Research on competitive adsorption laws of multiple molecules in coal seams containing fracturing fluid under thermal effect and displacement effect

In order to investigate the competitive adsorption pattern of N₂/CH₄ gases in coal under thermal and displacement effects, the study is based on the macromolecular model of bituminous coal, the adsorption behaviors of methane molecules, water molecules and fracturing fluid molecules under heated nitrogen injection are simulated by molecular dynamics and Monte Carlo methods, to investigate the interaction energy between fracturing fluid and coal surface molecules, and the microscopic laws of different gas injection temperatures, water contents and pore diameters on the competitive adsorption of N₂/CH₄. The results show that: the interaction between fracturing fluid molecules and coal molecules is 18.469 kJ/mol, the van der Waals interaction force between the systems is 23.732 kJ/mol, the electrostatic interaction force is -598.125 kJ/mol, and the stability of the system between the fracturing fluid molecules and the coal molecules is poor, which provides conditions for the injection of heated nitrogen to increase the seepage of the de-blocking; in the process of heated nitrogen injection, along with the increase of temperature, the adsorption capacity of N₂ relative to CH₄ in the model is enhanced; the increase in water content leads to a significant decrease in the adsorption of both CH₄ and N₂, and the adsorption capacity of CH₄ remains stronger than that of N₂ in the model; pore size has a significant effect on N₂ adsorption, the dominance of CH₄ begins to decrease with the increase of pore size, and the effect of N₂ replacement to drive out CH₄ is obvious when the pore size is 4 nm.