Molecular dynamics investigation of DME assisted CO2 injection to enhance shale oil recovery in inorganic nanopores

In view of the unsatisfactory improvement effect of pure CO2 injection on the shale oil recovery in microscopic pores, the molecular dynamics method was used to simulate the process of dimethyl ether (DME) assisted CO2 injection in nanopore to recover shale oil. Firstly, cosolvents with different functional groups were selected to assist CO2 injection to compare the enhanced oil recovery (EOR) effect of shale oil. Subsequently, the microscopic mechanism of DME promoting CO2-EOR effect was systematically revealed. Then, the effects of temperature, pressure, DME molar content and shale oil composition on the EOR mechanism of CO2-DME mixture were further studied. Finally, the sensitivity difference of EOR effect to these influencing factors was compared. The research results showed that the EOR effect of DME-assisted CO2 injection (42.96%) is the best, followed by acetone (38.61%), propane (33.38%), ethanol (32.42%) and pure CO2 (22.43%). The microscopic EOR mechanism of DME-assisted CO2 injection includes improving the solubility and competitive adsorption capacity of CO2, as well as reducing oil viscosity and two-phase interfacial tension (IFT). The EOR effect of CO2-DME mixture is the best when the system temperature rises to 100℃, but excessive high temperature has a negative impact on CO2 solubility and intermolecular interaction. The increase of system pressure increases the fluid density, shortens the mass transfer distance, and reduces the IFT, which promotes the solubility, miscibility and competitive adsorption capacity of CO2-DME mixture, thereby continuously improving the shale oil recovery. Although the increase of DME content can improve the competitive adsorption capacity and reduce the IFT, it also reduces the solubility and diffusion capacity. The EOR effect is best when the mole fraction of DME reaches 40%. The increase of the proportion of non-hydrocarbon compounds and heavy hydrocarbon components reduces the solubility, miscibility and competitive adsorption capacity of CO2, and the negative impact of the former is greater than that of the latter. The EOR effect of DME-assisted CO2 injection on shale oil is most affected by pressure, followed by DME mole fraction, temperature, and oil composition.