Relationship Between Micropore Structure of Different Coal Ranks and Methane Diffusion

The diffusion of methane (CH4) from coal, which is a crucial factor affecting the production of coalbed methane, exhibits dependence on pore structure. In the present study, the effects of pore structure of different coal ranks on the diffusion coefficient of CH4 from coals were investigated. The CO2 sorption, CH4 isotherm sorption and desorption experiments as well as fractal theory were carried out to evaluate the micropore structure of low- to high-rank coals and the characteristics of CH4 diffusion. The results revealed that the specific surface area (SSA), pore volume (PV) and fractal dimension (D-f) associated with micropores in coal showed a trend of first decrease and then increase with increase in coal rank, and their SSA and PV were controlled mainly by pore diameters of 0.55-0.6 nm, 0.8-0.83 nm and 1.2-1.3 nm. The micropore structure of high-rank coal was relatively more developed. The Langmuir volume (V-L) also showed a trend of first decrease and then increase with increase in coal rank. The SSA, PV and D-f associated with micropores influenced strongly the CH4 adsorption capacity of coal. In addition, the CH4 quantity diffused, and initial diffusion coefficient (D-0) showed a trend of first decrease and then increase with increase in coal rank. The differences among the diffusion coefficients of coals of different ranks are attributed mainly to variations in pore structures. As the SSA, PV and D-f of the micropores increased, the D-0 and D-150s also increased. Pore structure has a certain effect on the diffusion coefficient in a short time. The tortuosity of different rank coals was different, and the length of the diffusion channel of CH4 in the coal was also different. With increase in tortuosity, the diffusion amount of CH4 decreased exponentially. The research results are conducive to improving the relevant theory of gas migration in coal and are of great significance to the safe mining of coalbed methane.