Effect of CO2-assisted surfactant/polymer flooding on enhanced oil recovery and its mechanism

D oilfield is a typical heavy oil reservoir affected by bottom water and has the characteristics of high porosity, high permeability, and strong heterogeneity. However, water flooding leaves oil behind due to unfavorable mobility ratios and capillary forces. The layer (well section) with poor or no liquid entry ability originally has been improved in the process of surfactant/polymer flood. However, fluid entry distribution interlayers were not improved obviously in the middle and late stages. To further improve the recovery efficiency of surfactant/polymer flooding on bottom water reservoirs, the CO₂-assisted surfactant/polymer flooding development method had been proposed. The optimal composition of the surfactant/polymer system was selected by evaluating the oil-water interfacial tension and viscosity. Three-dimensional cores with bottom water were used in CO₂-assisted surfactant/polymer flooding experiments to improve oil recovery. On this foundation, the screening of oil displacement agents, optimization of oil production well location, and injection pressure were carried out. Based on the comprehensive analysis of the produced liquid, the CO₂ assisted surfactant/polymer flooding to enhanced oil recovery mechanism was analyzed. The results showed that the surfactant named BS had a low oil-water interfacial tension of 3.79 × 10^-3 mN/m. In addition, salinity decreases the viscosity of the surfactant/polymer solution. It was found that conducting CO₂ huff and puff after chemical flooding can improve oil recovery. Among the four chemical agents, CO₂-assisted the anionic nonionic surfactant and the salt-resistant polymer system flooding had the highest recovery. Moreover, the location of the oil production well, that is, the distance between it and the bottom water layer, has an impact on the recovery. During the experiment, 2.5 cm was the farthest distance between the oil production well and the bottom water layer, at which point the recovery was highest at 70.9%, and the bottom water breakthrough rate was 2.4%. These results provide theoretical and technical support for the development of CO₂-assisted surfactant/polymer flooding and its application in wells.