Remaining oil distribution in models with different heterogeneities after CO2 WAG injection: Visual research by nuclear magnetic resonance technique

Water alternating gas (WAG) injection is a widely used strategy for enhancing oil recovery (EOR) during gas flooding, and the mechanisms, operating parameters, and influencing factors of which have been extensively studied. However, with respect to its capacity in expanding macroscopic sweep volume under varying heterogeneities, the related results appear inadequate. In this research, three cores with different heterogeneities were used and flooded by the joint water and CO2 WAG, then the effects of heterogeneity on oil recovery were determined. More importantly, the cores after CO2 WAG injection were investigated using the nuclear magnetic resonance (NMR) technique for remaining oil distribution research, which could help us to understand the capacity of CO2 WAG in enlarging sweep volume at different heterogeneities. The results show that the presence of heterogeneity may largely weaken the effectiveness of water flooding, the more severe the heterogeneity, the worse the water flooding. The WAG injection of CO2 performs well in EOR after water flooding for all the cores with different heterogeneities; however, it could barely form a complete or full sweep throughout the low-permeability region, and un-swept bypassed regions remain. The homogeneous core is better developed by the injection of the joint water and CO2 WAG than the heterogeneous and fractured cases.