Study on the law of CH4/N2 adsorption-desorption-diffusion-seepage and the coal deformation characteristics under triaxial stress

The adsorption-desorption-diffusion-seepage characteristics of CH4/N2 are important indicators to measure the gas extraction capacity and N2 injection on gas displacement effect. In order to study the adsorption-desorption-diffusion-seepage law of CH4/N2 and the deformation characteristics of coal in the whole process of the experiment, the adsorption-desorption-seepage experiments of CH4/N2 were carried out. The timeliness characteristics of gas adsorption-desorption-seepage and coal deformation were analyzed. The influence of gas volume fraction on gas adsorption-desorption-seepage and coal deformation was clarified. The quantitative relationship between gas adsorption-desorption-seepage capacity and coal deformation were clarified. The space-time characteristics and anisotropic characteristics of coal deformation were discussed. A coal strain model for the whole process of CH4/N2 adsorption-desorption-seepage experiment was established. The results showed that under the same injection pressure of gas, the adsorption capacity/desorption capacity of pure N2 was the least, and the equilibrium time of adsorption/desorption was the shortest. As the increase of CH4 volume fraction in the mixture, the gas adsorption capacity/desorption capacity increased, and the adsorption/desorption equilibrium time prolonged. The adsorption capacity/desorption capacity of pure CH4 was the largest, and the equilibrium time of adsorption/desorption was the longest. The seepage capacity of pure N2 was the largest. As the volume fraction of CH4 in the mixed gas increased, the gas seepage capacity decreased, and the seepage capacity of pure CH4 was the least. The coal deformation produced by adsorption-desorption-seepage of CH4/N2 has timeliness, spatiality and anisotropy. The diffusion coefficients of CH4 and N2 in column coal had linear and quadratic function with pore partial pressure. The axial/circumferential adsorption strain and desorption strain of coal had a primary and quadratic function relationship with the gas volume fraction. The coal axial and circumferential of seepage strains had linear and cubic functions relationship with the gas volume fraction. The axial/circumferential strain of coal had a quadratic, primary, quadratic function relationship with the adsorption capacity, desorption capacity and seepage capacity, respectively. The coal stress in the process of gas adsorption-desorption-seepage covered the expansion stress of adsorbed gas to coal matrix, the compression stress of free gas to coal matrix, and the expansion stress of free gas to coal microporous fracture. Considering the three kinds of deformation effects by two state gases, a coal strain model in the whole process of CH4/N2 adsorption-desorption-seepage experiment was established. © 2023 Academia Sinica. All rights reserved.