Difference analysis of methane diffusion properties between tectonic coal and primary coal

Primary coal destroyed by geological stresses will form different types of coal structures, and the accumulation and migration of coalbed methane(CBM) are affected by the changes in pore structures, adsorption/desorption, diffusion and permeability. In this paper, the methane isothermal adsorption experiment, and low-temperature liquid nitrogen and carbon dioxide adsorption experiment were carried out on the samples of primary coal and tectonic coal of No.3 coal seam of Zhaozhuang Coal Mine in Qinshui Basin, so as to analyze the adsorption/desorption properties and pore structure characteristics of primary coal and tectonic coal. The methane diffusion difference and control mechanism of primary coal and tectonic coal were revealed by using unipore and bidisperse models. The results show that the adsorption capacity of CH₄ in the coal increases with the increase of the damage of the coal structure, and the desorption and diffusion rates of tectonic coal are better than those of primary coal. The diffusion coefficients of primary coal and tectonic coal decrease with the decrease of pressure, and in the whole diffusion stage, the diffusion coefficient of tectonic coal is higher than that of primary coal. In the pressure range above 5.0, 1.5-5.0 MPa and below 1.5 MPa, gas diffusion mainly occurs in macropores, mesopores and micropores, corresponding to Fick diffusion, transition diffusion and Knudsen diffusion respectively. The fitting degree of the bidisperse model for desorption data is significantly higher than that of the unipore model. It is revealed that the gas diffusion in coal changes in three stages: rapid diffusion stage(S1), attenuation stage(S2) and slow diffusion stage(S3). The effective diffusion coefficient shows an obvious positive correlation with temperature. The higher the temperature, the greater the effective diffusion coefficient. However, in the low-pressure stage, the influence of pore structure on methane diffusion in coal is higher than that of temperature. The research results provide a theoretical basis for the exploration and development of CBM in tectonic coal areas.