Permeability Distribution Characteristics With Mining Stress Perturbations And Its Effect On Gas Flow

Combined with the evolution equation of permeability and softening modulus and dual-porosity medium permeability evolution equations base on PM model, this paper model the permeability evolution equations under mining stress perturbations on the basis of experiment of coal containing methane under loading-unloading conditions, while model a gas-solid coupling model of coal containing gas. Using COMSOL Multiphysics simulate the distribution of permeability and gas pressure in coal body on driving face and preservation of coal gas by low-permeability zone in coal seam.Analysis of coal sample loading and unloading permeability experiment shows that: Permeability decreases sharply in the initial stage of loading, and then tends to a constant; Permeability increases slowly in the initial stage of unloading, and then rapidly. Unloading produces new fractures. With the increase of the amount of unloading confining pressure, the permeability variation experiences three stage of slow increase, growing faster and rapid increase; Permeability change of unloading process isn’t inverse process of loading, permeability in the initial stage of unloading is lower than loading, followed by a rapid increase and exceed the loading process.Relative to the original coal seam permeability, permeability in front of working face showed the zonal distribution characteristics. At a distance of 1.5m of coal wall, coal mass is high permeability zone because coal mass yield failure affected by disturbance stress which reduces gas pressure and gas pressure gradient, as a safety belt preventing the outbursts. At a distance of 1.5m to 2.75 m, coal mass is increase permeability zone because it is in front of the stress concentration peak point and plastic yield. At a distance of 2.75 m to 20 m, coal mass is decrease permeability zone and elastic coal, because it has low permeability it can storage coal gas and form high gas pressure gradient which accumulate energy to predisposing outbursts,.According to numerical simulation results of one-dimensional flow model of coal gas with a low-permeability zone, the extent of permeability decreases and area size of low-permeability zone is the key factors affecting the abnormal methane enrichment effect. In low permeability coal seam with a permeability of 0.1, 0.01 times of normal coal seam, gas pressure inside and outside of low-permeability zone will be extremely differences after gas outflow from coal seams. The low-permeability zone will imped gas outflow from coal seam surrounded by it. Gas content variation with various permeability properties of the low-permeability zone is consistent in the early. Gas content of coal seam with low-permeability zone is higher than without low-permeability zone because low permeability conditions and starting pressure gradient. On one hand the energy consumed of outburst of low-permeability zone is lower than that without low-permeability zone, on the other hand the accumulated energy for outburst of low permeability-zone is higher than that without low permeability-zone because the low permeability-zone has a high gas pressure and pressure gradient. With the increase of gas pressure, the accumulated energy and its energy density for outburst increase, distressed area reduced, the accumulated energy of coal damage area has a substantial increase, which prepares the danger of outburst.