Study On The Mechanism Of Hydraulic Punching Gas Permeability And Method Of Effect Evaluation In The Nuodong Coal Mine

Hydraulic punching is one of the key technologies to enhance the effect of gas drainage in coal mine.Based on the theoretical analysis,this paper constructed the theoretical model of the hydraulic punching gas permeability,and revealed the mechanism of pressure relief and permeability increase for hydraulic punching.The COMSOL Multiphysics software was used to simulate the effect of hydraulic punching in soft high outburst-prone seams.Based on the electric response experiment platform,a hydraulic punching evaluation system was constructed,and the effect evaluation critical index was also proposed.Finally,the theoretical model and lab test data were verified through field tests.The details are shown as follows:Based on the Hoek-Brown failure criterion,the mechanical properties of coal around the borehole was analyzed,and the variation law of the pressure relief area after drilling was obtained.through analyzing the formation,development and expansion of hydraulic fracture,the calculation equation of crack propagation length was derived.Meanwhile,based on the mass conservation equation,Langmuir equation and Darcy's law,the seepage equation of coal containing gas was established.Considering the dynamic change of porosity and permeability,the fluid/solid coupling model of borehole gas drainage was constructed.On the basis of the above theoretical model,the COMSOL Multiphysics software was used to numerically simulate the effect of pressure relief and permeability increase in the process of hydraulic punching in the Nuodong Coal Mine.The scheme for the arrangement of hydraulic holes in the Nuodong was obtained:1 t/m coal output,each row of 6 holes,spacing of 8.48m.Based on the DC electrical response,the simulation system of hydraulic punching laboratory was established,the impact of load,water and fissure on the apparent resistivity of coal and rock was studied,revealing the variation rule of apparent resistivity of coal and rock mass during the punching process.The average variation coefficient ? of apparent resistivity was proposed as the effect evaluation index of gas extraction.And the evaluation grading standard of hydraulic punching effect was set.Finally,according to the results of these studies,hydraulic punching of No.17 coal seam in Nuodong coal mine was carried out,and the effect was evaluated by DC electrical method.The results show that the effective range of hydraulic punching in the depth is-25m-45m,and in the direction is 35m?75m.The influencing radius is about 20m.The average variation coefficient ? of apparent resistivity was 93%in examination area,and the grade of evaluation about hydraulic measures is grade ?.This study proved the effect of pressure relief and gas permeability enhancement was significant in this area.The results of this study are useful and have great significance for the prevention of coal and gas outburst in soft outburst-prone coal mines.At the same time,it provides scientific basis and theoretical guidance for the comprehensive and effective evaluation of the effect of gas control.