Molecular Dynamics Simulations Of Interfacial Characteristics Among CO₂-Crude Oil-Water System With High Water Cut

CO₂ flooding can greatly enhance the oil recovery in high water cut period,which is one of the most promising ways to replace water flooding.The residual oil is often surrounded by water in high water cut period.At present,the study of the microscopic equilibrium mechanism of CO₂-crude oil-water system is not clear,which is the basic scientific problem that affects the effect of CO₂ flooding on improving oil recovery.In this paper,molecular dynamics simulation is used as the main means to study the microscopic mechanism of CO₂ break through the water film with high water cut and CO₂-crude oil microscopic mixing process,which can provide the basic theoretical support for CO₂ flooding mine applications.Firstly,we have simulated the properties of pure CO₂ and the interfacial interfacial characteristics of CO₂,crude oil,water interacted with each others,and verified the reliability of the molecular dynamics simulation method and the force field model.Then,the molecular model of CO₂-water-crude oil system with high water cut was established,and the mechanism of CO₂ break through the water film and the change of interfacial characteristics of the system were studied.Finally,the molecular model of CO₂+crude oil and water system was established,and the influence of CO₂ breaked through the water film on the interfacial chatacteristics of crude oil-water system was studied.The results demonstrated that when the water film thickness ratio is relatively small,CO₂can rapidly break through water film and dissolve with crude oil,and some of the CO₂accumulated at the crude oil-water interface,the water molecules were moved to both ends of the system.The interfacial tension,interface width,CO₂ interfacial accumulated density,the interfacial aera molecules numbers and so on,which change significantly with water film.With the increase of the water film thickness ratio,the ability of CO₂ to break through the water film is weakened,and the crude oil can not penetrate the water film to be extracted by CO₂.When the water film thickness ratio is 0.5,the interface parameters of system change trend is slowed down.Combined with the microcosmic process of CO₂ break through the water film,it can be concluded that the boundary of CO₂ break through the water film is the water film thickness ratio is 0.5.The temperature and pressure have little effect on the microcosmic process of CO₂ break through and the interfacial characteristics of system.In the CO₂+crude oil and water system,CO₂ molecules accumulated at the interface,which is equivalent to the existence of a CO₂ adsorption layer at the interface area of oil and water.With the increase of the mole fraction of CO₂,the distribution of octane molecule in interface area is changed from parallel to the interface to perpendicular to the interface,which can provide the space of the accumulation of CO₂ in the interface area,and enhance the degree of accumulation of CO₂ in the interface area.The interfacial tension of system decrease linearly with the increase of the mole fraction of CO₂,and the interface width,the interfacial aera molecules numbers and CO₂ interfacial accumulated density increase with the increase of the CO₂ mole fraction,and the change range is larger.The effect of temperature on the interfacial characteristics is relatively large,however,the pressure has little effect on the interfacial characteristics of the system.