Study On The Mechanism Of Coal Damage Evolution And Coal Seam Gas Pemeation Under Different The Mining Conditions

During the safety production of coal mining history, the gas has long been regardedas first one of all disasters. Improving permeability of coal seam is very important toenhance the effect of coal seam gas extraction and to reduce safety risks as well as toextract the gas as a clean energy. Therefore, the damage evolution and the penetrationmechanism of coal and rock have been the focus and difficulty of coal mine safetyresearch in China. In order to know more about the damage evolution and permeabilityof coal, using coal from Songzao K1, K2seam of Chong qing Songzao LLC and ShanxiJincheng Anthracite Coal Industry Group Co., Ltd. affiliated Zhaozhuang mine3#coalseam as the reseach objects to conduct experiments. Based on coal and rock triaxialstress state, cyclical mining disturbance, and gas extraction from coal seam step-downconditions, the coal and rock pore fracture characteristics and its adsorption desorptioncharacteristics, damage evolution and penetration mechanism under triaxial stress state,the periodic mining disturbing and gas step-down extraction process weresystematically studied, multi-field coupling permeability model and the permeabilitysensitivity was also systematically analysed by conbining the laboratory test andtheoretical research. And a series of useful disscussion was made from the theoreticallevel by combining with rock mechanics, seepage mechanics and damage mechanics.Through the research, some progresses were made in the following aspects:1) The native bore and fractures of coal were analysized. By using HCA-pressurevolumetric method, the gas adsorption characteristics of coal at different temperaturesand the variation law of adsorption constants a, b with temperature were experimentalstudied.2) Mechanics and seepage characteristics tests were carried out at differenttemperatures, particle sizes, gas pressures (temperature) under triaxial stress state and atthree different unloading rates under loading-unloading stress state in different miningmethods. The deformation characteristics, permeability during the evolution of damageand the relationship between deformation and permeability of coal were discussedrespectively.3) Using the concept and method of damage statistics, a statistical damageconstitutive model considering triaxial stress state and temperature was established itspracticality and rationality were verified by test data. The results show that the theoretical curve considering the temperature of coal and rock statistical damageconstitutive model can better reflect the actual trend of coal and rock damage evolution.The theoretical calculations data of the statistical damage constitutive model can bebetter fitted with experimental measurement data.4) The coal and rock damage evolution and penetration mechanism under cyclicalmining disturbance were studied by experiments at different temperatures, particle sizes,stabilization time. The relationship between main stress, permeability and axial strainduring load-unload stage were systematically discussed. The deformation andpermeability characteristics and their relationship with time under three conditions in asingle-cycle were anlyzed. Permeability changes lags volumetric strain changes atdifferent stability time.5) Through the periodic load tests under different temperature conditions, energyevolution mechanism of coal deformation were discussed.From the perspective ofenergy, study on energy conversion relationship and strain energy conversion rate atdifferent temperature during the periodic load were highlighted. The changing law ofabsorbed energy, elastic strain energy and dissipation energy with cycles andtemperatures were analyzed. The damage evolution equation of coal and rock based onthe analysis of energy dissipation was verified, and the related parameters were tested.And this equation describes the damage evolution process of coal and rock duringcompression. The relationship between energy dissipation and damage was discussed.6) The deformation and seepage features during the reducing of gas pressure wereanalyzed by gas pressure reducing seepage tests at different stresses and temperatures(CH4, He). Influence from the adsorption expansion and desorption matrix shrinkagedeformation of coal and rock, effective stress on permeability under different stressstates was discussed. Trend of coal and rock strain with the gas pressure droping andtemperature under different temperatures was analyzed. Coal and rock permeabilitydecreases first and then increases with the gas pressure droping, and also does with thetemperature increasing. The relationship between volumetric strain and permeabilitywas also analysed. Variability of coal and rock cleat compression coefficient wasstudied.7) Based on the permeability model of Jishan Liu and Shi-Durucan as well as therelationship between the effective stress and the strain of homogeneous porous mediuand isotropic thermoelastic, considering stress, temperature, adsorption and desorptionof gas and improving its algorithm and conditions, multi-field coupling permeability model was established. The new permeability model is proved to reflect the changingtrend of the measured permeability well by verifying the test data and making a contrastwith P&M and S&D models. By defining the gas pressure sensitivity coefficient andtemperature sensitivity coefficient, the function relation between the permeability andgas pressure, temperature based on the sensitivity coefficient was deduced. Andsensitivity analysis of permeability under different gas pressures and temperatures werestudied.