Dynamic behaviors and recovery mechanisms of large-PV CO₂ flooding in multi-scale reservoir systems

• A multi-scale experimental–numerical framework was established to elucidate the EOR mechanism of large-PV CO 2 flooding under miscible and near-miscible conditions. • The confined effect in nanopores significantly reduced interfacial tension and enhanced CO 2 –oil miscibility, promoting microscopic miscible mobilization in low-permeability reservoirs. • The permeability contrast threshold (∼5:1) was identified as a critical control on interlayer displacement balance and overall recovery efficiency. The efficiency of CO 2 flooding for enhanced oil recovery largely depends on whether the injected gas can effectively penetrate the matrix surrounding the main flow channels. However, realizing efficient utilization in complex reservoirs with multi-scale pore–throat structures and large fractures remains a major scientific challenge. This study integrates long-core flooding (5, 1, and 0.5 mD), artificial-core experiments (0.5–5 mD), and numerical simulations to investigate miscible and near-miscible CO 2 flooding under large pore-volume (large-PV) injection. Results show that decreasing pore throat size significantly reduces oil–gas interfacial tension and widens the two-phase region, leading to a marked reduction in residual oil saturation. Confinement effects attenuate capillary trapping while enhancing CO 2 diffusion and dissolution in fine pores, enabling microscopic miscible displacement. Large-PV injection strengthens the synergy between macroscopic sweep and microscopic mobilization. Continuous CO 2 flooding yields greater incremental recovery as permeability decreases, with the 0.5 mD core improving from 28.97 % to 80.7 % (+51.73 %). These findings indicate that high permeability layers contribute most of the production gains during CO 2 –WAG, whereas low-permeability layers demonstrate much higher displacement efficiency. In addition, reservoir heterogeneity is confirmed to be a critical factor controlling CO 2 flooding performance. Reducing permeability contrast greatly improves low-permeability layer utilization, increasing the total recovery factor from 0.61 to 0.74. However, this improvement trend exhibits a critical threshold at a permeability contrast of approximately 5:1. Overall, large-PV injection and confined-fluid behavior jointly promote a dynamic balance between macroscopic sweep and microscopic miscibility. By maintaining the injection pressure close to but slightly below the miscible point and optimizing the water–gas alternation ratio, CO 2 flooding performance in low-permeability and heterogeneous reservoirs can be maximized.