Study On Gas Storage, Diffusion And Migration In Shale Nanopores Based On Molecular Simulation

Shale gas is rich in reserve and is widely distributed.It is a kind of unconventional energy with great potential.But compared with the conventional oil and gas,the research related to shale gas is relatively limited.The reason is that,on the one hand,a history of exploitation of shale gas is relatively short.On the other hand,the porosity and permeability of density shale is very low.The micro effect of shale gas is obvious.The research content of conventional experimental methods is limited,and experimental results are not accurate.Therefore,it is necessary to explore new methods and theory.The shale nanopores model was built by molecular simulation.The effects of pressure,temperature and pore size on methane storage and migration in shale were studied.The results show that,due to the adsorption effect,the average density of methane in shale nanopores is greater than the bulk density of methane in the same conditions.Methane density in 2nm organic pore is about 170 kg/m³in 10MPa and 303K,while the bulk density is 74 kg/m³.The difference between the two kind density increases and then decreases with the pressure increasing.The stronger adsorption ability of the media is,the greater the difference between two kind of density is.The distribution of methane in nanopores is non-uniform.There is an adsorption layer at surface,and the adsorbed layer has a certain thickness and density.A calculation model is proposed considering the shale gas adsorption layer.In the calculation example,there is an error 0²6%between two models about the calculation results,which is related to the pressure and adsorption capacity of media.Shale gas production is simulated with pressure decreasing.The results show that the recovery of methane in different size pores is different.When the pressure is reduced from 40MPa to 10MPa,CH₄ recovery is about 35%in 1nm pores and is about 63%in 4nm pores.Methane in large pores is produced gradually with pressure decreasing.However,methane in small pores has a low recovery at the beginning of pressure drop,and is mainly produced in the low pressure.There is competitive adsorption between CO₂ and CH₄.And the sorption capacity of CO₂ is much larger than that of CH₄.The adsorption heat all was less than 42kJ/mol,belonging to physical adsorption.Diffusion coefficient of CH₄ in confined space is less than the diffusion coefficient of non confined space under the same conditions.The diffusion coefficient of CO₂ is less than that of CH₄.The migration of methane in nanopores is the result of joint action of multiple mechanisms,including diffusion,surface diffusion,slippage and so on.The contribution of various factors to the total flow flux is different when the pressure,temperature and pore size is different.Smaller the pore size is,more obvious the contribution of diffusion is.The effect of diffusion will fall with the pressure increasing.