Evaluation of the Driving Effect of the CO₂ Viscosity Enhancer Composite System in Extra-Low Permeability Sandstone Reservoirs

CO2 flooding is an important technology to enhance oil recovery and realize effective storage of CO2 in ultra-low permeability reservoir. However, due to poor reservoir properties, strong interlayer heterogeneity, and unfavorable mobility ratio of CO2, gas channeling easily occurs, resulting in low recovery. Chemically assisted CO2 thickening technology has been developed to control the gas flow rate and improve the CO2 repulsion effect. Through solubility and viscosity enhancement tests, the CO2 viscosity enhancer composite system is preferably constructed and then combined with the core drive experiments, the effect of CO2 viscosity enhancer composite system on oil drive in homogeneous and non-homogeneous cores is evaluated, the correlation between the drive efficiency and viscosity enhancement and solubility of the system is analyzed, and the mechanism of recovery enhancement is explored. The results show that the preferably constructed CO2–ASA-LAP composite system has a good effect for improving the gas drive effect under simulated formation conditions, and its improvement effect is positively related to the solubility and viscosity increase of the system. Combining oil repelling efficiency and economic considerations, ASA:LAP = 1:1 is selected as the optimal CO2 viscosity enhancer composite oil repelling system. For homogeneous cores, the CO2–ASA-LAP combined system drive can increase the recovery rate by 6.65% as compared with CO2 flooding. For heterogeneous cores, when the permeability difference is 5, the comprehensive recovery factor of the CO2–ASA-LAP system flooding is 8.14% higher than that of CO2 flooding. When the permeability difference increases from 5 to 10, the comprehensive recovery factor of the CO2–ASA-LAP system flooding increases by 1.85%.The injection of the CO2–ASA-LAP system has some injurious effect on the permeability of the reservoir core, and the smaller the permeability, the greater the degree of injury. The mechanism of the CO2–ASA-LAP system to improve recovery includes increasing CO2 viscosity, improving the oil repelling flow ratio, blocking high seepage channels, initiating low seepage residual oil, enhancing CO2 dissolution, and expanding the oil repelling effect.