Enhanced oil recovery via CO₂ flooding in tight reservoirs: A pore-scale analysis

CO₂ flooding has become a key technology for enhancing oil recovery in tight reservoirs, with great application potential. However, certain microscopic mechanisms of this technology still need to be further clarified. In this work, a multi-component and multi-phase lattice Boltzmann model based on the pseudopotential scheme is constructed considering different CO₂ flooding behaviors and verified for both immiscible and miscible phases, showing good agreement. On this basis, the effects of capillary numbers, extreme wetting at different velocities, Péclet numbers and injection patterns under fractured conditions on the CO₂ flooding process are systematically investigated. The results show that a larger capillary number enhances the displacement effect, whereas an excessively large value tends to cause viscous fingering, leading to accelerated CO₂ breakthrough. High-velocity extreme wetting conditions result in a higher displacement effect than low-velocity conditions. Moreover, an increase in displacement velocity weakens the wetting effect dominated by capillary force, thereby reducing the difference in oil recovery observed under high-velocity extreme wetting conditions. Different Péclet numbers dominate different fluid transport mechanisms. When the Péclet number is around the unity, the synergistic effects of molecular diffusion and viscous flow are balanced, jointly dominating fluid transport. The pore-fracture combined injection mode integrates the advantages of pore and fracture injections and effectively delays CO₂ breakthrough in the fracture system, resulting in an optimal displacement effect. This model can be extended to research on multiphase flow in tight and shale reservoirs as well as CO₂ geological sequestration.