Integrated pore structure analysis and methane adsorption and desorption investigation in deep multi seam coal systems

The pore structure of coal serves as the primary domain for gas adsorption and migration. Studying the pore structure characteristics holds significant implications for gas disaster prevention and control. This research focused on soft and hard coal samples from an abnormal gas emission seam and adjacent seams in Huainan Xieqiao Coal Mine. Quantitative characterization of pore structures was performed using low temperature nitrogen adsorption (LTNA), low-field nuclear magnetic resonance (NMR), and gas adsorption and desorption experiments to evaluate their impacts on gas desorption capacity and rate. The results demonstrate that the pore morphology of soft coal predominantly exhibits cylindrical pores, ink bottle pores, and slit shaped pores, whereas hard coal contains cylindrical pores, wedge shaped pores, and partially open slit shaped pores. As coal fragmentation increases, the pore volume, specific surface area, and fractal dimension exhibit progressive increases, indicating enhanced structural irregularity. The T₂ spectrum of the coal seam displays a tri-peak structure. The abnormal coal seam exhibits the highest NMR porosity but the lowest effective porosity 0.57% and permeability 0.013 × 10⁻³ mD demonstrating a distinct "high porosity and low permeability" behavior. This behavior aligns with extensive micropore development and poor pore connectivity, suggesting that coal permeability is predominantly governed by effective porosity and pore connectivity. Gas adsorption and desorption processes are predominantly governed by micropore volume and specific surface area. An increase in both parameters enhances gas adsorption capacity and the initial desorption rate, with correlation coefficients ranging from 0.95 to 0.98, indicating a strong positive correlation. The micropore fractal dimension (D_f2) reflects pore surface heterogeneity. However, it exhibits no significant correlation with gas adsorption or initial desorption velocity.