Research On Pressure Relief Features And Gas Migration Regularity Of Single Thick Coal Seam Under Condition Of Lower Protective Layer Mining

For an outburst coal seam,pressure relief gas drainage combined with protective layer mining is the most effective and rational treatment technology.In a coal mine area with a single thick coal seam,which has no adjacent coal seams,the one of key problems is how to choose the right stratum as protective seam.Fractures development,gas migration and seepagecharacter of protective seam mining of Zhongtai Coal Company were researched with theoretical analysis,numerical simulation,simulation study experiment and field investigation based on key strata theory.The main results were showed as follows.(1)Regional gas treatment method in complex protective seam mining were proposed according to the geological condition of Hebi mining district,which is characted with single thick,high methane low seepage and no protective seam mining.Feasibility analysis of protective mining shows that protective mining is feasible.The maximum height of caving zone is 7.58 m.The height of fracture zone is between 30.57 to 48.37 m.The protected coal seam was located in the top of fracture zone or the bottom of bending zone.There was no destructive impact on the continty of main coal seam.(2)The simulation study shows that protective mining is feasible.There are four fracture zones along mining direction,including fracture zone opening,fracture developing,farcture shrinking and fracture closing.The protected coal seam was located in the top of fracture zone or the bottom of bending zone.There was no destructive impact on the continty of main coal seam.There was no step subsidence on main coal seam.Strike pressure pressure relief angle is 70°.Tendency pressure pressure relief angle is beteween 55° to 64°.The maximum dilatation ratio reached to 26.37‰.(3)Permeability model of coal was built based on the dual physics phole-fracture medium theory,which includes effective stress and adsorption swelling rate.Regulary between permeability and stress on protective seam mining was obtained.Methane permeability increases by 3 times,which is consistent with field investigation.(4)The following is the results of seepage of coal samples.Gas permeability rate drcreases gradually with the increasing of confining pressure on samples.The permeability of coal samples decreased at first,and then increased slowly during pre-peak strength phase with the increasing of swelling deformation.During post-peak phase,numbers of gas seepage passages increase;cross-section area of passages expands;speed of seepage improves remarkably.Almost linear relationship exists between permeability and swelling ratio.There are 6 gase seepage phases according to different strength stages,including stable asorbsion phase,seepage decreasing phase,increasing slowly phase,increasing jumply phase,increasing steadily phase and increasing sharply phase.(5)Establishing gas solid coupling dynamic permeability model,which is based on fluid mechanics and flow solid coupling mechanics theory;Using multi-physics coupling software COMSOL Multiphysics,analysis of mining and drilling drainage relief under the double condition of gas flow field characteristics and relief effects.The results show that :with protection layer face continue to promote,in mining unloading pressure and drilling drainage(drilling field spacing 40m)dual role,protecting seam free gas and desorption gas continuous to adopt empty area and drilling in migration and protected layer gas pressure decreases,pressure relief range continues to expand.When the working face advances to 140 m,the pressure relief and drainage under the dual role,the protected layer pressure relief gas pressure range reduced to highlight the pressure below 0.74 MPa.(6)Site rock crack detection and drilling peep results show that after the protective seam mining,the protective layer is the largest expansion reached 27.4 ‰;The protective layer within the scope of the relief bore multiple ring to the shaft-circumferential crack staggered,some parts of the cell wall severely damaged,shallow separation more obvious,the protective layer outside the scope of the relief drilling fewer cracks,many holes wall smooth,It shows that the protective layer mining plays a very good role in relieving the pressure,and the seam of the protective layer and the roof and floor of the coal seam is fully developed,which is beneficial to the gas migration and the gas drainage of the protected layer.(7)The parameters of gas drainage results show that :protecting gas drainage layer parameter variation and the protective layer of rock pressure behavior law corresponds,from 0 to 10 m range of the protective layer is drawn from the front of the protective layer,from 10 to 40 m range is the best extraction area,from 40 to 60 m range for pumping and mining area,60 m after the failure area for pumping;The optimum gas drainage area of the drilling field is pushed to the front of the drill field 3m to the drill field in front of the 21 m to the left and right side of the drill field;Maximum residual gas content after mining protection layer 1.83m3/t,below the critical value of coal and gas outburst,the outburst danger of coal seam can be eliminated effectively by the mining protection layer combined with pressure relief gas drainage.It provides a new way for the exploitation of gas control in a single thick coal seam in Hebi mining area.