| In the process of coal mining,the stress in the coal seam is redistributed due to the disturbance of the surrounding stope,resulting in the change of the coal seam void structure,which affects the mechanical properties and permeability characteristics of the coal body,thus affecting the safety of operation and mining efficiency.And with the increase of buried depth,mining disturbance has a more significant effect on the mechanical and permeability characteristics of coal.it is of great significance to study the stress state,pore and fracture structure and permeability evolution of protected mining coal seam during the mining disturbance process in the upper protective layer.In this thesis,the mining conditions of Ji15-31030 mining face at 1000 m in Pingmei No.12 Coal mine are taken as the research object,and the evolution of stress field and seepage field in the mining disturbance process of protected mining seam in the upper protective layer is studied by integrating similar material model experiments,seepage experiment,and nuclear magnetic resonance experiment.The stress loading path of protected coal seam mining is proposed,the pore structure evolution of coal is obtained under this path,and the multi-scale fractal dimension characteristics units model is further proposed.A permeability model was established based on the Multi-scale fractal dimension characteristics units model,and the permeability model was verified by experimental data.The main conclusions are as follows:(1)In the mining process of Ji14 coal seam,the distribution of roof stress of J15 coal seam is similar to that of J15 coal seam.The peak stress of the coal seam and roof of Ji15 coal seam under the coal in front of the working face increased slowly with the working face advancing.The stress of coal seam and roof in Ji15 coal seam below the coal pillar in open-off cut increased with the working face advancing.The stress of Ji15 coal seam below the goaf presented a "V" type distribution at the initial stage of working face advance.With the working face advancing,the continuous collapse of the overlying strata in the goaf continued to compress the goaf,and the stress of the coal seam and roof of the Ji15 coal seam below the goaf increased and began to show a "W" type distribution.In the process of protection layer mining disturbance,the stress of Ji15 coal seam goes through three stages: stress concentration in front of working face,rapidly pressure relief after working face pushing,and stress recovery after goaf compaction(2)Through the analysis of vertical stress evolution based on similar material model experiments and the analysis of horizontal stress of the protected coal seam,the stress loading path of protected coal seam mining was proposed.Then the seepage experiments under the stress loading path of protected coal seam mining were carried out.In the process of mining disturbance in the upper protective layer,the greater the disturbance stress withstood by the coal sample,the greater the peak failure stress and the greater the volume strain in the mining process of the coal sample.After mining disturbance,volume expansion point appeared earlier than the conventional triaxial test.The mining disturbance of the upper protective coal mining made the state of coal sample in the protected layer transition from elastic state to plastic state.The permeability of the protected coal seam did not show a single change trend with mining disturbance.The confining pressure of group M coal sample was 35 MPa,and the influence of confining pressure on permeability was greater than axial pressure.The confining pressure of group N coal samplea was 20 MPa.The alternating confining pressure and axial pressure had a major influence on permeability during the mining process of the protected layer,and the permeability curve showed a "W" shape.(3)The pore structure of coal samples under different deviatoric stress showed fractal characteristics.The fractal dimension D of coal samples varies with axial deviatoric stress in three stages.Stage I,the fractal dimension D decreased.At this stage,the pores compressed,the number of pores and the volume of pores decreased,which resulted in the space structure of pores from complex to simple.Stage II,stable growth stage.The fractal dimension D increased with axial deviatoric stress increasing,and they had a linear relationship.Due to the disturbance stress of protective coal seam mining,the damage had been appeared in the coal sample,volume expansion point appeared earlier.In this stage,the damage accumulated and the new pores and micro cracks occurred,which complicate the pore structure.Stage Ⅲ,rapidly changing stage.The fractal dimension of macropore and micropore increased rapidly,and the fractal dimension of mesopore decreased.When the deviatoric stress closed to the peak stress,the coal sample stayed in the plastic phase.The severe damage occurred in coal sample.The number of micropore increased,which complicate the micropore structure.The pore connectivity had been enhanced.(4)The pore structure in the whole scale has no obvious fractal characteristics.However,the pore structure has prominent fractal characteristics in different local scale ranges.Based on the multi-scale fractal characteristics of pore structure,multi-scale fractal dimension characteristics units model had been proposed,which can simulate pore structure distribution in any scale range.A multi-scale fractal permeability model is established based on multi-scale fractal dimension characteristics units model.The calculated results base on multi-scale fractal permeability model and the measured permeability based on experiments are in the same order of magnitude.The permeability result predicted by multi-scale fractal permeability model is better than the classical single fractal permeability model,which shows that the multi-scale fractal permeability model can describe the seepage characteristics with complex pore structure.The multi-scale fractal permeability model has a simple form and clear physical significance.It is easy to be applied to predict the permeability of porous media,especially for low-permeability porous media.(5)Based on the field test of Pingmei No.12 Mine,the evolution of coal seam pressure and seepage field of Ji15 coal seam under the mining disturbance of the upper protected layer Ji14-31070 working face had been studied.The pressure change of the protected layer Ji15 coal seam showed obvious lag effect compared with the advance degree of the protected layer Ji14-31050 working face.Gas pressure and gas flow rate were closely related to the pressure change of coal seam and had strong coupling characteristics.The pressure of coal seam played a leading role in the gas flow process of gas pressure monitoring borehole. |
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