Influence of impurities on reactive transport of CO2 during geo-sequestration in saline aquifers

Abstract

Carbon capture and storage (CCS) is a promising technology to reduce greenhouse gas emissions by storing CO2 in geological formations. Storing less pure CO2, which has low percentages of impurities, can reduce the cost of the process. However, impurities can affect the dynamics of CO2 propagation and its trapping mechanisms in porous media, potentially impacting the overall effectiveness of the storage process. Therefore, this study investigates the effect of impurities on the reactive transport of CO2 during geo-sequestration in a saline aquifer using a reliable reactive transport model. The effect of co-injected H2S and CH4 in the CO2 stream on CO2 plume migration, CO2 trapping mechanisms, minerals dissolution and precipitation, and cap rock integrity, was studied here. To do so, reactive transport modeling was performed using a compositional simulator based on actual data from Sleipner storage site. The model was run for pure CO2 and mixtures (CO2 with 5 wt% H2S and CO2 with 5 wt% CH4). The results showed that the impurities co-injected with CO2 affect the CO2 plume migration in the aquifer. For instance, injecting the CO2-CH4 mixture increased the CO2 horizontal migration below the cap rock compared to the pure CO2 injection case. However, the buoyancy effect of the injected gas was slightly reduced by adding H2S to the CO2 stream. The impurities influenced the pH of the system, and hence minerals dissolution and precipitation, and cap rock porosity changes. The impurities (CH4 and H2S) injected with CO2 were found to suppress an anticipated increase in the cap rock porosity and accelerated the mineral trapping of CO2 compared to pure CO2 storage. While, among the studied cases, the CO2-CH4 mixture case had the highest structural trapping and the lowest solubility trapping of CO2. Thus, this study suggests that storage of impure CO2 can enhance mineral trapping, maintain cap rock sealing, and change CO2 distribution in porous media while reducing separation costs.