Research on coal matrix pore structure evolution and adsorption behavior characteristics under different thermal stimulation

The technique of thermal stimulation for coalbed methane extraction is considered an important approach for increasing coalbed methane production capacity in the future. This study focuses on the impact of the thermal evolution of pore structure in coal on gas adsorption characteristics. The low-pressure CO2 and N2 physical adsorption data were analyzed using classical thermodynamic methods and density functional theory. The results showed a decrease in pore volume and an increase in average pore size. The fractal dimensions obtained from the Frenkel-Halsey-Hill and Sierpinski fractal models indicate that the micro-scale spatial structure of coal becomes simpler, and the surface becomes smoother. The high-pressure volumetric method was employed to determine the gas adsorption parameters of the coal, and a comparison was made between the calculated gas adsorption parameters based on the micro-pore structure parameters of the coal samples. It was found that the variation pattern of micro-pores in the coal samples correlated highly with the methane adsorption capacity. The decrease in adsorption capacity increases the proportion of free-phase methane, which helps enhance the early-stage methane extraction rate. Improving gas migration pathways avoids the limitation imposed by diffusion in the later stages of coalbed methane extraction.