Mining Stress-fracture Evolution Of Lower Protective Layer Mining Study On Gas Migration Law

As China’s main energy,coal plays an vital role in China’s economic development.The safety problems such as mine gas outburst seriously restrict the safe and efficient intensive production of coal enterprises,and gas control has become an the difficult problem in mine safety production urgently awaits solve.Based on the occurrence state of’three low and one high’coal seam in China,the mining of protective layer and relief gas extraction have become effective measures for gas control.The selection of the first mining protective layer,the evolution law of surrounding rock fractures,the pressure relief effect of mining and the migration law of gas in the extraction process are the key scientific problems of safe and efficient co-mining of coal and gas.Therefore,this paper takes the mining of the lower protective layer in Chajiaotan Mine as the engineering background,and the above problems are studied by using the methods of theoretical analysis,numerical calculation and field measurement.Combined with the research results,a set of comprehensive gas control methods were formed,which guided the field gas control engineering practice.Through the combination of theoretical analysis and engineering field measurement,it is verified that protective layer mining and pressure relief gas drainage can eliminate the outburst risk of protected coal seam and ensure the safe mining of protective layer.The main research contents and results are as follows:（1）The gas outburst risk comparison and mining feasibility analysis of C₂₅and C₁₉coal seams were carried out,and the first C₂₅coal seam was selected.By calculating the equivalent layer spacing,it was concluded that C₂₅coal seam was a long-distance lower protective layer.Based on the key layer theory and discrete element numerical simulation,the influence of key layer fracture on the evolution of coated rock fractures in the mining of lower protective layer is studied.The results show that during the mining of C₂₅coal seam,the key layer on the overlying strata will be broken up layer by layer,resulting in the continuous upward development of fractures.When the C₂₅coal seam is excavated for 90 m,the height of the‘three zones’affected by the barrier of the main key layer is basically stable.（2）Based on the mechanical theory of elastic-plastic mechanics and seepage mechanics,the expressions of permeability-volume stress,gas pressure-fracture porosity and fracture porosity-volume strain in the process of mining disturbance are studied,and the coupling model of stress field,gas diffusion field and gas seepage field in the process of gas flow is established,which provides a theoretical basis for COMSOL to simulate gas migration in the process of extraction.（3）The pressure relief effect of C₁₉coal seam after C₂₅coal seam mining was studied by FLAC^3D.The results showed that the stress of C₁₉coal seam experienced the process of loading-unloading-stress recovery-stability.According to the relative expansion deformation,C₁₉coal seam was divided into sufficient pressure relief zone and insufficient pressure relief zone.The comparative analysis of gas drainage effect before and after pressure relief in C₁₉coal seam was carried out by COMSOL,and the reasonable spacing of drainage boreholes in different pressure relief areas of C₁₉coal seam after pressure relief was studied.The numerical simulation results show that the gas extraction effect after pressure relief is obvious.C₁₉coal seam fully pressure relief area drainage borehole spacing should be 30 m,not fully pressure relief area drainage borehole spacing should be 5 m.（4）The comprehensive gas control scheme of upper and lower cross-layer drainage,borehole drainage and buried pipe drainage in goaf is proposed.The residual gas pressure of C₁₉coal seam,the relative expansion deformation,the pure amount of pressure relief gas extraction and the gas concentration of return air flow in C₂₅coal seam were measured.The measured data show that the mining of protective layer C₂₅coal seam can effectively improve the extraction effect of pressure relief gas in protected layer C₁₉coal seam,and play a good outburst elimination effect on C₁₉coal seam.At the same time,it prevents the pressure relief gas into the C₂₅coal seam mining face,and ensures the safe mining of C₂₅coal seam.