Effects of Pore Structure on Methane Adsorption and Diffusion Properties in Varying-Rank Coals Modified by Tetrahydrofuran

The low permeability and difficult desorption characteristics of deep coal seams lead to low coalbed methane (CBM) recovery efficiency. Tetrahydrofuran (THF) modification can improve CBM recovery efficiency, but its mechanism remains to be further studied. Low-temperature nitrogen adsorption, methane isothermal adsorption, and methane initial velocity tests were carried out on three different ranks of coal samples. Combined with Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), Langmuir, and Frenkel-Halsey-Hill (FHH) theoretical models, the effects of THF modification on pore structure, methane adsorption, and diffusion characteristics of coal were investigated, and the quantitative relationship between the three changes was further analyzed. The results show that after THF treatment, the proportion of micropores in the experimental coal samples decreases while that of mesopores increases, accompanied by the reduction of specific surface area, the enlargement of average pore size, the decrease of fractal dimension D1, and the tendency of the pore surface to be smoother. The maximum reduction in ultimate methane adsorption capacity reaches 26.56%, and the initial methane diffusion velocity decreases significantly. However, THF modification does not alter the methane adsorption mechanism and emission mode of the coal matrix. The Langmuir volume constant increases with the rise of fractal dimension D1, and the initial methane diffusion velocity presents a linear positive correlation with the Langmuir volume constant. THF reduces methane adsorption sites and improves pore connectivity through a dual mechanism of "dissolution and pore expansion" and "surface modification", which promotes the desorption and seepage of adsorbed methane. The relevant results provide theoretical support and technical insights for the efficient extraction of CBM in low-permeability and high-gas coal seams.