Investigating clogging and scaling in carbonate aquifer thermal energy storage systems using batch and column experiments

Recently, Aquifer Thermal Energy Storage (ATES) systems gained increasing attention as a suitable storage method for local and temporary surplus thermal energy in aquifers. Among others, the success of ATES depends on the properties of the aquifer like hydraulic permeability, thermal conductivity, and porosity. During ATES operation, different pressure and temperature conditions above and below the surface can cause clogging and scaling processes, eventually leading to operational and maintenance issues or failures.

In the research project „UnClog-ATES“ (funded by the Federal Ministry of Education and Research of Germany - BMBF), clogging and scaling processes are investigated on an interdisciplinary basis (microbiology, geology, hydrogeology, and geochemistry) and influencing factors for carbonate aquifers are determined. Based on the findings, countermeasures (e. g. scaling inhibitors or CO₂ addition) are (further) developed.

ATES conditions (pressure, temperature, hydraulics, and chemical composition) are systematically simulated: While 1-D column tests serve to model the transport processes taking place in the real system, 0-D batch reactor tests allow varying the hydrochemical environment and rock compositions. Two different ATES-relevant rocks are used as representatives of limestone in general: i) Jurassic limestone from Upper Malm, Germany ("Treuchtlinger Marmor”; mainly calcite) as a representative rock for the Malm, as well as ii) marble from Hammerunterwiesenthal, Germany (“Erzgebirgsmarmor”; calcite and dolomite). Water samples from the same Erzgebirge marble quarry are used as fluid. Batch and flow tests are conducted in cycles at ATES-typical temperatures between 5 °C and 60 °C. Quasi-continuous monitoring of fluid parameters in measuring cells and performing comprehensive hydrochemical and geochemical analyses before, during, and after the tests allows monitoring of alterations in hydrochemical and geochemical conditions. Hydrochemical analysis results of the first batch experiments (shaking “Treuchtlinger Marmor” with distilled water for approx. 30 days at 5, 40 and 55 °C) showed a decrease in the concentrations of Calcium and Magnesium with increasing temperature. Using a larger grain size showed an even greater decrease.

Accompanying the experiments, hydrochemical modeling is used to quantify the processes and to estimate the experimental parameters a priori. The simulation results with PHREEQC first showed an equilibrium of the aforementioned rock and fluid materials at 15 °C and subsequently precipitating calcite while heating stepwise. Further simulations indicate that this precipitation can be prevented by adding a certain amount of dissolved CO₂.

UnClog-ATES intends to contribute to optimizing the prediction accuracy of hydrogeochemical reactions and to the creation of methods both for estimating the clogging potential and for developing and testing possible countermeasures.