Multiphysics Coupling Simulation Study Of Coal Seam Gas Diffusion Process

According to the characteristics of multi-scale diffusion of coal seam gas,gas is transported and diffused in various ways in the matrix.This paper studies the gas diffusion process comprehensively through the theoretical analysis of coal matrix diffusion,experimental study on pore structure characteristics,experimental study on gas diffusion law and numerical simulation of coal bed methane diffusion and migration.The basic parameters of coal samples were obtained by bulk density measurement,industrial analysis and adsorption constant of “a” and “b”.The pore characteristics and pore size distribution of coal samples were obtained by mercury pressure experiment and low temperature liquid nitrogen adsorption experiment.According to the theoretical analysis,the gas diffusion law of different pore characteristics coal is obtained.Combined with the variable diameter capillary model,the critical diameter is used as the basis for division,and the effective diffusion coefficient of various diffusion modes is obtained according to the proportion of different diffusion modes.The experiment platform of gas diffusion at high temperature and high pressure is established.The coal samples are divided into 0.25mm-0.5mm,0.5mm-1mm and 1mm-2mm.Different pressure and temperature conditions are applied to the coal samples.The influence of particle size,pressure and temperature on the diffusion and diffusion speed of coal gas is studied respectively.The sensitivity of temperature and pressure to the diffusion of coal gas is analyzed.It is concluded that under low pressure,the gas diffusion quantity has a great influence on temperature The change of degree is not sensitive;under high pressure,the influence of gas diffusion on temperature is more sensitive.In order to study the adaptability of different diffusion models,the experimental data are fitted with the classical diffusion model and the dynamic diffusion model respectively.The results show that the dynamic diffusion model can fit the actual diffusion better,and the fitting accuracy is high,which can make up for the shortcomings of the simulation accuracy of the classical diffusion model.Combined with the effective coefficient and dynamic diffusion coefficient model based on the pore characteristics of coal,the gas migration and diffusion field equation of coal seam considering multi-scale diffusion of matrix is established,and the dual pore medium is improved.Taking Honghui coal mine in Guizhou Province as an example,a numerical model is established to simulate the gas drainage in coal seam.Through the numerical simulation results,the gas diffusion flow in coal matrix and gas flow in coal fracture are obtained.At the same time,the gas extraction from the coal seam of Honghui coal mine is carried out,and the daily gas flow and velocity of the borehole are recorded.The actual data are compared with the simulation results.The model can simulate the actual situation of drilling gas extraction,and can provide guidance and reference for the development of deep coal bed methane and the prevention and control of mine gas.