Microcosmic Mechanism Of Methane/Nitrogen Adsorption In Coal Based On Molecular Simulation

Most coalbed methane reservoirs in China are characterized by low permeability,low pressure and low gas saturation,and methane displacement by nitrogen fracturing is one of the stimulation measures adopted at present.However,when methane is displaced by nitrogen,differences in macromolecular structure of coal lead to differences in adsorption location,adsorption capacity and displacement capacity,etc.In order to reveal differences in adsorption of nitrogen and methane by macromolecular structure of coal and adsorption mechanism,it is necessary to conduct in-depth studies from the perspective of molecular structure.In this paper,the coal samples from Pingdingshan No.6 Mine（PDS）,Hebi No.4Mine（HB）and Changping Mine（CP）of Shanxi Province were taken as research objects,and vitrinite reflectance,elemental analysis,¹³C NMR,FTIR and XPS photoelectron spectroscopy were tested respectively.With the help of Origin data analysis and Materials Studio numerical simulation software,the spectra of infrared spectrum and electron energy spectrum were analyzed by combining peak fitting,and the three-dimensional structure model of macromolecules of coal samples was constructed and optimized,and the energy composition of the model was analyzed.Sorption Isotherm module of Materials Studio software was used to simulate isothermal adsorption of coal samples,and the variation characteristics of adsorption capacity of CH₄,N₂ and CH₄/N₂ mixtures with pressure and adsorption position in coal macromolecular structure were obtained.Forcite module is used to calculate the change of action energy when the amount of gas adsorbed by the molecular structure of coal changes,obtain the strength of the adsorption of gas by the structure of each functional group,and analyze the optimum adsorption and displacement pressure of each pore.The results show that the coal macromolecular structural formulas of PDS,HB and CP coal are C₆₉H₄₃NO₃,C₁₆₆H₉₃N₃O₈ and C₁₆₉H₁₀₅N₃O₇,respectively.The molecular weights were 976,2257 and 2289,respectively.The surface areas were1328.87（?）²/structure,2563.07（?）²/structure and 2831.90（?）²/structure,respectively.With the increase of coal metamorphism,the molecular mass and surface area of coal increase.When CH₄ was adsorbed alone or 50%CH₄+50%N₂ was adsorbed,the order of adsorption capacity of coal molecular structure from strong to weak was aromatic ring structure>oxygen-containing functional group>pyridine nitrogen>aliphatic structure.Aromatic ring structure>pyridine nitrogen>aliphatic structure>pyrrole nitrogen>oxygen-containing functional group;Aromatic ring structure>pyrrole nitrogen>aliphatic structure>oxygen-containing functional group.When N₂ was adsorbed alone,the adsorption capacity of PDS coal was as follows:oxygen-containing functional group>aromatic ring structure>pyridine nitrogen>aliphatic structure.When 50%CH₄+50%N₂ adsorption is mixed,the metamorphism degree of coal is different,and the pressure of N₂ displacement methane is also different.When PDS coal achieves the best displacement effect,the pressure is 7.5MPa.HB coal and CP coal are 6 MPa.When displacement pressure is 7.5 MPa,the adsorption capacity of pyrrole nitrogen to nitrogen is enhanced.The research results can lay a foundation for the study of micro mechanism of nitrogen methane displacement in coal with medium and high metamorphic degree.