Gas Emission Characteristics of Remaining Coal under Complex Stress Conditions in Abandoned Mines

It is of great significance to achieve the efficient development of gas resources in the coal of abandoned mines. However, the multisource gas emission characteristics of coal under complex stress remain unclear. Therefore, in this paper, gas emission experimental platforms for the creep of triaxial coal cores and uniaxially confined coal were independently designed and built first. Then, gas emission experiments for the creep of coal cores and crushing coal under different deviatoric stresses were carried out based on the self-built experiment platform. In the gas emission experiments for the creep of the coal core, the influence of coal damage during the creep process on gas emission characteristics was analyzed in combination with the changes in axial strain, acoustic wave velocity, and gas diffusion ratio. In addition, in the gas emission experiments for the creep of crushing coal, the influence of the creep on gas emission characteristics was analyzed in combination with the changes in axial strain, porosity, fractal dimension, and gas diffusion ratio. The research results show that (1) the influence of the creep deformation of coal cores on gas emission is mainly reflected in the accelerated creep stage, rather than the decelerated creep and stable creep stages. The deviatoric stress affects the gas diffusion ratio at a certain creep time by changing the internal predamage magnitude of coal. Accelerated creep's impact on gas emission diminishes as matrix pore pressure depletes during stable diffusion; (2) the influence of the creep deformation of crushing coal on gas emission is mainly reflected in the recrushing degree of the crushing coal during the decelerated creep stage, which in turn affects the magnitude of the gas diffusion ratio at a certain creep time. The recrushing effect of the coal particles inside the crushing coal intensifies when the deviatoric stress increases, resulting in a longer time required for the crushing coal to reach deformation equilibrium. Although the porosity decreases significantly under high deviatoric stress, the gas emission rate still shows a gradually increasing trend. Therefore, the crushing effect of the coal particle is the main controlling factor affecting the gas diffusion ratio in combination with void compaction during the creep deformation of crushing coal.