Deformation And Permeability Distribution Rule Of Overlying Outburst Seam In Exploiting Extra-Thin Protective Seam By Drilling Method And Its Application In Pressure Relief Gas Drainage

Based on the theory analysis, with the colligation analysis method of the similar material simulation test, the numerical simulation and the field test, movement and deformation of the overlying coal-rock mass, the mining-induced fractures growth and distribution feature of the protected coal seam, the change of ground stress and deformation of the protected coal seam, the permeability change and permeability distribution of the coal seam, the rule of pressure relief gas flow, and its application in pressure relief gas drainage are studied systematically. The results have an important significance for solving problems of outburst prevention and deep safe mining in the similar mining geological conditions.The results show that, coal seam No.21 is the strong outburst dangerous coal seam in Cuimiao Coal Mine and the gas parameters of eliminating outburst must be controlled below 0.32MPa or 8m3/t. According to the special mining geological conditions and coal seam No.21 with low permeability, high gas concentration, and strong outburst risk, we present the regional gas control technique of the extra-thin protective coal seam drilling combined with pressure relief gas drainage of the protected coal seam used by the coal auger.Ground stress, deformation, and permeability change and permeability distribution of the overlying protected coal seam induced by drilling the extra-thin protective coal seam are studied used by the seam mining and gas permeability simulation test-bed. The results show that, the overlying coal-rock mass is integrally bending and sinking, without forming any caving zone and the obvious fault zone. The protected coal seam is in the bending zone, a large number of bed separation crannies are created induced by mined, and the obvious vertical joint crannies are not created between the protected coal seam and the protective coal seam. Ground stress of the protected coal seam decreases in a great degree. Measured expansion deformation value in the full stress-relaxed area of the protected coal seam is 10.5mm and the relative expansion deformation is 1.31‰. Gas flow regularity accords with Darcy law in the protected coal seam, which is verified by the test, and then the rule of permeability change and permeability distribution of the protected coal seam is studied.The numerical simulation results show that, the average value of ground stress in the full stress-relaxed area of the protected coal seam is 3.4MPa, which decreases 32% compared with the original ground stress. The expansion deformation is 11mm and the relative expansion deformation is 1.4‰. The deformation of the protected coal seam is characterized by 1) compression, 2) rapid expansion, and 3) reaching a stable expansion value. The results show that, mining height of the extra-thin protective coal seam has a great influence on the ground stress and expansion deformation of the protected coal seam. The change rule of the seam permeability, pressure relief gas flow, and gas drainage rule of the protected coal seam are studied used by RFPA. The results show that, the extra-thin protective coal seam drilling makes the seam permeability improve rapidly, increasing by 407 times compared with the original permeability. The pressure relief gas flow condition is formed which is desorption, diffusion, and percolation. Gas flows in the open crannies along the protected coal seam. Gas pressure around a penetration borehole through the protected coal seam obviously decreases with the increase of mining distance and the drainage time.The results of the field test show that, used by the method of the extra-thin protective coal seam drilling combined with pressure relief gas drainage, the gas pressure of coal seam No.21 decreases from 0.75MPa to 0.15MPa, the gas content decreases from 13m³/t to 4.66m³/t, the expansion deformation is 12mm, and the relative expansion deformation is 1.5‰. The permeability coefficent increases from 0.047m²/MPa²Â·d to 18.928m²/MPa²Â·d, increasing by 403 times, the gas drainage rate is up to 64%, and the outburst risk of the protected area in coal seam No.2₁ is eliminated. In addition, the results obtained by the method of the theory analysis, the similar material simulation, and the numerical simulation are well fit to the results of the field test.