Mineralization potential of water-dissolved CO 2 and H 2 S injected into basalts as function of temperature: Freshwater versus Seawater

International audienceMineralization of freshwater-dissolved gases, such as CO 2 and H 2 S, in subsurface mafic rocks is a successful permanent gas storage strategy. To apply this approach globally, the composition of locally available water must be considered. In this study, reaction path models were run to estimate the rate and extent of gas mineralization reactions during gascharged freshwater and seawater injection into basalts at temperatures of 260, 170, 100, and 25 °C. The calculations were validated by comparison to field observations of gas-charged freshwater injections at the CarbFix2 site (Iceland). The results show that more than 80% of the injected CO 2 dissolved in freshwater or seawater mineralizes as Ca and Fe carbonates at temperatures ≤170 °C after reaction of 0.2 mol/kgw of basalt, whereas at 260 °C much lower carbon mineralization rates are observed in response to the same amount of basalt dissolution. This difference is due to the competition between carbonate versus noncarbonate secondary minerals such as epidote, prehnite, and anhydrite for Ca. In contrast, from 80 to 100% of the injected H 2 S is predicted to be mineralized as pyrite in all fluid systems at all considered temperatures. Further calculations with fluids having higher CO 2 contents (equilibrated with 9 bar pCO 2) reveal that i) the pH of gas-charged seawater at temperatures ≤170 °C is buffered at ≤6 due to the precipitation of Mg-rich aluminosilicates, which delays CO 2 carbonation; and ii) the most efficient carbonation in seawater systems occurs at temperatures < 150 °C as anhydrite formation is likely significant at higher temperatures