Stability Study Of Bedding Drainage Borehole Based On Finite Element Strength Reduction Method

When drilling in soft coal seam with high gas,soft-hard composite coal seam and outburst coal seam,the drilling depth is often restricted due to the phenomenon of sticking and pinching,while the occurrence of Coal Gun,caving hole,spray hole and blocking hole,etc.,also causes the poor formation of hole wall,the low completion rate and short life of Borehole lead to the difficulty of gas control,which seriously affects the effect of gas drainage in some high gassy and outburst mines.In this paper,the mechanical analysis of coal around the hole is carried out based on the principle of elastoplastic mechanics.Based on the simulated coal seam stress monitoring system,the variation characteristics of stress around borehole in coal seam are studied,and the relationship between in-situ stress and the size of coal pressure relief zone around borehole is discussed.A fluid-solid coupling model of borehole is established by using ANSYS software,and the plastic zone distribution of coal around borehole is studied under different factors.The results show that:(1)When other conditions are the same,the larger the in-situ stress,the larger the stress relief area,the more unstable the coal seam drilling,and the pressure arch around the hole moves to the deep coal body.(2)The range of the inner boundary of the pressure arch is negatively related to the stability of the Factor of safety.The larger the inner boundary,the smaller the Factor of safety of the stability of the borehole,and the worse the stability of the borehole.(3)The peak stress in the horizontal direction is much greater than the peak stress in the vertical direction,so the arch foot and abutment of the pressure arch should be located on both sides of the borehole.(4)By fitting the stability Factor of safety under the influence of different factors,the formula for estimating the Factor of safety of borehole stability without considering the interaction of various factors is obtained.(5)The Grey theory is used to analyze the primary and secondary relations of the factors affecting the borehole stability,and the order from high to low is Poisson’s ratio,cohesion,gas pressure,elastic Modulus,internal friction angle,in-situ stress and borehole diameter.(6)Using Poisson’s ratio,cohesion and gas pressure as the three factors to analyze the borehole stability Factor of safety,the regression equation related to borehole stability Factor of safety is calculated by means of uniform test,it provides a basis for quantitative evaluation of borehole stability.