Diffusion Process of Methane in Coal Particles and Its Relationship with the Pore Scale

Diffusion is the primary migration form of coal seam gas. This paper establishes a conceptual diffusion model to describe microdiffusion based on the diffusion form and the pore scale. First, the researchers obtained the pore structure characteristics of 10 coal samples through low-pressure carbon dioxide and nitrogen adsorption experiments. Through quantitative calculations and literature collection, it determines the main diffusion space, that is, a network of pores ranging from 1.5 to 100 nm. Second, the main diffusion space is further divided into three parts: pores ranging from 1.5 to 50 nm, from 50 to 70 nm, and from 70 to 100 nm, where Knudsen diffusion, transition diffusion, and Fick diffusion, respectively, control the gas transport. The three parts are connected in series with parallel pores inside each part. The average pore diameters of the parallel pores in the three parts are calculated as 26, 60, and 85 nm, and the equivalent total pore volumes are 6.52 X 10(-4) to 1.71 X 10(-2) cm(3)/g, 1.72 X 10(-4) to 1.77 X 10(-3) cm(3)/g, and 1.49 X 10(-4) to 1.69 X 10(-3) cm(3)/g, respectively. It shows that the part where Knudsen diffusion occurs has more and smaller pores than the other two parts. Finally, the relation between the cumulative methane diffusion volume in 30 s, and the pore volume in the diffusion zone of 10 coal samples is analyzed using a linear regression method. The results showed that the pore volume in the Knudsen zone has the highest influence on the diffusion flow, with a correlation of 0.914-0.925. This research provides a theoretical basis for gas migration under in situ conditions.