Hydrochemical Reactions during Aquifer Thermal Energy Storage (ATES) in Carbonate Aquifers

Aquifer Thermal Energy Storage (ATES) systems are gaining attention as a method to store surplus thermal energy in aquifers. However, during ATES operation, changes in pressure and temperature conditions can initiate clogging and scaling processes, leading to operational and maintenance issues or failures. In the “UnClog-ATES” project (funded by the BMBF, Germany), we investigate clogging and scaling processes in carbonate aquifers and develop countermeasures such as scaling inhibitors or CO₂ addition through an interdisciplinary approach that combines microbiology, geology, hydrogeology, and geochemistry.

Aiming at carbonate aquifers, we used two types of limestone: i) Jurassic limestone from Upper Malm, Germany ("Treuchtlinger Marmor”; primarily calcite) and ii) Marble from Hammerunterwiesenthal, Germany (“Erzgebirgsmarmor”; mainly composed of calcite and dolomite). Water samples from the Erzgebirge marble quarry served as fluid phase in all experiments, which were conducted at ATES-relevant temperatures (5–60 °C).

Shaking experiments (0-D) assess the influence of hydrochemical environments and rock compositions on rock and fluid alteration. A series of time-dependent shaking experiments at 5, 40, and 60 °C revealed that, with Erzgebirgsmarmor, Ca concentrations in fluid decrease over time at all three temperatures, while Mg concentrations increase. Conversely, Treuchtlinger Marmor exhibits the opposite behavior. PHREEQC modeling of the 60 °C experiments predicts precipitation of dolomite, calcite, aragonite, and vaterite.

1-D column experiments systematically simulate ATES conditions, including temperature and chemistry to model transport processes. Preliminary results at 12 °C with Treuchtlinger Marmor indicate precipitation of dolomite, calcite and aragonite. Early findings from a 40 °C test run comparing both carbonate rocks show differences over time and compared to the results at 12 °C in pH, electric conductivity and alkalinity.

These results highlight the need of further site specific investigations to enhance our understanding of hydrochemical processes and reactions during ATES operations. Findings from this study will improve the prediction of dissolution and precipitation processes and the development of effective countermeasures for clogging and scaling during ATES in carbonate aquifers.