An experimental study of factors influencing gas seepage in confined broken anthracite

The void structure evolution of broken coals in a caving zone directly affects the gas seepage characteristics of the zone, further influencing the gas migration and enrichment patterns in abandoned coal mine goaves. With the second generation of compaction-seepage-CT scanning experimental system for broken coals independently designed and developed, this study performed 162 sets of permeability experiments on broken anthracite samples with five particle size ranges under varying gas pressures, axial stresses, and void fractions. Key findings are as follows: (1) When gas migrated in broken anthracite, its pressure and flow rate kept dynamically changing until the adsorption equilibrium was reached. (2) Under a low Reynolds number, a quasi-starting pressure gradient was required for gas flow in broken anthracite. This gradient ranged between 158.89 and 1408.64 Pa/m, increasing with an increase in axial pressure and a decrease in void fraction. (3) The broken anthracite exhibited permeability ranging from 10⁻¹² to 10⁻¹⁰ m², increasing logarithmically and exponentially with increasing gas pressure and void fraction, respectively. (4) A larger particle size corresponded to larger initial voids in broken anthracite samples within the same loading space, more conducive to gas flow. Therefore, under the same void fraction, the permeability of broken anthracite increases with particle size. (5) The average changing amplitude of the permeability of particles with varying sizes all decreased with a decrease in the void fraction. For particles with a larger size, the changing amplitude decreased more significantly under the same decrease in the void fraction. To achieve the surface extraction of coalbed methane (CBM) from an abandoned coal mine goaf, ground drilling should be preferably arranged in a U-shaped high-permeability CBM-rich zone composed of broken coals in a caving zone and circular overlying zones at the longwall panel in the vertical direction.