Geochemical effects of SO2 during CO2 storage in deep saline reservoir sandstones of Permian age (Rotliegend) – A modeling approach

• Novel approach to SO2 solubility calculation for the use in geochemical modeling.
• Identification of mineral reactions tendencies related to the presence of SO2.
• Higher amounts of dissolved SO2 lead to more pronounced mineral reactions.
• Implications of SO2-affected reactions for porosity evolution and mineral trapping.

Geochemical modeling was used to assess the impact of sulfur dioxide (SO2) in impure carbon dioxide (CO2) streams on fluid–rock interaction during storage in siliciclastic rocks of Permian age (Rotliegend). We focused on the impact of SO2 on the porosity evolution, as well as on the trapping of CO2 as solid mineral phase. Due to the lack of a validated approach to calculate SO2 solubility in highly saline brine at CO2 storage conditions, different calculation approaches were tested against experimental literature data. Our novel approach employs a pressure and temperature adjusted Henry's constant with salinity correction. Depending on the solubility calculation approach and on the consecutive reactions of dissolved SO2, different SO2 concentrations in the brine were calculated. Based on the results of these solubility calculations, a low and a high SO2 concentration case were defined for geochemical modeling to assess the impact of different SO2 concentrations on fluid–rock interactions in a CO2 storage reservoir. An exemplary Rotliegend sandstone composition was chosen reflecting compositions of potential target horizons in the North German Basin. Short-term dissolution of carbonates and, in the presence of SO2, subsequent precipitation of sulfur-bearing minerals results in a decrease in porosity. A precipitation of sulfates near the injection well may lower injectivity. With time, dissolution of alumosilicates further provides cations to the aqueous solution for a precipitation of secondary carbonates. The presence of SO2 modifies this long-term interplay between silicate and carbonate minerals. The higher the SO2 concentration in the brine, the lower the amounts of newly formed carbonates, i.e. the mineral trapping of CO2, and the lower the overall porosity decrease. Hence, the assessment of SO2 dissolution in the formation brine at subsurface conditions is crucial for predicting mineral reactions and porosity evolution during geological storage of impure CO2.