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

Abstract

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.