Pore-Scale Modeling of CO ₂ -Enhanced Oil Extraction from Shale Nanopores with Thermodynamic Consistency and Phase Separation

CO2 injection has emerged as a promising technique for enhancing the shale oil recovery. During oil extraction from nanopores, phase separation of the CO2–oil mixture can significantly influence shale oil production. However, current studies on oil extraction always ignore the role of dynamic phase change at the pore scale. In this study, a multicomponent multiphase lattice Boltzmann model was applied to simulate the phase separation of the CO2–oil mixture during extraction. Our simulations reveal three distinct stages of oil extraction from a dead-end nanopore. In the first stage, oil is steadily extracted when the pressure remains above the bubble point. As phase separation begins, liquid flow declines sharply due to the strong capillary resistance. In the third stage, residual oil is either extracted or trapped, depending on pore size and wettability. We further investigated oil extraction from a nanoporous medium. Unlike dead-end pores, improved pore connectivity suppresses the emergence of the three extraction stages. Once phase separation occurs, oil transport in small pores is severely impeded by confinement effects. In contrast, a large fraction of oil is extracted from large pores, contributing over 75% of total recovery. Oil–wet surfaces promote oil extraction from large pores but hinder oil recovery in small pores. Moreover, increasing the CO2 concentrations consistently improves oil extraction in both types of pores.