Feasibility and mechanism of miscible oil recovery in unconventional reservoirs using CO2-enriched hydrocarbon gas mixtures: Insights from phase behavior and compositional simulation

• This work explores the EOR mechanisms and feasibility of CO 2 -hydrocarbon gas co-injection. • Gas mixture properties and phase behavior are significantly improved by CO 2 enrichment. • Mixing mass transfer in porous media is further investigated under miscible and immiscible status. • Co-injecting associated gas with limited CO 2 contributes to boosting the EOR-GCS process. CO 2 -enriched hydrocarbon gas mixtures take advantage of the low miscibility pressure of CO 2 and the compositional similarity of hydrocarbons, enabling enhanced oil recovery (EOR), reducing CO 2 emissions, and mitigating carbon source limitation. However, the potential principles of their multi-component mass transfer in oil remain unclear. In this paper, the EOR performance and mechanisms of CO 2 -hydrocarbon gas co-injection in unconventional reservoirs are clarified by integrating phase behavior with compositional simulation. The physical properties of CO 2 -hydrocarbon gas mixtures and their phase behaviors with oil are studied in a fluid model. The impacts of different gases on oil–gas miscibility, gas front migration, and mixing mass transfer are analyzed using a slim-tube simulation study. The EOR mechanisms of CO 2 -hydrocarbon gas co-injection and its feasibility in reservoirs with carbon source limitation are explored based on compositional simulations. Results show that regulating the CO 2 ratio in the gas mixtures improves both oil–gas miscibility and carbon utilization efficiency. The comparison between gas front migration and oil component distribution under miscible and immiscible status indicates that CO 2 -hydrocarbon gas miscible flooding enhances CO 2 dissolution, promotes mass transfer, and delays gas breakthrough. Gas dissolution, oil expansion, component extraction, oil–gas miscibility and energy replenishment are essential mechanisms that boost the synergistic CO 2 -hydrocarbon gas-EOR process. Nearly 52.53% CO 2 is stored availably, while the rest is produced after the end of 20-year EOR. This impure CO 2 injection technology combines abundant associated gas with limited CO 2 for oil production, which provides engineers with valuable insights into methane management and carbon reduction in unconventional reservoirs.