The geochemical effects of O2 and SO2 as CO2 impurities on fluid-rock reactions in a CO2 storage reservoir

Costs for CO2 capture could be reduced if CO2 gas impurities can be co-injected and do not adversely affect the long-term CO2 containment. This project, as a part of the Callide Oxyfuel Project, investigates the geochemical impacts of the CO2 impurities SO2 and O-2 on mineral-fluid reactions in a siliciclastic reservoir. In a single-well push-pull field experiment CO2-saturated water with and without impurities was injected into the reservoir. The injection water was allowed to interact with minerals in the reservoir for three weeks, during which water was back-produced and sampled on three occasions. Four soluble tracers were added to the injection water to estimate the proportions of injection and formation water in the back-produced water. Redox state, speciation and reaction pathway modelling are used as part of the data interpretation. Once injected, SO2 (67 ppm vol/vol, initially as a dissolved impurity in CO2) was dissolved and oxidised, leading to sulphate formation. The alkalinity of the injection water counteracted any substantial decrease in pH, which would otherwise occur due to sulphuric acid formation, thus inhibiting additional mineral dissolution. After being injected, O-2 (6150 ppm vol/vol, dissolved impurity in CO2) led to immediate oxidative dissolution of pyrite. Consequently, the SO42- concentration increased rapidly and dissolved iron is predicted to precipitate as hematite. Overall, the impact of CO2 impurities was minimal.