Study On Stress Change Characteristics And Arch Formation Characteristics Of Coal Body Around Hole Before And After Secondary Drilling In Gas-bearing Coal Seam

The poor stability and short effective life of drainage borehole in soft coal seam often lead to the awkward situation of low gas flow and low pure quantity.The research and development of hydraulic operation machine for gas drainage hole provides the possibility to realize the repair of the old hole,but the law of stress change of coal body around the hole before and after drilling dredging is not clear.Based on this,the characteristics of stress change and arch formation of coal around the hole before and after drilling in gas-bearing coal seam are studied by means of mechanical analysis,numerical simulation and laboratory tests.The influence factors of the plastic zone of coal and rock around the hole are analyzed by the principle of elastoplastic mechanics,and it is found that the radius of plastic zone is mainly affected by three factors: ground stress,cohesive force of coal body and internal friction angle.Combined with bulk mechanics and the calculation method of the weak surrounding rock of Taisha foundation,the dynamic moving transformation of the arch caused by the bearing capacity,thickness and borehole instability of the pressure arch around the hole is analyzed.Based on the stress monitoring test system of simulated coal seam,the effects of coal hardness,gas equilibrium pressure and simulated in-situ stress on the pressure arch of borehole before and after secondary drilling were studied.The results show that :(1)after secondary drilling,the pressure relief zone on the left and right sides of the borehole expands,and the stress increasing zone(i.e.the position of the arch foot of the pressure arch)moves to the deep part of the coal seam.(2)The greater the equilibrium pressure of gas adsorption,the greater the simulated in-situ stress and the longer the distance between the area of coal body stress increasing around the hole and the deep part of coal seam after secondary drilling.(3)After the second drilling,the coal debris is taken out of the hole,but the gas emission capacity is not improved.(4)The small cavity of the hole in the hole is large,which is located in the upper part of the hole,and the crack propagation of coal sample is obvious.Based on the UDEC software,model of borehole dredging is constructed,and the evolution process and the influence of borehole dredging are studied.The results show that :(1)Before the second drilling,the pressure arch of coal seam often has the characteristics of "double arch ",and " shallow arch " plays a major role in the stability of drilling.(2)After secondary drilling,the "double arch" structure of the borehole in the coal seam with low in-situ stress and high Young’s modulus is more stable.