The Permeability Evolution Characteristics Of Loaded Coal And Its Application In The Drainage Of Pressure-relief Gas

Coal seam mining with the pressure-relief method is to release the stress loaded on thecoal body, resulting in the increase of new fractures, change of the structure and permeabilityof the coal body, and then realization of the gas drainage through permeability increasing. Inthis paper, the fracture evolution and permeability variation of coal during the loading andunloading process were studied by the method of theoretical analysis, laboratory experiments,numerical simulation and engineering research comprehensively, and the fracture evolutionlaw and the corresponding model were obtained and applied to study the permeabilitydistribution and gas drainage in underlying protected coal seam during the pressure-reliefmining. The main conclusions of this paper are as follows:(1) Permeability experiments of the standard cylinders nature coal samples were carriedout under the different unloading stress path similarly as the loaded characteristic ofpressure-relief coal mining. The permeability ratio which reached7.5:1in different beddingdirections were obtained and the cause of the difference of permeability evolution during theunloading process was analysed. The initial damage become bigger and the ultimate bearingstrength reduced at the same unloading point of stress, when the sample has weakcompressive strength induce the damage aggravate increase and the massive generation ofnew fractures in the process of unloading, which induce the permeability increase to greaterthan that in loading process. The experimental phenomena and new knowledge of unloadingincrease permeability mechanism were obtained.(2) Based on the theory of damage mechanics, unloading mechanics and seepagemechanics, the model was built by RFPA2D-Flow to study the characteristics of fractureinitiation, expansion, cut-through and permeability in different pre-fracture direction coalsamples under the action of loaded-unloaded. The fracture evolution law and permeabilityvariation of coal during different unloading are obtained, the fracture evolution andpermeability change were coincident each other. We compared and analyzed the experimentalresults, and discovered that if the sample had many damage micro structure in unloading point,the extension speed of along the bedding plane faster, the number of break-point larger, alarge number of fractures easier produced and breakage deformation, accompanied by thepermeability increasing sharply.(3) According to the permeability experiment and fracture evolution simulation of coalsamples under loading condition, we could conclude that the permeability decrease with theincrease of effective stress during the process of loading; and the permeability increase withthe decrease of effective stress during the process of unloading, but the unloading process is not a simple inverse process of loading. Furthermore, the relationship of the permeability andeffective stress under unloading condition was established. Compared the whole permeabilityevolution process, conceptual models under three different paths of permeability evolution ofcoal under different conditions of stress were proposed, and the elastic permeability model,plastic permeability model and unloading increase permeability model were built, whichcorrespond to the deformation of loaded coal.(4) According to the characteristics of coal seam in Dalong coal mine, the rule of miningdeformation and the variation of stresses on protected coal seam were obtained by the modelconstructed by FLAC3Dsoftware based on testing mining area. Combined with permeabilityevolution experiment and model, and used the Surf function from Matlab, the permeabilityspatial distribution of underlying protected coal seam was obtained. The permeability ofpressure-relief mining influence area could be divided into original permeability area, elasticpermeability area, plastic permeability area, unloading increase permeability â…  area andunloading increase permeability â…¡ area. On the basis of the research results, we take thebottom lane through coal seam drilling for pressure-relief gas drainage, and optimize thelayout parameters of gas drainage drilling boreholes.