Capillary number effects on two-phase flow and residual oil morphology in water and supercritical CO₂ displacement: A microfluidic study

The capillary number (Ca) is a key parameter that quantifies the balance between viscous and capillary forces during fluid flow in porous media, significantly influencing fluid behavior and residual oil morphology in enhanced oil recovery. In this study, water and supercritical CO2 flooding experiments were conducted using a microfluidic chip at 75 °C and 22 MPa. Two-phase flow and residual oil morphology under both water and supercritical CO2 flooding were compared and analyzed at different capillary number. The results show that, at the same capillary number, water flooding exhibits more fingering, while CO₂ flooding displays more diffusive and unstable flow patterns. At high capillary numbers (lg Ca = −4.2), residual oil in water flooding is mainly in the form of emulsion, while in supercritical CO2 flooding, it is predominantly in droplet form. When the capillary number increases from −5.2 to −4.2, residual oil saturation decreases by 22.3 % in water flooding and 32.5 % in CO₂ flooding. The oil recovery factor in CO₂ flooding is 17.0 % higher than in water flooding at low capillary number conditions (lg Ca = −5.5), and 18.1 % higher at high capillary number conditions (lg Ca = −4.2). These findings provide valuable insights into the mechanism of capillary number on residual oil recovery efficiency.