Microscopic Simulation Of Flow Of Shale Gas In Nanopores Under Multiphase Coexistence

With the increasing contradiction between energy supply and demand at home and abroad.the development of new energy has become an urgent problem to be solved.As a new energy source,shale gas is in line with the needs of current development.There are many states of shale gas,a small part of which exists in dissolved natural gas,and most of the rest are adsorbed on the surface of organic matter and rock particles or in pores and cracks.The structure and types of pores are also diverse,mainly composed of shale kerogen,clay minerals and the like.The shale gas reservoir is a multi-scale complex system containing micro-nano pores,microcracks and cracks.The pores in the shale are generally on the nanometer scale,so the study and analysis of the seepage mechanism of shale gas in the micro-nano pores can To lay a solid foundation for the development and exploitation of shale gas in the future.In this paper,molecular dynamics methods were used to simulate the flow mechanism of single-phase and multi-phase fluids in micro-nano pores.The effects of different driving accelerations and different pore sizes on the flow of single-phase water and methane in micro-nano pores were studied.The flow mechanism of gas-water two-phase fluids in micro-nano pores under different water saturations was simulated.Flow characterizations such as density,potential energy,velocity,self-diffusion coefficient,slip velocity,and flow rate were analyzed and discussed.The main work and results of the research are as follows:(1)Using kaolinite pores as the shale reservoir matrix,the flow law of single-phase methane and water in micro-nanopores was simulated by molecular dynamics method.When a small driving acceleration is applied to methane and water,they all form a stable adsorption layer at the wall surface;however,when a large driving acceleration is applied,the density of the first adsorption layer of methane is lowered,and the bulk density is rising.High,while the density curve of water molecules is basically unchanged,and the potential energy curve of water molecules is basically unchanged.This also shows from the side that the driving acceleration is basically not effective for water molecules.Both methane and water exhibit a higher first adsorption layer in the small pores and a larger bulk density in the large pores,because as the pores increase,the methane and water molecules are affected by the wall.Less,resulting in a higher speed,increased collision with the density of the wall adsorption layer molecules,resulting in a decrease in the density of the adsorption layer.Methane slips at the wall surface.With the increase of driving acceleration and pore size,the velocity and slip length of methane show an increasing trend.The water molecules form strong hydrogen bonds with the hydroxyl groups on the wall.It causes no slip of water molecules,but its speed still increases with the increase of aperture and driving acceleration.Since the volumetric flow rate and the velocity of the fluid are positively correlated,the flow rates of methane and water are increased to varying degrees.The diffusion coefficients of methane and water molecules increase with the increase of pore size.The diffusion coefficient of methane is proportional to the driving acceleration,but the diffusion coefficient of water molecules does not change with the increase of driving acceleration.(2)The flow mechanism of gas-water two-phase fluid in micro-nano pores under different water saturations was simulated by molecular dynamics method.Water molecules adsorb to the surface of kaolinite more preferentially than methane molecules.This phenomenon indicates that kaolinite is a hydrophilic rock.The presence of water molecules limits the flow of methane molecules,causing the velocity of methane molecules to decrease with increasing water saturation,while water molecules have a greater number of water molecules at higher water saturations,leading to their The velocity is positively correlated with water saturation.The study also found that the velocity of methane is still parabolic under different water saturation,and the velocity of water molecules cannot be described by the classical poiseulle flow model.The flow rate and diffusion coefficient of water molecules are proportional to the water saturation,while the flow rate and diffusion coefficient of methane are inversely proportional to the water saturation.