numerically investigate the effects of ATES-induced boundary conditions on mineral precipitation and dissolution, with a focus on porosity alterations in saline groundwater environments. Results are presented as contour plots visualising precipitation and dissolution trends as a function of temperature and carbon dioxide or oxygen concentration in anoxic aquifers with low or negligible carbonate content.

The findings aim to enhance the understanding of deep ATES applications and develop strategies for mitigating risks to ensure sustainable operation. This research provides valuable insights into the challenges and opportunities of deeper ATES systems in siliciclastic formations, contributing to the broader goal of decarbonizing Germany’s energy sector through innovative geothermal solutions.