Coupling Mechanism Of Stress-Fracture-Flow In High Gas Coal Seam Group And Its Impact On Pressure Relief Extraction

The combination of pressure relief mining and gas drainage is an effective method to realize the simultaneous extraction of coal and gas in high gas coal seam group.The coupling mechanism of stress-fracture-flow is the basic problem of pressure relief mining in high gas coal seam group,which directly affect the success and effects of pressure relief mining.Thus,in this doctoral dissertation,comprehensive methods including theory analyses,laboratory experiments;numerical simulations,field observations,etc.,are employed to establish the stress-fracture-flow coupling model of repeated mining and their coupling mechanism is analyzed.Based on this model,the evaluation method of pressure relief effect is put forward and the temporal and spatial evolution of the surrounding rock stability and the permeability evolution are identified.The research results provide a theoretical basis for the simultaneous extraction of coal and gas in high gas coal seam group.The following innovations can be obtained:(1)An experimental system is developed to measure the permeability of anisotropic coal and rock mass affected by mining;the mechanism of stress-flow of coal and rock mass with different mining damage fracture is revealed.The test of axial and radial stress permeability of coal is realized with the directional permeability experimental system.According to the different damage degree of the three overlying strata zones,the classification and preparation method of rock and coal samples with different mining damage fracture were proposed.The time-space relationship of the permeability evolution of the different damaged coal and rock during pressure relief mining and gas drainage is tested.Permeability-stress models of coal and rock samples under repeated mining conditions were established.According to the permeability-stress model,the absolute and relative stress sensitivity coefficients were proposed to evaluate stress sensitivity and evolution characteristics of the different damaged coal and rock.(2)A numerical methodology was proposed to determine the fluid-solid coupling parameters for fractured coal and rock mass,and the coupling mechanism of stress-fracture-flow is illustrated.Discrete element numerical models of coal samples with different fracture structure were built.A numerical method of calibrating joints and fractures parameters was proposed.The isotropic and anisotropic characteristics of the coal permeability are studied,and the influence mechanism of the fracture on the coal permeability stress sensitivity is obtained.The internal relation between morphology and parameters of fracture and permeability of fractured coal samples is mastered.The triaxial fluid-solid coupling stress-fracture-flow evolution characteristics of fracture coal sample are analyzed.The main reasons for the difference of the axial permeability to the confining pressure and axial pressure under the condition of deviatoric stress are described.(3)The stress-fracture-flow coupling model for the surrounding coal and rock mass is established,and the parameters design method of pressure relief mining and gas drainage is put forward.The reliability of the model was verified by the experiment of triaxial fluid-solid coupling and the measured results of gas extraction in coal seam.Using this model,quantitative analysis of permeability evolution and distribution characteristics in the three overlying strata zones is realized.The flow path of pressure relief gas during the pressure relief mining and gas drainage are obtained;and the gas pressure evolution distribution characteristics of protected coal seam are also clarified.The minimum and reasonable mining thickness of the upper protective coal seam in Hancheng coal mines and the lower protective coal seam in Huainan coal mines are determined.The relationship between the coal seam spacing and the mining height of the protective coal seam is expounded.The layout scheme of pressure relief gas drainage drilling is designed.(4)The evaluation model of the pressure relief mining effect is established,and the temporal and spatial evolution of overlying strata movement is identified.The different gas source model is established,and the evaluation method of pressure relief effect is put forward.The compaction time,compaction degree and permeability distribution characteristics of each area in the caved zone are quantitative analyzed.The temporal and spatial evolution law of the surrounding rock permeability during pressure relief mining is obtained.The inherent relationship between the compaction stress,compaction time and permeability of the caving zone in the gob is indicated.