Dynamic Migration Characteristics Of Methane In Mining-damaged Coal And The Multiphysics Coupling Mechanism

The coal mining industry in China has gradually moved into deeper areas,where the high stress and strong mining effect in the deep mines will cause severe damage to the coal seamsbody ahead of the working face.As a result,a large amount of adsorbed gas in the coal seams will be rapidly desorbed and released,leading to gas abnormal effusing and even coal and gas outburst disasters,which pose a serious threat to the safety of coal mining production.Therefore,it is of great significance to deeply understand the gas migration process and the inherent control mechanism of coal damage caused by mining in deep mines for accurate prevention and control of gas dynamic disasters.This thesis explores the crack propagation mechanism inside damaged coal seams under different stress pathsby combining laboratory experiments with theoretical modeling.The thesis also analyzes the dynamic process of gas desorption and diffusion in situ within damaged coal seams and the evolution of permeability during mining-induced damage.In addition,a multi-field coupling model for gas outburst considering the jump of permeability in mining-damaged coal seams is established to reveal the multi-field coupling mechanism of gas outburst in miningdamaged coal seams.The main research conclusions are as follows:Based on the stress process experienced by coal ahead of the working face,the simulated stress path under laboratory conditions was extracted to analyze the evolution of micro-cracks in the damaged coal and elucidate the mechanism of coal instability failure.The results show that during the mining process,particularly before instability and failure,a large number of tensile cracks are generated inside the coal,with the crack angle mainly parallel to the maximum principal stress direction.However,this type of crack does not directly lead to coal instability.As loading continues,many shear cracks are generated in the coal,mainly parallel to the internal friction angle,eventually leading to coal instability.After instability and failure,the coal remains under stress constraints.As the vertical stress continues to decrease,new cracks constantly emerge around the shear principal cracks,which connect with each other and eventually form acomplex network of cracks.During this process,the strength of the coal deteriorates continuously.Based on the changes in horizontal and vertical stress under the complete mining stress path,the impact of stress on methane desorption and diffusion in coal is divided into five stages.The initial rapid reduction stage : methane in the coal’s macropores rapidly desorbs and diffuses,primarily influenced by gas concentration gradients;The early slow reduction stage : methane in the micropores continues to desorb,still driven by concentration gradients,but the effects of mining stress begin to emerge;The midterm slow increase stage,desorption still mainly occurs in the micropores,and new pore fractures continuously form in the coal,which is significantly affected by mininginduced stress;The coal sample damage stage : when the vertical stress reaches the stress peak,the coal undergoes fracture deformation,the coal pore structure is destroyed,and the coal’s exposed surface area increases,leading to a rapid increase in desorption volume and rate;The post-damage stage : the desorption process is mainly influenced by mining stress and the adsorption capacity of original coal.Nine simulated mining stress paths were extracted based on the complete mining stress path,and the variation of coal permeability with gas pressure under nine simulated mining stress paths was studied.The results show that the permeability of coal is mainly affected by the effective stress in the horizontal direction,while the influence of the effective stress in the vertical direction is relatively small.The effective stress acts on the coal matrix,changing the distribution of coal pores and fractures,and affecting the permeability and dynamic mechanical properties of coal.When the effective stress in the horizontal direction increases,the constraints on the horizontal pores and fractures in the coal increase,the pores shrink,and a reduction in the size of the pore space occurs.The opening of fractures is reduced,and some fractures close,with an increase in the resistance encountered during gas transport and a decrease in the permeability of coal samples,while the dynamic strength increasesenhanced.When the effective stress in the vertical direction increases,the tension on the vertical fractures in the coal increases significantly,leading to an increase in the opening of fractures,a decrease in the velocity of p-wave,a decrease in the resistance encountered during gas transport,an increase in permeability,and a decrease in dynamic strength.Based on the elastic coal permeability equation,a mining-induced damage coal permeability model was constructed by introducing permeability jump coefficient and damage factor.The validity of the mining-induced damage coal permeability model was verified by simulating the permeability data under three simulated mining stress paths.The results showed that the coal damage and permeability gradually increased under the simulated mining stress path,and the permeability calculation results of the model matched well with the experimental test results.The fluctuation range of the damage jump coefficient under the three simulated mining stress paths was 10～15,and the fluctuation range of the damage factor was 0～0.5.A multi-physics coupled model considering the heterogeneous mechanical properties of coal was developed based on the coupled processes of stress field,fracture field,and gas flow field.The spatiotemporal evolution laws of each physical field during the gas extraction process in the mining-induced area were investigated.The external parameters of the model(vertical stress,distance from the borehole to the working face),coal mechanical parameters(cohesion,internal friction angle),and mining parameters(coal excavation distance)were studied using a single-factor analysis method to determine their effects on gas migration and borehole extraction efficiency in the mining-induced area.The results showed that under the influence of mining-induced stress,the damaged zone in front of the working face extended towards the depth of the coal seam until it connected with the depressurization damage zone of the gas extraction borehole.The smaller the cohesion and internal friction angle and the closer the borehole was to the working face and the larger the coal excavation distance,the greater the extent and degree of damage to the mining-induced coal body and the better the depressurization and permeability enhancement effect of the gas extraction borehole.The mining-induced coal body was found to have stress concentration,which seriously affected the extraction efficiency of the borehole.The thesis contains 92 figures,15 tables and 190 references.