The Characterization Of Coal Fracture Structure And Its Influence On Seepage Characteristics Under Cyclic Loading And Unloading

The coal seam within the mining influence range of the protective layer of coal seam group will be affected by the mining stress of different coal seams.In this paper,the existing research on stress path change of coal mining is reviewed.Based on the existing influence stress path of protective layer mining and the stress distribution law of coal body in the protected layer,the paper puts forward the cyclic loading and unloading stress path of different mining influence strength under the condition of multi protective layer mining of coal seam group.On this basis,the bituminous coal sample was drilled in the experiment,and the experiment was carried out in a self-made gas injection and displacement gas tester.The micro characterization of fracture structure under the action of repeated mining is studied and analyzed based on the repeated mining stress path of the coal seams caused by the mining of the working face of the above protective layer by laboratory experiment and theoretical analysis.The influence of repeated mining times,effective stress and pore pressure on coal seepage characteristics in the process of mining stress is explored.The main achievements are as follows:(1)There is a great difference in the fracture distribution of coal body before and after the influence of repeated mining stress in coal seam group.The combination of average gray value method and depth learning segmentation method can achieve very good segmentation results.The three-dimensional model of the fracture structure of the coal body is successfully obtained.The internal fracture network of the coal body before and after the fracture is affected by the repeated mining stress has good self-similarity.The fractal number of fracture network increases before and after fracture.(2)Under the stress path of repeated mining with high stress concentration,the coal permeability decreases first and then increases with the increasing of the mining influence strength of the protective layer,until the coal is broken.When the axial deviant stress is constant,the relation between permeability and effective stress is exponential function.The stress concentration coefficient is linearly correlated with the fracture compressibility coefficient.The relationship between permeability and effective stress pore pressure under repeated cyclic loading and unloading and under different deviatoric stresses is studied.(3)The change of coal permeability is influenced by the intensity of simulated repeated mining.When the mining stress exceeds the critical value,the permeability of coal sample increases with the increase of loading and unloading cycle,and vice versa below the critical value.When the intensity of coal body affected by mining decreases gradually,the permeability changes from increasing to decreasing.(4)With the increase of pore pressure,permeability variation curve is divided into three stages: lower stage,stable stage and higher stage,the relationship of a quadratic function.When the stress environment by low deviatoric stress increases as high deviatoric stress,longer stable stage,can reduce goodness-of-fit.Different paths and unloading cycle for the first time seepage data analysis showed that the increase of confining pressure axial pressure are will cause lower permeability,at the same time will also lower the overall flow of pore pressure sensitivity.Cyclic loading and unloading of different axial deviatoric stress paths have different effects on coal sample permeability.There are 74 figures,7 tables and 89 references in this paper.