Research On Technology Of Relieved Gas Drainage Using Surface Boreholes In A Single Extremely Thick Coal Seam Of Weijiadi Coal Mine

The ultra thick coal seam goaf gas distribution and gas abnormal emission rules during the period of the pressure have been studied systematically in this paper.The ground drilling control goaf gas flow field technology is put forward to replace underground roadway roof extraction technology.The influence of drilling hole position to goaf air leakage and the gas concentration extraction flow are studied. The main technical problems in the process of drilling construction are analyzed and summarized.The drilling gas production rule and gas flow change characteristics are analyzed.The research results are actually used in Weijiadi Coal Mine in Gansu Jingyuan.FLUENT software simulates goaf gas distribution and gas abnormal emission rules during the period of pressure:(1)High concentrations area of oxygen and low concentration area of methane respectively existence near working face;(2)Gas concentration is increased by exponential function at strike direction and lower near ventilation roadway and intake airway reversely.Also gas concentration grows faster at the 0~50m on the vertical direction;(3)Gas emission has three stage which includes“speed up and quickly reduce and slow down” during the movement of impacted roof.The increase of the gas concentration of working face has a“delayed effect”, which means the increase of gas concentration and the peak hour are later than the moment of roof sudden sinking.A reasonable layout of drilling is determined and the main technical problems in the process of drilling construction in the ground are analyzed and summarized:(1) The best location of surface borehole is 30 m from ventilation roadway and 36 m from the coal seam floor;(2) The shale mechanics and chemical coupling analysis model is established and the shale wall rock failure criterion is given.The effect of formation water on gas drilling is analyzed and the treatment countermeasures have been also given;(3) The main influencing factors of borehole deviation during gas drilling are studied. The causes of deviation during gas drilling are analyzed and control measures are put forward.The research results mentioned above have been carried out in Weijiadi Coal Mine of Jingyuan Group, Gansu province, which shows the effect of pressure relief gas drainage with surface boreholes is remarkable: The pure gas drainage quantity of the four boreholes in the testing area has totally reached 629.256 ×104m3.The gas concentration of return air decreases to 0.2% to 0.3%.The drainage flow of the high level suction roadway is greatly reduced, which can be replaced by the ground drilling.