Study On The Law Of Negative Pressure Distribution For Bedding Pre Gas Extraction Borehole Under The Creep Action

Borehole pre-pumping coal seam gas is widely used in regional outburst prevention,the negative pressure has two kinds of functions in the process of gas drainage: the first,the gas drainage from the coal seam to the borehole,the second,the gas drainage from the borehole into the pumping production line.Because of the gas flow in the borehole to generate the frictional resistance and aperture unbalanced deformation to generate local drag force,caused the phenomenon of negative pressure drop and even positive pressure.The scholars study the negative pressure distribution in the bedding borehole,which mainly uses the field test method and the mathematical model method.And the measured method has a strong pertinence and accuracy,but the operation is more complex;pre-pumping working face is located in the influence of mining stress,and the mathematical model is mostly not considering the effect of mining stress on the borehole creep deformation,the result of simulation and the actual deviation is great.So proven the distribution of negative pressure in the borehole under creep action is beneficial to the optimization of gas drainage in coal seam.It has important significance to the high efficiency extraction and the elimination process.In this paper,using the method of drilling cuttings to study on the distribution laws of the stress in the pre-pumping coal seam;establishing viscoelastic-plastic softening model around the borehole in the pre-pumping coal seam,and obtained the space-time evolution laws of the aperture of the borehole;building the gas flow model around the borehole;application of Comsol Multiphysics software to simulate the gas pressure of coal around the borehole,indirect influx of drilling gas volume;building the gas flow model in the borehole,concluded that the negative pressure distribution law in the pre-drainage gas drilling hole under the effect of creep.The main conclusions of this paper are as follows:(1)Under the action of mining stress,the aperture appears narrow to varying degrees,in the stress relief area to a certain extent reduced;in the stress concentration area,borehole collapse phenomenon in a short time;original stress area in a short time obvious shrinkage,and borehole collapse phenomenon appeared after a period of time.(2)The borehole of the 3103 North working face of Hebi eight mine,gas drainage for first days to fifth days,the pressure difference between orifice and borehole bottomincreased from 270 Pa to 3.3kPa;with the time going on,on the 7 days,the negative pressure value to become 0 in the section of borehole;on the 9 days,almost no negative pressure in the borehole.