Fingering crossover and enhanced oil recovery mechanism of water-alternating-CO2 injection

Combined with strengths of water and CO2 flooding, water-alternating-CO2 (WAG) injection stabilizes displacement front and suppresses viscous fingering. Optimal injection factor and water–CO2 slug ratio (W:G) from both oil field and micromodel were given in previous studies. By analyzing residual oil types and fingering patterns at steady state, enhanced oil recovery (EOR) mechanism of WAG was revealed by comparing with water and CO2 flooding, respectively, regardless of interactions between three phase fluids. However, when oil, water, and CO2 are simultaneously present, the three-phase flow is much more complex, fingering is time-varying, and the patterns are different. Previous knowledge of EOR mechanism of WAG cannot indicate the fingering characteristics in three-phase flows and patterns crossover, resulting in the optimal W:G not being well explained. We simulated a WAG displacement by varying W:G and pressure difference between the inlet and outlet ΔP. EOR mechanism of WAG and optimal W:G were obtained by analyzing time-varying fingerings and patterns crossover in three-phase flows. The results showed that (1) fingering patterns change significantly before and after CO2 injection. At low ΔP (100 < ΔP <1000 kPa), capillary fingering converts to viscous fingering, and at high ΔP (ΔP > 1000 kPa), viscous fingering converts to stable displacement; (2) fingering crossover is discrepant at various W:G, which most easily occurs when W:G = 1:3 and 1.5:3; and (3) EOR capacity of WAG injection is significantly promoted when the fingering pattern changes. These results are helpful to better understand the dynamic displacement mechanism of WAG injection and are important for design and optimization of WAG injection operation.