The Key Technology And Application For Pressure Relief And Permeability Increasing By Hydraulic Flushing In “Three Soft” Coal Seam

The permeability can`t be increased by pressure relief after mining protective coal seam for “three soft” single outburst coal seam, meanwhile because its top-bottom coal seam is soft and easy to be broken badly, the fluid fracture is not feasible, the coal output、pressure relief and permeability increasing by hydraulic flushing is the best way to realize the extracting goal. A set of rapid pressure relief for permeability increasing technology which is fit for “three soft” coal seam is put forward through laboratory test,field test,theoretical analysis and numerical modeling using a series of new hydraulic flushing equipment based on the analysis of the former hydraulic flushing technology system in this article.First through using Hoek-Brown failure criterion, the mathematical model of stress distribution around the borehole before and after the hydraulic flushing is established, the plastic range around the borehole is calculated, the sphere of influence of hydraulic flushing is confirmed based on high-pressure hydraulic jet coal broken mechanism. Second the limited spacing of arch during drilling hole is calculated, the mechanics of granular media is introduced aim at the stress arch after hydraulic flushing; the influence of moisture content to the arch caused by drilling is analysed through direct shear test which provide the basis for the stabilization of the arch before and after flushing. Hereby the pressure relief and permeability increasing mechanism of hydraulic flushing in “three soft” coal seam is discussed. Finally the industrial test is implemented in “three soft” coal seam in Zhengzhou coal industry LLC Daping and Gaocheng mine, a set of permeability increasing technology by multi-stage and multi-method is initial shaped which respectively is rapid drilling with medium air pressure in the first stage, the integration technology of drilling and flushing of high-pressure hydraulic jet during the drill recede, the repeatedly repaired for permeability increasing technology in aged hole during unite drill stage, the naked well technology by positive displacement motor in exposed hole and the permeability increasing technology by high-pressure hydraulic jet.The results of field application show that the radius of borehole is increased by 1.5 times, the breathability coefficient of coal seam is increased by 2 times; the gas extraction volum is enhanced by three times by mult-stage permeability increasing technology in the exposed hole which is temporarily sealed; the gas extraction volum is increased by 0.4~4.2 times after the aged hole is repaired.