Study On Oil-CO₂-Water System Distribution In Nanopores Of Shale

As an important unconventional oil and gas resource,shale oil has great exploitation potential.CO₂ injection is an effective EOR（enhanced oil recovery）method.However,after tens of thousands of cubic meters of fracturing fluid is injected into the formation during horizontal well volume fracturing and the flow back rate of fracturing fluid is low in the production process.All of them resulting in increased water saturation near the well zone,which seriously affects the EOR effect of CO₂ injection.The important prerequisite for breaking through this bottleneck is to understand the occurrence laws of oil-CO₂-water in nano-micron pores throat.At present,relevant theoretical research have not clarified the influence mechanism of the water phase in nano-micron pore throats,but it has been recognized that neglecting the coupling effect of the water phase will lead to cognitive deviation of the occurrence laws of multiphase fluids.In this paper,the nanopores shale oil occurrence model is established by molecular dynamics method to study the effects of pore size,crude oil composition and rock composition on shale oil occurrence state.The adsorption behaviors of H₂O,CO₂,CH₄ and SDBS with crude oil in different sizes of nanopores are simulated under typical shale reservoir conditions（353 K and 40 MPa）,and the occurrence laws and adsorption mechanisms of different solvents/shale oil are revealed by density distribution and energy calculation.Finally,the oil-CO₂-water model is constructed to simulate the fluid adsorption process under the coexistence of oil-CO₂-water in pores of different sizes,and the distribution pattern of fluid under the coexistence of oil-CO₂-water in nano pores of shale is clarified.The simulation results show that the smaller the nano pore size,the higher the proportion of adsorbed shale oil in the pores,and the higher the density.Aromatic hydrocarbons are more easily adsorbed on shale surfaces.The adsorption ratio of shale oil in dolomite pores is similar to that in siltstone pores.When the same volume of solvent is injected,CO₂ or CH₄ is scattered around the system,H₂O is aggregated on the surface as a group,and SDBS is aggregated in the adsorbed oil layer.Density distribution and C atom calculation show that SDBS has the best desorption effect on adsorbed crude oil in pores,followed by H₂O,CH₄ and CO₂.This is mainly due to the CO₂ swelling of crude oil,which reduces the distance between the adsorbed oil and the rock surface,resulting in a significant increase in the binding energy and difficult desorption.The oil-CO₂-water simulation results show that CO₂ is mainly distributed at the oil-water interface,and the CO₂ injection amount and nano pore size have significant effects on the distribution of shale oil,and the efficiency of CO₂ desorption of adsorbed shale oil.In this paper,the adsorption characteristics and distribution of shale oil-CO₂-water three-phase system in nano-scale pore throats were studied,and the microscopic mechanism of CO₂ injection to improve shale oil production was revealed from pore scale...