Study On Fluid Occurrence And Flow Characteristics In Shale Pores Based On MD

Compared with conventional reservoirs,shale oil reservoirs have a wide range of nanoscale pores,and the occurrence state and flow law of shale oil are complex.At present,the research wall of shale oil using molecular simulation method mostly adopts single mineral and the shale oil model is mostly single component hydrocarbon.Therefore,in this paper,the molecular simulation method is used to study the adsorption and flow mechanism of multicomponent shale oil in the combined wall model and the variable cross-section pore model closer to the reservoir situation,and the occurrence form and flow law of shale oil in nanopores under different influencing factors are analyzed.The pore slits of organic graphene,inorganic quartz and composite wall were constructed,and the shale oil molecules were simulated by using branched alkanes,naphthenes and n-hexanoic acid with the same number of n-pentane,n-octane,n-dodecane and main atoms.The occurrence state of oil molecules in nanopores was studied by molecular dynamics method,and the effects of shale oil composition,wall wettability,shape factor and other influencing factors on the occurrence state of shale oil were analyzed.It is found that:（1）the shale oil molecules in the pores are cyclically fluctuating and asymmetrically distributed along the pore center,and the thickness of each adsorption layer is about 0.4–0.5 nm;（2）Pressure has no obvious effect on the adsorption of alkanes in pores,and temperature has a great influence on it.Low reservoir pressure,high reservoir temperature,large pore size,strong hydrophilic wall,high shape factor,low molecular weight and weak polarity of shale oil are beneficial to the desorption of shale oil molecules;（3）The adsorption of alkane molecules increased by 18.18%in the wall of the complete hydroxyl fossil quartz combination,which was contrary to the pore law of quartz,and the adsorption and transfer of n-hexanoic acid and cyclohexane were observed;（4）Under the same pore contact area,the pore shape factor decreases by 0.0171,and the proportion of adsorbed phase in pores increases by 29.93%.The non-equilibrium molecular dynamics method was used to simulate the flow of shale oil in the pores of graphene and quartz.The effects of temperature,displacement pressure and pore width on the flow of alkanes were analyzed and discussed.The results show that:（1）The velocity profile of shale oil in the pores of graphene is similar to that of piston,and the velocity profile of shale oil in the pores of quartz is sliding parabolic.The flow velocity increases with the increase of temperature and driving force,and the slip length increases first and then decreases with the pore size to a stable value of 0.62 nm;（2）The velocity profiles of shale oil in the two end-face pores ofα-quartz structure are different.Negative slip occurs in the pores ofαend-face,and the friction coefficient between hydroxyl fossil quartz wall and fluid is the smallest;（3）The pore velocity profile of the left and right combination wall is vacant piston-shaped.The fluid viscosity at the wall of the upper and lower combination wall pores is0.534m Pa·s,and the effective viscosity fitting is 0.417m Pa·s.The interlayer combination wall reduces the thickness of the quartz structure,and the shale oil slip velocity is reduced to87.63×10^-5?/fs;（4）The permeability value considering boundary slip and viscosity heterogeneity is 2.509×10^-3μm²,which is less than that considering only boundary slip velocity and greater than that considering only adsorption layer.The research results on the adsorption and flow characteristics of shale oil in nanopores in this paper can provide certain theoretical support for shale oil to consider the occurrence state and flow law of multi-component mineral wall pores.