Experimental evaluation of the impact of CO2/N2 mixture on residual water saturation during CO2 storage in saline aquifer.

Residual water critically constrains CO2 geological storage efficiency, yet we discover that elevated CO2 concentration in gas mixtures systematically reduces its saturation. By revealing that N2 co-injection paradoxically enhances storage safety despite increasing residual water, this work provides transformative strategies for optimizing carbon sequestration with impurities. Specifically, in this study, nine sets of core-flooding experiments were conducted using three different ratios of a CO2/N2 gas mixture (50 % CO2 + 50 % N2, 75 % CO2 + 25 % N2, and 99.99 % CO2). The experimental results indicate that an increase in the ratio of CO2 within the gas mixture leads to a progressive reduction in residual water saturation, with the order of saturation being 50 % CO2 + 50 % N2 > 75 % CO2 + 25 % N2 > 99.99 % CO2. Furthermore, a detailed mathematical relationship that delineates the connection between residual water saturation and drainage duration is presented, with coefficients a and b examined thoroughly. The incorporation of N2 into the CO2 mixture raises residual water saturation, as lower concentrations of CO2 correlate with increased residual water saturation. However, the presence of N2 effectively extends the CO2 breakthrough time, making it more difficult for CO2 to penetrate the rocks and thereby enhancing storage safety. As the concentration of CO2 in the gas mixture increases, a reduction in contact angle values and interfacial tension (IFT) occurs. This variation results in a decrease in capillary pressure, which facilitates the displacement or migration of fluids within the core pore space. Concurrently, the viscosity ratio of the gas phase to the liquid phase reduces the viscous resistance in the pores and improves displacement efficiency. Furthermore, the experimental results are primarily influenced by capillary forces. This study enhances the theoretical understanding of CO2 storage, and provides valuable insights for evaluating the feasibility of CO2 storage projects that include impurities.