Residual oil retention and stripping mechanism of hydrophilic pore surfaces during CO2 and N2 combined gas flooding

The mixed gas of CO2 and N2 can replace the great mass of the remaining oil in the center of the porous media channel, but the surface of the pore channels still retain more residual oil. Most of the previous studies focused on how to displace the residual oil in the center of the pore passages, while ignoring how to enhance the recovery degree of the residual oil on the pore surfaces. Moreover, there are few research results on the mechanism of residual oil retention and stripping on pore surfaces during CO2–N2 combined gas flooding, which need to be improved and supplemented. In this paper, the microscopic interaction process among gas phase, oil phase and pore surfaces were explored by molecular dynamics, the changes in microstructural between n-decane molecules and the spatial distribution of gas molecules and n-decane molecules in the system was clarified. The interaction energy between gas, n-decane and pore surfaces were analyzed to reveal the mechanism of remaining oil retention and stripping. The results indicated that when only CO2 in the gas phase, the oil films were easily divided by CO2 molecules, and the adsorption layer formed by CO2 molecules on the quartz surface could well detach the oil phase from the quartz surfaces. With the increase of the proportion of N2 in the mixed gas, the interaction energy between the gas and n-decane phase and between the gas phase and pore surfaces decreased, and the capacity of the gas phase to peel off the oil films from the pore surfaces was weakened. When the proportion of N2 in the gas phase exceeded 25%, the gas phase has no ability to peel off the oil films from the quartz surfaces. The research results have guiding significance for the CO2-N2 combined gas flooding to further tap the remaining oil.