Stability and enhanced oil recovery performance of CO2 in water emulsion: Experimental and molecular dynamic simulation study

Enhanced oil recovery by CO2 flooding is conducive to energy and environmental development. However, the geological storage capacity and oil recovery of CO2 are relatively low due to its low viscosity, gravity segregation, and reservoir heterogeneity. CO2 in water (C/W) emulsion can reduce CO2 mobility and increase the volumetric efficiency of CO2. This paper investigated the stability and recovery performance of C/W emulsion. The results show that the C/W emulsion formed by AOT was more stable than the C/W emulsion formed by SDS. CO2 molecules were difficult to penetrate through AOT layers due to two hydrophobic chains in AOT molecules. Emulsion stability tests show that C/W emulsion was more stable with higher pressures and lower temperatures. The AOT hydrophobic chains were tightly packed at high pressure, thus increasing the thickness of the AOT layer, which can increase the stability of the C/W emulsion. Enhanced oil recovery by C/W emulsion at 8, 10, and 12 MPa was 23.72%, 34.88%, and 39.27%. The produced oil's density, viscosity, and asphaltenes content after C/W emulsion flooding were much lower than the original crude oil. Asphaltenes in residual oil were higher than in the original crude and produced oil. The micromodel test and MD simulation study indicate that CO2 could extract light components from the crude oil, including saturates and aromatics. As a result, the asphaltenes molecules would aggregate and precipitate. The light saturates molecules absorbed by the oil-water interface could form oil in water droplets and produce during the C/W emulsion flooding. The heavy components, including resins and asphaltenes, remained in the oil phase.