Mineral dissolution and precipitation reactions and their net balance controlled by mineral surface area: An experimental study on the interactions between continental flood basalts and CO2-saturated water at 80 bars and 60 °C

Abstract Continental flood basalts are currently considered as unconventional CO2 storage reservoirs given several knowledge gaps remain. This study addresses the question whether subvertical fractures and joints in the storage complex and its overburden may lead to long-term CO2 leakage or whether commencement of self-sealing can be expected at an early stage of geochemical reactions. The latter occurs when mineral precipitation outcompetes mineral dissolution. The nature of secondary mineral precipitation and the change of ion concentrations over time were studied in a batch reactor experiment where basalt wafers with a low surface area were exposed to CO2-saturated formation water at a pressure of 80 bars and 60 °C. Primarily stilbite (a zeolite) and montmorillionite (a smectite) were observed in suspension and kaolinite, Fe-oxide and Na-nontronite were additionally identified as authigenic minerals on the surface of the basalt wafers at the end of the incubation. These observations are in agreement with the state of mineral saturation based on the water composition throughout the incubation. Carbonate minerals were undersaturated throughout the incubation suggesting limited carbon mineral trapping capacity under the given conditions. A mineral nucleation phase (day 1 to 6) dominated by cation release due to dissolution and a mineral growth phase (day 8 to 44) dominated by cation uptake due to precipitation were identified. The latter resulted in the formation of considerable mineral mass, which was deposited almost completely on the wafer surface.