Experimental Study on the Sweep Patterns of CO₂ Flooding in Reservoirs with Interlayer Heterogeneity

CO₂ flooding is widely applied in low-permeability reservoirs due to its good injectivity, miscibility with crude oil, and ability to diminish the interfacial tension and viscosity between oil and gas. However, strongly heterogeneous reservoirs can lead to phenomena such as fingering and gravity override during the displacement process, which significantly reduce the sweep area. To address this, this study systematically investigates the mechanisms influencing CO₂ miscible flooding sweep efficiency under interlayer heterogeneity using a 2D, three-layer, equally thick, heterogeneous visualized physical model. The study compares oil displacement efficiency under three injection-production strategies: sequential activation of high-, medium-, and low-permeability layers, simultaneous activation, and accelerated flooding (1.0 mL/min). High-speed imaging technology is used for real-time monitoring of the CO₂ front migration, revealing the dynamic coupling mechanism of gravity segregation and mass transfer. Experimental results show that at low injection rates (0.1 mL/min), gravity override leads to early gas cap formation at the top of the high-permeability layer, restricting the gas-phase sweep area. In contrast, high injection rates (1.0 mL/min) enhance horizontal displacement forces, suppressing override and increasing the sweep area in the high-permeability layer. Additionally, the migration of CO₂ from the low-permeability layer to the medium-permeability layer intensifies under gravity segregation, and its recovery is strongly influenced by injection rates. The simultaneous activation strategy significantly improves recovery in the medium-permeability layer through synergistic vertical mass transfer and horizontal pressure difference, achieving a total recovery of 85.47%, making it the optimal injection-production strategy. This study provides a theoretical basis for optimizing CO₂ flooding in interlayer heterogeneous reservoirs, emphasizing the critical role of injection-production rates and interlayer coupling effects on sweep efficiency.