Experimental And Numerical Simulation Analysis Of Gas Injection Displacement CH₄based On Dynamic Change Of Permeability

China is a big country of coal production.Due to complicated geological conditions and low permeability of coal seam,mine gas has been a threat to mine safety production.At the same time,gas is a kind of clean energy with high calorific value.Therefore,it is of great significance to realize efficient extraction and utilization of coal seam gas.This article around the coal seam N2 injection displacement CH4technology systematic simulation experiment is carried out,with seepage mechanics,surface chemistry and adsorption study,based on the theory of gas state equation is deduced in the process of gas injection displacement dynamic evolution model of porosity and permeability,and using the finite element numerical simulation of coal seam gas injection displacement experiment is simulated,and the permeability dynamic evolution model verification,perfected the theory model of gas injection displacement seam CH₄.In this paper,the gas injection displacement experiment of CH₄ in coal seam was carried out under different pressure of N₂ injection.The results showed that the breakthrough time of N₂ was advanced with the increase of gas injection pressure,but there was still a long period of CH₄ output in the gas outlet after N₂ breakthrough,indicating that the displacement of CH₄ in coal seam by N₂ injection was not on one surface.The relationship between the displacement volume ratio,displacement efficiency and the volume fraction of the gas outlet is measured,which provides a basic reference for the field gas injection test.In the process of displacement,the dynamic change of permeability is tracked and the experimental study on the permeability of coal seam by different gas injection sources is carried out,which provides experimental data for the verification of the dynamic evolution model of permeability in the following text.It is assumed that the flow field of binary gas in the coal mass is composed of the diffusion motion of adsorbed gas in the coal matrix and the percolation motion of free gas in the cracks between the matrix blocks,and the Fick diffusion law and Darcy percolation law are followed respectively.After gas injection,the original adsorption and desorption equilibrium of CH₄ in coal seam is broken,and mass exchange of N2-CH4 binary gas will occur in the pore-fracture system.Based on the above principles,the dynamic evolution model of the porosity and permeability of the displaced coal seam is established by considering the influence of the effective stress change of coal and the effect of adsorption expansion and desorption.Based on physical simulation experiment of the reflected binary gas adsorption,desorption,diffusion and percolation mathematical model,using COMSOL Multiphysics numerical simulation platform,the different N₂ injection pressure,under the condition of different initial permeability under the condition of the numerical simulation research,with the result of physical simulation,and verify the validity of the obtained dynamic evolution model,improve the gas injection displacement of coal seam CH₄ theory of numerical simulation.