Dimethyl Ether-Assisted CO ₂ Miscible Flooding in Tight Reservoirs: MMP Reduction, Displacement Mechanisms, and Reservoir-Scale Performance

Carbon dioxide (CO 2 ) miscible flooding is an effective enhanced oil recovery (EOR) technique for tight reservoirs, but its application is often constrained by an excessively high minimum miscibility pressure (MMP). This study examines dimethyl ether (DME) as a cosolvent to reduce the MMP of the CO 2 -crude oil system and improve displacement efficiency, with emphasis on phase equilibrium and transport mechanisms. The MMP of the DME-CO 2 -crude oil system is determined using two complementary numerical approaches: the multiple mixing-cell (MMC) method based on rigorous thermodynamic flash calculations and one-dimensional slim-tube simulations capturing dynamic multiphase flow and mass transfer. The two methods show good agreement at DME concentrations ≤30 mol %, but diverge significantly at higher concentrations. This indicates that dynamic displacement effects (e.g., viscous fingering and finite mass transfer rates) become increasingly important relative to thermodynamic phase behavior, providing key insights for scale-up to reservoir flow. Under reservoir conditions, DME-assisted CO 2 flooding increases oil recovery by 2.18%, 3.88%, and 5.66% at DME concentrations of 10, 20, and 30 mol %, respectively, compared with pure CO 2 flooding. Mechanistic analysis shows that DME promotes rapid local miscibility, reduces oil viscosity, and enhances CO 2 dissolution, lowering oil saturation in the transition zone. Permeability sensitivity analysis further reveals a nonmonotonic trend: incremental oil recovery first increases and then decreases with rising permeability, peaking at approximately 1.2 mD. This study provides insight into how a cosolvent modifies phase behavior and transport properties, offering theoretical guidance for designing more efficient miscible gas injection processes in tight reservoirs.