Microfluidic experimental study of CO₂-water-oil three-phase flow in porous media under both immiscible and miscible conditions

Understanding the mechanisms of CO₂–water–oil three-phase flow is crucial for enhancing oil recovery and improving CO₂ storage efficiency. In this study, a high-pressure, high-temperature microfluidic system is utilized to directly visualize pore-scale CO₂–water–oil interactions under both immiscible and miscible conditions. Under immiscible conditions (20 °C, 5.0 MPa) at a high water-cut stage (>70 %), injected CO₂ forms intermediate oil films at gas–water interfaces, promoting preferential flow and residual oil mobilization through coalescence–migration and dual/multiple displacement events. Flow path evolution, driven by capillary instabilities and snap-off, leads to a 9.69 % increase in oil recovery and 20.82 % CO₂ storage. Under miscible conditions (40 °C, 8.0 MPa), supercritical CO₂ (scCO₂) rapidly diffuses into oil, enhancing extraction and mixing, thereby displacing oil from isolated regions and achieving 19.54 % recovery, with 68.94 % CO₂ storage. Notably, scCO₂ can penetrate water films and expand trapped oil in blind-end pores, dynamically thinning the water layer even in high water-cut conditions. A theoretical model describing scCO₂ penetration through water films is developed and validated against experiments, enabling the prediction of breakthrough times across varying film thicknesses.