Deformation and failure characteristics of gas drainage drilling-reaming coal mass in non-uniform stress field

Clarifying the internal relationship between pore structure and methane adsorption and diffusion in coal is the basis for quantitative evaluation of methane desorption. The multi-scale pores in coal were quantitatively characterized by the low-pressure nitrogen and carbon dioxide adsorption methods, and the occurrence sites and migration channels of adsorbed and free methane were determined as filling pores with size of 0.38～1.5 nm and diffusion pores with size of 1.5～100 nm, respectively. A dual-pore series diffusion model of adsorbed methane in filling pores and free methane in diffusion pores was established. The methane diffusion coefficient and pore structure parameters were obtained by fitting the methane desorption data of coal particles. The results indicate that the calculated methane limiting adsorption based on multi-scale pore methane adsorption model matches the measured value well. More than 90% of adsorbed methane is stored in filled pores in the form of micropore filling, and the methane adsorption ability in coal depends on the filled pores. During methane desorption, the adsorbed methane first diffuses in filled pores, and then flows out of coal in free phase through the diffusion pores. The adsorbed and free methane diffusion coefficients are in the order of magnitude of 10⁻¹³ m²/s and 10⁻⁶ m²/s, the equivalent pore lengths of filling and diffusion pores are 0.109～2.855 μm and 0.525～3.106 mm, and the corresponding equivalent pore numbers per unit mass of coal are 10¹⁶～10¹⁷ g and 10¹⁰～10¹² g, respectively. Due to more filled pores with shorter pore lengths and higher adsorbed methane concentration, the mass transfer rate of adsorbed methane in coal is much faster than that of free methane in diffusion pores, and the methane desorption in coal is mainly controlled by the free methane diffusion process in diffusion pores.