Study on the pore structure evolution and microscopic seepage characteristics of coal under high pressure air blasting

The pore structure in coal seams has a significant impact on the occurrence and migration characteristics of coalbed methane. The High Pressure Air Blasting (HPAB) is one of the main feasible technologies to improve the efficiency of unconventional gas extraction. Currently, there is little research on the visualization of the evolution of pore structure and seepage characteristics in coal under HPAB, resulting in unclear understanding of the formation of 3D pore network structure in coal under HPAB, the interconnection of pores and fissures, and the mechanism of gas seepage and diffusion under HPAB. Therefore, it is necessary to study the evolution characteristics of pore structure and seepage characteristics of coal under HPAB. The HPAB test was carried out based on a self-developed HPAB device. Statistical analysis of crack propagation law of coal under HPAB from a macroscopic perspective. During the experiment, a microscopic visualization model of coal before and after HPAB was established using CT scanning and Avizo software. The vessel axial skeleton algorithm in Avizo software was adopted to extract the connectivity channels between connected pores and fissures, which realizes the 3D visualization of coalbed methane seepage in the spatial topology structure at the microscopic scale. The seepage, migration, diffusion process and distribution law of coalbed methane in the micro-pore structure of coal have been studied. The mechanism of coalbed methane seepage and migration has been revealed from a microscopic perspective. The research results indicate that: (1) There are four main cracks on the coal surface that approximately run along the direction of principal stress under HPAB and confining pressure. The direction of crack development and propagation in coal under HPAB is influenced by natural defects such as joints and bedding. (2) Before HPAB, the representative elementary volume (REV) of coal has a high degree of curvature in the connectivity channel of pore and fissure. After HPAB, the degree of curvature in the REV is significantly reduced, with a decrease of up to 26.72%. (3) The original micro-pores and newly formed fractures developed and expanded outward under HPAB, forming a relatively developed 3D fracture network channel. Compared with before HPAB, the streamline distribution inside the REV is denser and more numerous. The maximum flow rate and maximum velocity of coalbed methane migration have increased by 112.90 times and 5.24 times, respectively. The research results provide experimental and theoretical basis for improving the efficient extraction of coalbed methane in low-permeability coal seams by HPAB.