Unveiling the Beneficial Effects of N2 as a CO2 Impurity on Fluid-Rock Reactions during Carbon Sequestration in Carbonate Reservoir Aquifers: Challenging the Notion of Purer Is Always Better.

CO2-enhanced oil recovery (CO2-EOR) is widely used in reservoir development, but its implementation is often limited by scarce pure CO2 sources and high carbon capture costs. Flue gas from steam injection boilers typically contains 10-15% CO2 and 80-85% N2, both of which serve as effective gas displacement agents. Injecting flue gas or CO2/N2 mixtures into reservoirs can reduce carbon emissions, sequester CO2, and enhance recovery. Therefore, this study proposes a concept of enriching rather than capturing flue gas for storage, with a focus on how N2 as an impurity affects the safety of CO2 storage. This study examines interactions between gas mixtures with varying CO2 enrichment rates (with N2 impurities) and minerals. It performs microarea analyses of aged rocks using ultradepth-of-field microscope and atomic force microscopy (AFM), assesses calcite wettability via contact angle tests, and evaluates CO2 storage column height under experimental conditions. Results indicate that with a CO2 enrichment of at least 50%, adding N2 creates additional dissolution pits on calcite surfaces. The calcite matrix exhibits optimal water-wettability at 50-75% CO2 enrichment, facilitating greater CO2 storage column heights. This suggests coinjecting N2 and CO2 can enhance long-term CO2 storage safety and reduce capture costs.