Methane Adsorption Capacity of Deep Buried Coal Seam Based on Full-Scale Pore Structure

Abstract

Coalbed methane primarily exists as adsorbed gas within the microscopic pores and fractures of coal. However, the complex pore structure of deep coal seams and its quantitative relationship with methane adsorption capacity remain unclear. This study investigated nine samples from a coal seam in the Ningwu Basin, representing different burial depths, including five middle-shallow and four deep burials. This was accomplished through a series of experiments, including high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTGA–N₂), low-pressure carbon dioxide adsorption (LPGA–CO₂), and high-pressure (30 MPa) methane isothermal adsorption (HPGA–CH₄). The study revealed the characteristics of the pore structure in deep coal seams and their differences compared to those in middle-shallow coal seams. Moreover, it clarified the mechanism by which the pore structure influences CH₄ adsorption capacity. Given the differences in methane adsorption mechanisms at various pore scales, a novel method for quantitatively assessing the methane adsorption capacity using pore structure parameters is proposed. The results showed that the micropore pore volume and specific surface area of the deep coal seam were significantly higher than those of the middle-shallow coal seams. In contrast, the development of mesopores and macropores was relatively limited. The CH₄ adsorption capacity of a coal seam was calculated using pore structure parameters across multiple scales, considering the coexistence of two-dimensional “filling adsorption” and three-dimensional “monolayer adsorption” mechanisms. The calculated capacity V_L’ closely matched the measured value of V_L, with error of less than 10%. The degree of micropore development is the main factor influencing the accuracy of this method. Therefore, using pore structure parameters at different scales to calculate methane adsorption capacity is effective and feasible for deep coal seams with extensive micropore development. This study established a connection between microscopic pore structure and macroscopic methane adsorption capacity, offering a novel method to determine the methane adsorption capacity of deep coal seams.