Research on comprehensive characterization of deep coal full aperture structure and burial depth effect

With the increasing depletion of shallow mineral resources, deep mining has become important; however, basic research on deep resource development remains insufficient, and basic laws are unclear. In particular, comprehensive research on the pore structure characteristics of deep coal bodies at different scales is needed, and how burial depth affects pore structure characteristics should be investigated. Herein, this study performed mercury intrusion porosimetry, liquid nitrogen method, and low-temperature carbon dioxide adsorption method to systematically determine the pore structure characteristics of deep coal and comprehensively analyze coal pore complexity based on the Menger Sponge, Frenkel-Halsey-Hill (FHH), and density function models. The influence of depth on coal pore structure was also discussed. In the study zone, the stage pore volume was mainly concentrated in the macroporous (43.6-83.4%) and mesoporous Sect. (16.3-54.7%) and sparsely distributed in the microporous Sect. (0.3-2.1%). The accumulated pore volume decreased with an increase in coal sample burial depth, with a gentle curve in the micropore section and obvious fluctuations in the mesopore and macropore sections. The main distribution of stage surface area was in the mesoporous Sect. (79.5-91.8%), followed by the microporous (7.8-19.8%) and macroporous Sect. (0.3-0.6%). The accumulated surface area curve fluctuated greatly in the mesoporous and microporous sections, and that in the macroporous section was relatively flat with limited distribution. According to the Menger Sponge model, the fractal dimensions of the macroporous (D_mac) and mesoporous (D_mes) sections ranged from 2.4 to 2.8 and 3.7-3.9, respectively, with obvious fractal characteristics; D_mes was higher than D_mac, indicating that the structure of small and medium-sized micropores is more complex and irregular. The fractal dimension in the high-pressure zone of the FHH model (D_HP) ranged from 2.8 to 2.9, and that in the low-pressure zone (D_LP) ranged from 2.1 to 2.4, indicating that coal pore structure under low-pressure conditions is relatively simple. The fractal dimension of the density functional model ranged from 2.5 to 3.0, closer to 3, indicating that the pores are relatively complex and rough. A single-variable analysis revealed a strong correlation between burial depth and aperture parameters. This is the first study to reveal the relationship between burial depth and aperture parameters. Moreover, characterization of critical depth based on aperture structure was performed, providing a feasible method for quantifying "critical depth." Our results enrich our understanding of the structure of deep coal bodies and serve as a reference for deep mining engineering.