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

CO flooding is an important technology to enhance oil recovery and realize effective storage of CO in ultra-low permeability reservoir. However, due to poor reservoir properties, strong interlayer heterogeneity, and unfavorable mobility ratio of CO, gas channeling easily occurs, resulting in low recovery. Chemically assisted CO thickening technology has been developed to control the gas flow rate and improve the CO repulsion effect. Through solubility and viscosity enhancement tests, the CO viscosity enhancer composite system is preferably constructed and then combined with the core drive experiments, the effect of CO 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 CO-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 CO viscosity enhancer composite oil repelling system. For homogeneous cores, the CO-ASA-LAP combined system drive can increase the recovery rate by 6.65% as compared with CO flooding. For heterogeneous cores, when the permeability difference is 5, the comprehensive recovery factor of the CO-ASA-LAP system flooding is 8.14% higher than that of CO flooding. When the permeability difference increases from 5 to 10, the comprehensive recovery factor of the CO-ASA-LAP system flooding increases by 1.85%.The injection of the CO-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 CO-ASA-LAP system to improve recovery includes increasing CO viscosity, improving the oil repelling flow ratio, blocking high seepage channels, initiating low seepage residual oil, enhancing CO dissolution, and expanding the oil repelling effect.