Unveiling the Impact of SOx and NOx in CCS: Implications on Injectivity, Mineralization, and Long-Term CO2 Storage

Abstract This study investigates the impacts of SOx and NOx impurities in CO2 streams on key stages of Carbon Capture and Storage (CCS), including injection performance, mineralization, and long-term storage behavior. Excluding these impurities from simulations can lead to significant gaps in understanding reservoir dynamics and may underestimate geochemical and operational risks. This work aims to provide actionable insights for designing CCS systems that ensure safe and efficient storage while accommodating impure CO2 streams. Advanced numerical simulation tools, specifically CMG's GEM compositional reservoir simulator, were employed to model reactive transport and simulate various trapping mechanisms, including structural, residual, solubility, and mineral trapping. The simulations captured key geochemical processes such as acid formation, mineral dissolution, and precipitation under a range of operational scenarios. In addition to the base case with 5% SO2 and 5% NO2 impurities, two additional cases were analyzed to isolate the effect of each impurity: one with 10% SO2 and no NO2, and another with 10% NO2 and no SO2. The results demonstrate that the presence of SOx and NOx impurities alters the balance of CO2 trapping mechanisms. Impurity-bearing scenarios showed reduced contributions from solubility and residual trapping but exhibited enhanced structural containment and an increase of approximately 1% in mineral trapping relative to pure CO2 injection. Mineralization was greater in the NO2-only case compared to the SO2-only case; however, the mixed scenario (5% SO2 + 5% NO2) produced the highest overall increase. This enhancement is attributed to impurity-induced acidification, which accelerates mineral dissolution and promotes carbonate precipitation, leading to more permanent CO2 immobilization. These findings highlight that overlooking impurities may underestimate the role of mineral trapping and misrepresent long-term storage behavior. The study underscores the importance of explicitly accounting for SOx and NOx when forecasting capacity, injectivity, and containment security in CCS projects.