Abstract: Deep saline aquifers offer substantial CO₂ storage potential, and exploring the CO₂-water two-phase displacement mechanisms in tight sandstone is crucial for efficient and secure CO₂ storage in saline aquifers. CO₂-water two-phase displacement experiments were conducted on two low-permeability sandstones with distinct pore structures. Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques were employed to quantitatively and visually analyze the distribution of gas and water phases during the drainage and imbibition processes. The study examined the impact of core pore size distribution, heterogeneity, and permeability variations on the two-phase displacement characteristics. The research indicates that in the drainage processes, CO₂ is preferentially captured in larger pores, and cores with a larger proportion of large pores exhibit higher CO₂ saturation. However, during the imbibition processes, CO₂ captured in the cores with a higher proportion of micropores and lower permeability are less likely to be displaced by water, resulting in higher CO₂ storage efficiency. The local permeability changes of rock cores have a significant impact on the final residual gas distribution, and areas with lower permeability have higher final residual gas saturation and storage efficiency. This study can further improve the theoretical system of reservoir selection, efficient storage, and safety assessment in CO₂ saline aquifer storage projects.

Key words: CO₂ storage in saline aquifer, two-phase displacement, pore structure characteristics, local permeability, residual gas saturation