Study On Stability And Optimal Arrangement Of Boreholes Drilled By 1000m Drilling Machine For Gas Extraction In Yuwu Coal Mine Tunneling Faces

It is one of the effective methods to control the gas in coal mine by using directional long drilling.But soft coal seam is widely distributed in China,and drilling collapse is of common occurrence,greatly reducing the ratio of finishing borehole and the efficiency of gas drainage management.Therefore,the drilling stability problem is worthy of an in-depth study of the important issues.This paper takes the drilling of the working face of Yuwu coal mine in Changzhi of Shanxi Province as the research object,and the mechanism of borehole collapse was studied through Elastic-plastic theory analysis,finite element numerical simulation and cases study.And the drilling trajectory design method and drilling arrangement were optimized,which based on the research results.The main conclusions are as follows:(1)The elastoplastic mechanical model considering the softening characteristics of rock is established,and the elastoplastic solution of the surrounding rock is obtained.It is shown that the plastic radius of the soft coal seam in Yuwu coal mine is 1.99 times of the drilling radius,and plastic zone radius and stress influence radius of roadway are 5.02 m and 17.09 m respectively,which is equivalent to 2.01 times and 6.84 times the equivalent radius of roadway.According to the analytical solution,the softening comprehensive coefficient N is proposed,which is a function of shear strength characteristic parameter,deformation characteristic parameter and original rock stress and characterize the softening of the hole wall.The softening comprehensive coefficient N is taken as an indicator of borehole collapse,which indicates the softening degree of hole wall.The larger the value,the easier the hole collapse,and there is a critical value for the identified coal and rock.When the surrounding rock reaches the critical value,the rupture zone appears,and then the damage occurs.The critical value of the softening comprehensive coefficient of coal seam in Yuwu coal mine is 2.5.(2)The COMSOL Multiphysics multi-physics coupling analysis software was used to simulate the stress-strain law of boreholes.The lower the strength of the rock,the greater the original gas pressure,the larger the drilling radius,the longer the pumping time,the larger the plastic zone of the borehole rock and the worse the drilling stability.After the drilling,the tangential stress around borehole is concentrated away from the drilling center at 2.3r and the stress concentration coefficient is less than 2.When the local stress lateral pressure coefficient is not 1,the plastic zone and stress around borehole no longer appear circular distribution.It is more prone to produce stress concentration in the horizontal direction on both sides of the borehole resulting in collapse and the plastic area is elliptical with the long axis in the horizontal direction.(3)The mechanism of drilling collapse is obtained by comprehensive comparison and analysis according to elastic-plastic mechanics model and its analytical solution and numerical simulation results.The strength of coal decreases due to the softening effect of boreholes;gas pressure and adsorption expansion stress decreased,causing significant increase in effective stress and stress concentration around borehole.When the stress exceeds the ultimate strength of the coal,the rock around borehole produces a very unstable crushing zone,and then collapse.(4)Based on the mechanism of borehole collapse and the stress and strain of the elastic-plastic zone surrounding rock and combined with Yuwu coal mine S2108 heading face mining conditions,in order to prevent the borehole collapse,The top(bottom)plate comb directional long drilling was proposed to pre-excavated gas of surrounding rock in drifting tunnel by “horizontal” or “vertical” arrangement.The main hole will be arranged in a stable rock formation,and the branch hole is in-depth coal seam to ensure the extraction effect.(5)For long(bottom)board comb directional long drilling,the design method of “straight line-arc-straight line” three-stage oblique plane drilling trajectory is proposed,which converts the space drilling axis to the plane for design.The method is simple,rapid,and can meet the accuracy in engineering.(6)The proposed drilling arrangement and drilling trajectory design method well guide the gas control in Yuwu coal mine,which is of great significance to the similar coal seam.