Study On Mechanism Of Borehole Collapse And Improvement Measures Of Gas Drainage Borehole In Soft Coal Seam

The coal seam with solid coefficient is less than 0.8 is the soft coal seam, and high gas soft seam are widely distributed in China, simultaneously soft coal seam is known as the "breeding ground" of coal and gas dynamic disasters. Deep hole drilling in soft and high gas coal seam is a worldwide recognized difficult problem, which is one of the reasons that restrict the efficiency of gas drainage. Due to the special physical and mechanical properties of soft coal seam, soft coal seam gas pumping boreholes in the drilling process is easy to cause the hole is not deep, borehole deformation, collapse and other phenomena. This greatly reduces the gas pumping rate, severely limits progress of the project, affects the safety of production. Therefore, it is necessary to study the mechanism of the collapse of the hole in soft coal seam.In this paper, the N2202 working face of Yuwu coal mine as the project background, according to the firmness properties of soft coal seam is small, considering the coal pore fluid, based on the Mohr-Coulomb strength criterion, establish the horizontal borehole and inclined borehole collapse and failure linear elasticity mode. Analysis the mechanical conditions of borehole instability and the calculation of the critical collapse pressure,study the effects of the basic mechanical parameters, the drilling trajectory and the pressure of coal seam on the borehole collapse. By collecting the basic mechanical parameters of coal in N2202 working face of Yuwu coal mine,use FLAC3 D finite element numerical simulation method, comparative analysis the stress field, displacement field and plastic zone of the open hole and the supporting hole, study the influence mechanism of depth, lateral pressure coefficient and borehole diameter on borehole deformation. The main research results are as follows:(1)The original fluid pressure of coal is greater, the borehole collapse pressure is greater, the risk of collapse is greater, but the rate and amplitude of decay are also greater, the time to reach the stable is shorter. When drilling angle is under 60°, and azimuth angle is nearby 30°,the borehole is very unstable,At the same time, when the drilling angle exceeds 60°,showing exactly the opposite situation, when the azimuth angle is nearby 90°,the borehole is very unstable,when the inclination angle of borehole is 60°,the azimuth angle has little influence on the stability of the borehole. When drilling in soft coal seam, the most appropriate drilling angle is set to 22.5 degrees.(2)When the buried depth is less than 600 m, the effect of buried depth on borehole deformation is small; when the buried depth is greater than 600 m, the borehole deformation rate increases sharply with the increase of depth, borehole shrinkage is obvious, accelerate the increase in the speed of vertical displacement and plastic zone.(3)When the side pressure coefficient is 1.0, 1.5, 2.0, compared to the side pressure coefficient is 0.5, the maximum vertical stress increased by 1.4, 2.6, 3.05 MPa.(4)The increase of borehole diameter will result in the increase of the disturbance of the coal body, and the stability of the borehole is reduced.(5)Under the support condition, with the increase of the strength of support, the vertical displacement of borehole surrounding rock, The distance between the maximum vertical stress acting point and the drilling center and the plastic zone are decreased.(6)According to the above theoretical analysis and numerical simulation, buried deep in Yuwu Mining borehole stability is less affected, and coefficient of lateral pressure, borehole diameter, drilling support pressure is the main factors affecting the stability of the borehole. Therefore, it is recommended that in soft coal seam geological structure caused by comprehensive stress increased area, should take small diameter bored, increases the drilling support strength of the way to improve the borehole stability and security borehole gas drainage.