Gas Adsorption/Desorption And Flow Mechanism In Shale Matrix Pores

The accurate understanding of gas storage and transport behavior in shale nanopores is of great significance for the rational development of shale gas reservoirs.In this paper,first,based on the characteristics of organic and inorganic nanopores in shale,the current methods for studying the adsorption/desorption and seepage mechanism of shale gas are introduced,and it shows that the molecular simulation method can simulate the adsorption of shale gas from a microscopic perspective,which is quite different from the experimental method.Then,the shale organic and inorganic nanopores are constructed using molecular simulation technology to simulate the gas adsorption behavior.The effect of different factors on the amount of gas adsorbed is revealed.And the law of gas adsorption is summarized.Finally,we propose reasonable assumptions to construct the gas transport model in shale matrix pores based on theoretical research and molecular simulation results.The effect of various factors on gas transport capacity is analyzed.The results of molecular simulation show that the adsorption of methane in shale matrix pores is physical adsorption.When the mixture of CH₄ and CO₂ is adsorbed in the pores of organic matter,the CO₂ is preferentially adsorbed.As the temperature increases,the density distribution of CO₂ is greatly affected,while the density distribution of CH₄ is less changed.When the CH₄ is adsorbed in the inorganic nanopores,if the pore size is small,an adsorption peak will be formed in the center of pores,and a symmetrical adsorption layer will be formed in the pores when the crystal cell thickness increases to 18.5?.If the water molecules are adsorbed in the inorganic nanopores,the adsorption space reserved for methane will be reduced,and the diffusion coefficient of water molecules is significantly lower than that of methane molecules,which makes it difficult for water molecules to diffuse.The results of gas transport show that the effect of TOC on total apparent permeability is related to pore pressure.For organic nanopores,the surface diffusion and slip flow contribute more to the total transport than the Knudsen diffusion,and the slip flow is more affected by gas adsorption/desorption than the Knudsen diffusion.For inorganic nanopores,the slip flow dominates in gas transport,and the apparent permeability decreases with increasing relative humidity.When the relative humidity is higher than 0.5,the apparent permeability decreases more rapidly.