Comparative study on CO2-oil， water-oil two-phase and CO2-water-oil three-phase displacement experiments

To address the challenges of mobilizing residual oil and ensuring the stability of CO2 sequestration in high-water-cut reservoirs， based on CT scans of real sandstone， etching patterns were constructed and used to fabricate glass-etched micromodels. A high-temperature and high-pressure （50 °C， 6.0 MPa） visual microfluidic experimental system was then set up. Displacement experiments were conducted sequentially for CO2-oil and water-oil two-phase systems， as well as for CO2-water-oil three-phase systems， enabling real-time visualization of multiphase flow processes and quantitative calculation of saturation. The results show that the CO2-oil two-phase displacement is strongly governed by capillary forces under the same injection rate. Due to the adverse effects of the capillary force threshold difference and viscosity ratio， the displacement front advances in an unstable manner， making it prone to gas channeling. The residual oil saturation is 0.70， and the CO2 sequestration saturation is 0.30. The water-oil two-phase displacement advances in a piston-like manner. Under the combined effects of water blockage and capillary forces， the residual oil primarily distributes as “H”-shaped columnar slugs， with the saturation decreasing to 0.21. When CO2 is injected after water flooding， a dual-displacement mechanism of “CO2 displacing water and water re-displacing oil” is formed. The synergistic action of the wetting barrier from the water film and the spreading of the oil film along the interface effectively suppresses front instability and enhances the mobilization of residual oil， further reducing the residual oil saturation to 0.15 and increasing the CO2 sequestration saturation to 0.45. The study indicates that the strategy of “water flooding followed by CO2 injection” simultaneously achieves enhanced oil recovery and increased CO2 retention， providing valuable insights for optimizing CO2 injection strategies and improving sequestration safety in high-water-cut reservoirs.