Research On Distribution And Flow Characteristics Of Oil And Water In Shale Pores

Shale oil is a vital constituent part of unconventional oil and gas.Understanding the distribution and flow characteristics of oil and water in shale nano-pores is prerequisite for studying oil and water migration in shale pores and enhancing oil recovery,and is of great significance for shale oil exploration and development.In this paper,the pore size and mineral composition of shale samples were determined by low-temperature nitrogen adsorption experiment and X-ray diffraction analysis(XRD),and the fluid composition was determined by gas chromatography.The determination results show that the nano-pores of shale were mainly distributed between 1.0 nm and 18.0 nm,and the pore distribution curve had a peak around 4.8 nm;The mineral components of shale were mainly illite and quartz,the main carbon peak in the gas chromatogram was n C₁₉,and the n-alkane with the largest mass fraction was n C₁₉H₄₀.According to the experimental results,the molecular dynamics model of shale pores,alkanes and water was constructed based on the molecular dynamics simulation method,the occurrence state and diffusion characteristics of shale oil under different pore sizes,temperatures and pressures were simulated by equilibrium molecular dynamics,and then the distribution of oil and water under different wettability and pore sizes was studied;Finally,the oil-water flow characteristics in shale pores were investigated by non-equilibrium molecular dynamics simulation.The research results indicate that the continuous density distribution curve of single-phase oil was axisymmetric.With the increase of pore size,the density of each adsorption layer was enlarged,the average density of the adsorption layer increased,and the adsorption critical point was 3.2 nm;As the temperature increased,the density of adsorption layer decreased and the self-diffusion coefficient increased;When the pressure rose,the density of each adsorption layer changed slightly,and it had a strong influence on the diffusion coefficient of bulk fluid in a certain pressure range.In the oil-water density distribution curve,water molecules presented a symmetrical layered structure,and oxygen atoms in water were preferentially connected to the wall.The stronger the hydrophilicity,the more adsorption layers;the larger the pores,the larger the thickness of the adsorption layer;The oil density curves in the pores of strongly hydrophilic,weakly hydrophilic and mixed wetting shale were respectively piston-like,top-like and discontinuous.The parallel diffusion coefficient in the system was greater than the vertical diffusion coefficient,and the stronger the hydrophilic capacity,the weaker the diffusion capacity of water.The more hydrophilic the pore surface is,the more hydrogen bonds are formed,and the higher the degree of hydrogen bonding is;On the hydrophilic wall,the interaction between fluid and wall was mainly manifested as Coulomb force,while on the mixed wetting wall,the interaction between fluid and wall was mainly characterized by van der Waals force;The oil flow rate was the fastest in the mixed wetting pores,followed by weakly hydrophilic pores and strongly hydrophilic pores in sequence,which may be related to the interaction force and friction coefficient between fluid and solid wall.