A modelling study to evaluate the effect of impure CO₂ on reservoir performance in a sandstone saline aquifer

Carbon capture and storage (CCS) is expected to play a key role in meeting greenhouse gas emissions reduction targets. In the UK Southern North Sea, the Bunter Sandstone formation (BSF) has been identified as a potential reservoir which can store very large amounts of CO₂. The formation has fairly good porosity and permeability and is sealed with both effective caprock and base rock, making CO₂ storage feasible at industrial scale. However, when CO₂ is captured, it typically contains impurities, which may shift the boundaries of the CO₂ phase diagram, implying that higher costs will be needed for storage operations. In this study, we modelled the effect of CO₂ and impurities (NO₂, SO₂, H₂S) on the reservoir performance of the BSF. The injection of CO₂ at constant rate and pressure using a single horizontal well injection strategy was simulated for up to 30 years, as well as an additional 30 years of monitoring. The results suggest that impurities in the CO₂ stream affect injectivity differently, but the effects are usually encountered during early stages of injection into the BSF and may not necessarily affect cumulative injection over an extended period. It was also found that porosity of the storage site is the most important factor controlling the limits on injection. The simulations also suggest that CO₂ remains secured within the reservoir for 30 years after injection is completed, indicating that no post-injection leakage is anticipated.