Effects Of Methane On Shale Oil Interfacial Properties And Migration Behaviors By Molecular Dynamics Simulation

The unconventional petroleum resources mainly including shale oil and gas have become a highlight in global petroleum exploration and development.However,shale reservoir is mainly composed of nanoscale pore.Due to the nanometer size effect of nanopores,traditional migration mechanism of petroleum is no longer suitable for shale oil and gas.Shale oil is accompanied by a certain amount of associated gas in the process of formation and migration,and associated gas main component is mainly methane.Moreover,methane can affect the crude oil physical properties such as density,interfacial tension,and fluidity,which there are few related studies.And it has not been clear that the effect of methane on the migration mechanism of shale oil.Traditional experimental methods have been challenged to study the microscopic properties of oil and gas at the nanoscale.But molecular dynamics simulation can have dramatic advances in characterizing the micro dynamic process and molecular interaction.Therefore,molecular dynamics simulation was used to study the effect of methane content,temperature and pressure on the oil-water interface in this work.On this basis,we investigated the effect of methane on the micro migration process of shale oil in the nano-pore throat.First,the effects of the methane content on the oil-water interface were investigated.This study found that interfacial tension(IFT)of oil-water linearly decreased(from 42.18 mN/m to 37.75 mN/m)with the methane mole fraction increasing(from 0 to 0.8).Then we analyzed the interfacial microstructure of oil-water.As the methane mole fraction increased,the interfacial thickness and interfacial roughness increased.These leaded to a decrease of the IFT.Furthermore,the effects of the temperature and pressure on the IFT were also investigated.Our calculations indicated that the IFT decreases with the rising temperature.By comparison,the IFT was more sensitive to temperature when shale oil was accompanied with the methane.And the IFT increases with the strengthening pressure in shale reservoir environment.Second,the effect of methane on the oil transporting through the nano-pore throat was investigated.The simulation results showed that the oil drop was hampered during its transporting through the nano-pore throat by a large resistance.And the resistance was caused by the self-deformation of the oil drop,the drag between the oil droplets and the nanopore wall and the capillary resistance.However,with the methane content increasing,the threshold pulling force(TPF)of the oil through the nano-pore throat reduced.The causes of this phenomenon are as follows.On the one hand,because of the methane dissolving in the oil phase,interaction energy between oil molecules became weak and the volume of oil drop increased,which meant structure of oil drop could more easily change and enhanced the fluidity of the oil drop.On the other hand,the methane molecules accumulated at the interface of oil-water,which not only decreased the oil-water IFT,but also weaken the interaction energy between oil and nanopore.Meanwhile,it was reduced that the resistance of oil transporting through the nano-pore throat.The same effect appeared when the temperature increased.The results indicate that when crude oil associated gas is present,it facilitates the crude oil through narrow pore throats,promotes the migration of crude oil,and enhances oil recovery.These findings are helpful for understanding the interfacial properties and migration of shale oil in the nanopore at the molecular lever,and facilitate technological developments in exploration and development of shale oil and gas.