Microfluidic study on the pore scale displacement behaviors of CO2 and oil under miscible or immiscible conditions

CO2 injection into reservoirs presents an effective method for achieving greenhouse gas sequestration while enhancing oil recovery. Pore scale CO2 displacement behavior is crucial for understanding the oil recovery enhancing mechanism. In this study, CO2 flooding experiments were carried out in a microfluidic chip under miscible or immiscible conditions to investigate the flow dynamics and oil distributions. Dynamic oil recovery efficiency was quantified, and the effects of pressure and temperature on residual oil morphology were analyzed. Experimental result demonstrates that the elevation of temperature and pressure significantly enhances the displacement efficiency. As the pressure and the temperature increased from 4.0 to 8.0 MPa and from 40 to 60 °C, the CO2 displacement efficiency increased from 31.81% to 88.32% and from 33.99% to 51.27%, respectively. Under immiscible conditions, CO2 tends to flow preferentially through larger pores, leading to pronounced gas channeling and low displacement sweep efficiency. Concurrently, the CO2 bubble is snapped off when it passes through narrow throats and is confined by capillary force. Upon reaching the minimum miscibility pressure, gas channeling is suppressed, and a stable piston-like displacement front develops, resulting in a higher sweep efficiency compared to immiscible conditions. It is also found that the residual oil cluster contributed dominantly to the oil recovery rate. As pressure increased from 4.0 to 6.0 MPa, the proportion of oil cluster decreased significantly from 49.7% to 20.3%.