Flow Mechanisms And Theoretical Model Of Oil And Water In Shale Micro-nano Pores

Shale oil is widely distributed and abundant in our country,which is one important type of resources for the fossil fuel exploration and development in the future.Since numerous pores in shale are in micro-nano scale,the conventional theory and research methods are diffuclt to describe the fluids flow behavior in shale.Clarifying the geofluids flow behavior,understanding the effect of interactions between the surface and liquids,and the interaction between the oil and water to the fluids flow mechanisms,providing an efficient theoretical model to describe the oil and water flow considering the micro-scale effect could not only be benefit for shedding light on the underlying mechanisms of fluids flow in nanpores,but also promoting the development of shale oil efficiently.In this work,the flow behaviours of oil and water were revealed from the view of molecular sight.Firstly,the static structure and transport phenomenon of oil/water single phase and two-phase flow were revealed by molecular dynamic(MD)simulation.The effect of pressure gradient,pore size,oil and water compostions were analyzed systematically.The influence of wall-fluids interacitons and oil-water interactions to the geofluids migration was also depicted,in addition,the key parameters were obtained.The results indicate that although the multi-layer structure of water molecules is observed near the quartz surface,only the first adsorbed water layer is hard to be motivated,and the alkanes liquids could slip on the quartz surface.The density dip appears near the oil-water interface and the alkane molecules show obvious orientation,resulting in the liquid-liquid slip between the oil and water velocities.The interfacial apparent viscosity which is around 0.22 m Pa·s and lower than that of bulk oil and water viscosity,is derived to characterize such liquid-liquid slip behavior.Secondly,the fluids distributions and flow behavior were investigated in various clay nanopores,such as kaolinite,montmorillonite and illite.The results show that the cations adsorbed on the montmorillonite and illite surfaces which show negative charges.Althogh the water density profiles are different in various clay nanopores,the water molecules in the first adsorbed layer are nearly sticked,and the thickness of the stick layer is 0.34-0.40 nm.Thirdly,the model of realistic kerogen slitlike nanopore was constructed,and the fluids distribution and flow behavior were investigated.Furthermore,by comparing the flow phenomenon confined in graphene and kerogen nanopores,it’s discussed that whether the ideal carbon structure could be used to replace the kerogen.The results show that owing to the atomic roughness of the kerogen surface,the oil and water flow shows no-slip boundary on the kerogen surface.While in the perfect smooth graphene slit pore,the large slip makes the fluxes of oil and water derivate from those in kerogen nanopores in the several orders of magnitude.With the same roughness,the flow behavior of alkane in graphene nanopore is closed to that in kerogen,while the flux of water in the graphene nanopore is slightly larger than that in kerogen.Meanwhile,it’s observed that the static structure and hydrodynamic properties of the mixture in the oil-water interface region is the same in kerogen,clays and quartz nanopores,and the velocity jump is all observed.Finally,coupled with multiphysics,such as heterogeneity of fulid distribution,slip on the surface and slip at liquid-liquid interface,the theoretical model of oil-water laminar steady state flow was created,which could describe the oil-water two-phase flow velocities and flux in inorganic and organic nanopores in shale accurately.Considering the enhancement to non-wetting phase flow caused by the liquidliquid slip,we also provided the apparent oil viscosity which could be used in the multi-scale flow simulation.Our work could shed light on the underlying mechanisms of geofluids confined in shale nanopores,such as static structure and hydrodynamic properties,and provide theoretical support for the geofluids flow behavior and the efficient development of shale oil.