Geochemical impacts of CO₂ storage in saline aquifers with various mineralogy - Results from laboratory experiments and reactive geochemical modeling

Investigations of reservoirs, cap-rocks and traps in the Norwegian-Danish Basin have indicated a large potential for geological storage of CO₂ onshore Denmark. The possible reservoir rocks include a variety of sandstones with different mineralogical composition, ranging from the Bunter Sandstone Formation containing K-feldspar, clay minerals, calcite, and dolomite as primary reactive minerals to the Gassum Formation containing albite, clay minerals, siderite, and ferroan dolomite as the most reactive minerals. A laboratory and modelling study was carried out to investigate the geochemical response of five potential reservoir rocks to CO₂ storage in order to constrain predictions of mineral-CO₂ reactions prior to geological storage. The study includes hydrogeochemical experiments, petrographical and mineralogical analyses, and reactive geochemical modelling. During up to 13 months of exposure to CO₂ at reservoir conditions (70°C; 20 MPa), the five different rock samples show little mineral reactivity as compared to similar rock samples exposed to N2 for the same period of time. However, during the period covered by the experiments, dissolution of carbonates present in the host rock is observed both from petrographical analysis and geochemical analysis using speciation calculations. Thus, for the Bunter Sandstone Formation calcite dissolution is apparently taking place in the laboratory experiments while for the Gassum Formation samples ferroan dolomite and siderite dissolution are the dominant mineral dissolution reactions taking place. As a result of the speciation calculations, a procedure for back calculation of chemical analyses to true experimental conditions is suggested in order to reflect the correct saturation state of the pore water with respect to carbonates, primary silicates and aluminosilicates. It is suggested that performing such back calculations is essential to the understanding of the future evolution of the hydrogeochemistry of aquifers aimed at as CO₂ storage reservoirs.