Study On The Correlation Between Mining Distressing Law And Gas Migration Ahead Of Working Face

Affected by the mining, coal body destressing failure occurs ahead of working face. Distressing zone of coal body development fracture, gas by desorption and diffusion and gush out to the coal mining face, bring dangerous to coal mining working face for safe mining. In this paper, using the rock mechanics, seepage mechanics and other related knowledge, discusses the correlation between deformation failure destessing process and the gas migration ahead of working face, aims to grasp the coal destressing and the gas migration rule ahead of coal mining face, for gas drainage of destressing zone ahead of coal mining face to provide theoretical basis and guidance, in order to reduce the gas emission of destressing zone, thereby reducing the risk of gas. Through the technical route combined with laboratory test, theoretical analysis, numerical simulation, field test, provide a comprehensive data basis and theoretical support for the research work of this paper.First of all, use the TAW-2000 rock triaxial test machine for mechanical experiment of uniaxial, conventional triaxial, unloading confining pressure as well as loading-unloading conditions of coal. Through uniaxial, conventional triaxial, unloading confining pressure and loading-unloading conditions test of jincheng anthracite, respectively obtained the stress-strain characteristics, strength characteristics and failure characteristics under three kinds of stress path.In the conventional triaxial compression test, the confining pressure increases, the limited bearing loads and cohesion increases, internal friction angle decreases; high confining pressure increases the axial compressive deformation, the failure of coal sample is more intense. In the case of the same initial confining pressure, compared with the conventional triaxial test, unloading confining pressure tests peak secant modulus is smaller, secant Poisson’s ratio is larger; Coal sample easier to failure and failure more intense when unloading confining pressure, the lateral strain is larger, and the axial strain but little difference.Compared with the conventional triaxial experiment, internal friction angle increases, the cohesion decreases under loading/unloading conditions; under the same initial confining pressure, the compressive strength is reduced, thus easier to failure. In loading/unloading experiments, unloading confining pressure rate slower, the greater the degree of deformation and failure of coal specimen; the faster the rate of unloading confining pressure, the smaller the coal specimen fracture strength, and the shorter thetime required,, the coal specimen more easy to failure. In the process of unloading confining pressure, the deformation modulus of coal increased first and then decreased, the Poisson’s ratio continued to increase. Compared with the conventional three axis test, under the loading/unloading experimental conditions, the difference deformation modulus E50-Ecand the difference Poisson’s ratio μc-μ50 have a larger increase, and it demonstrated a greater degree of deformation and failure in loading/unloading experiments.From failure characteristics, in uniaxial compression test and the low confining pressure, coal sample is mainly shear failure, have obvious shear failure surface. With the increase of confining pressure, there is more than one failure surface, and have tensile crack and shear crack at the same time. With the increase of confining pressure(6MPa), show conjugate shear failure. Along with the confining pressure continues to increase, samples have no obvious failure surface, but showing the bulging phenomenon. In unloading confining pressure tests, have no obvious shear failure surface and have tensile crack and shear crack at the same time, and failure more intense. With the increase of initial confining pressure, failure showed ductile characteristic. Compared with the conventional triaxial experiment, coal specimen is more broken. Compared with the conventional three axis test, under the condition of unloading coal specimen is given priority to with tensile damage, no obvious smooth fracture surface. Due to large deformation, coal specimen is more broken. With the increase of initial confining pressure, coal specimen deformation is large.Coal body deformation process ahead of working face experienced compression process and dilatancy process, at the same time accompanied different gas migration process. Through laboratory test can better response the correlation of the expansion and compression and seepage process ahead of working face. In this paper, compression and dilatancy of coal and gas seepage properties were studied in the third chapter through the triaxial stress seepage experiment device. Based on the change of volume strain of coal specimen, the deformation and failure process is divided into compression and dilatancy stage. Dilatancy conditions are presented by volume strain increment. Theory analyzes the stress space form of compression/dilatancy(C/D) boundary, and C/D boundary of Jincheng anthracite is obtained through triaxial experiments under different confining pressures. The gas seepage change in the process of coal compression and dilatancy is studied using triaxial seepage test. During the compression stage, permeability and effective stress curve conforms to theformula1/ 20[1( /) ]t eK =K -ss. During the dilatancy stage, when the confining pressure is low, permeability and effective stress curve is in line with binomial formula. When the confining pressure is high, permeability and effective stress curve conforms to the formula/(1)ed C DK K ce-s= +. Confining pressure and gas pressure have significant effects on dilatancy boundary. The axial stress on the dilatancy boundary and stress ratio increases, and the permeability decreases with the increase of initial confining pressure. In addition, increases in gas pressure lead to anthracite dilatancy boundary in advance. Field test results show that the coal body in front of the working face has undergone the compression and dilatancy process, that is, the process of coal permeability change consistent with the triaxial seepage test.The coal body destressing zone ahead of working face is theoretically analyzed. According to the stress equilibrium equation in destressing zone of horizontal direction, combined with the Mohr-Coulomb criterion, derived the formula of distressed zone width including pore gas pressure. Analysis of the gas pressure, Biot coefficient, coal mining depth, thickness of coal seam mining, the mechanical properties of coal on the influence of distressed zone width. Analysis results show that in the horizontal direction, the influence of gas pressure on the distressed zone width shows the promoting role, distressed zone width expand along with the increase of gas pressure; in the vertical direction, the Biot coefficient has little effect on the distressed zone width, the influence of gas pressure on the vertical stress can be neglected in the distressed zone width calculation; The deeper the mining depth of coal seam and the thicker the thickness of coal seam mining, the greater the distressed zone width; the greater the internal friction angle and cohesive, the smaller the distressed zone width. Drilling stress sensor using field measurement of the distressed zone width ahead of a mine working face, and the results are basically consistent with the theoretical formula.The coal gas migration is a complicated process ahead of working face. First adsorption gas desorption from coal body to free gas, free gas diffusion through the pore fissure seepage process migration to working face. And the porosity and permeability of coal body decided to gas migration, at the same time, and the size of the porosity and permeability is associated with the original porosity and stress strain process. Because of the complexity of coal gas migration ahead of working face in distressing area, COMSOL Mulitphysics software provides a useful tool to simulate the coal body deformation-gas migration change process ahead of working face.Considering the skeleton volume change of coal, porosity model of dynamic change containing the initial porosity, total volume strain and skeleton volume strain was deduced. Based on the porosity model, according to the Kozeny-Carman equation, and considering change of coal skeleton surface area, the permeability equation include the initial permeability, volume strain, initial porosity and gas pressure was deduced. According to the mass conservation equation and Darcy’s law, gives the coal body deformation-gas migration equations ahead of working face, according to the Mohr-Coulomb criterion, the changes in porosity and permeability change equation and the equation of coal body deformation-gas migration equation, using COMSOL Multiphysics numerical simulation software analyzed changes of the stress,permeability and gas pressure ahead of working face. The results show that ahead of working face, the stress distribution consistent with the field test; Destressing zone range is consistent with the theoretical calculation; Permeability changes are consistent with the gas flow of field test; Gas pressure increased gradually with the distance of working face, until reaching the original gas pressure. With the passage of time, the destressing zone widened, the minimum permeability to the distant metastasis ahead of t working face, gas pressure relief influence scope expanding gradually.By experimental research and theoretical analysis, numerical simulation and field test, we recognize that coal sliding failure occurs in destressing zone ahead of working face, there are obvious dilatancy and destressing and permeability-increasing effect. Field measurement the change of stress and bore hole gas flow ahead of working face, determined the abutment pressure zone, destressing zone distribution. In distressing zone, because the permeability of coal increases, the average flow of gas is increased by 2~3 times. Based on destressing and permeability-increasing effect ahead of coal face, according to the different bore hole failure distance and destressing zone width, given the different angle of pre drainage bore hole destressing gas drainage quantity formula. Analysis shows that: bore hole angle increases, destressing gas drainage amount larger. Combined with field condition of N2105 face in a mine to calculate, the maximum hole angle is 21.4 °, Compared with the original vertical coal wall bore hole, destressing gas drainage can increase 978.5 m3 of single bore hole, expectations can effectively improve the coal seam gas drainage rate.Combined with a coal mine reality, considering the bore hole forming rate and blind area, destressing gas drainage quantity increases rapidly with the increase of Angle, as the angle increases, destressing gas drainage quantity decreases gradually, in the angle of 17.5°, destressing gas drainagquantity reached the maximum value.This paper has made the beneficial exploration in the correlation of coal body deformation failure distressing and gas migration process, especially put forward bore hole destressing gas drainage quantity formula ahead working face and optimize the bore hole Angle. Research results can provide guidance for bore hole destressing gas drainage.