Crystallization of calcium carbonate in a large-scale push-pull heat storage test in the Upper Jurassic carbonate aquifer

Crystallization of carbonates is a key process affecting the operation of geothermal facilities and aquifer heat storage systems. The crystals formed in an aquifer heat storage test in the Upper Jurassic carbonate aquifer were investigated at injection temperatures of 65 degrees C to 110 degrees C, with varying CO2 partial pressures, and varying Mg/Ca ratios. Water samples were directly filtrated, and analyzed by SEM/EDX. Complementary autoclave experiments were run. In the autoclave experiments with tap water, aragonite crystals dominated at all temperatures (45-110 degrees C). In the autoclave experiments with ultra-pure water, calcite crystals dominated at the same temperatures. In the field test, mainly calcite crystals were found up to temperatures of 90 degrees C. Only at very high temperatures of 110 degrees C aragonite crystallization prevailed. CO2 partial pressure varied especially between injection and production stages. Mg/Ca ratio varied through all stages, and depended on the dissolution of the rock matrix. Together with the autoclave experiments, this study suggest that temperature and Mg/Ca ratio had no influence on the crystallization, and only supersaturation affected the CaO3 polymorphs. We further assume that we produced initially injected crystals back during the following production stage. That results in the assumption that existing particles can maintain an equilibrium in the dispersion, and reduce precipitation on surfaces like pipes and heat exchangers.