The Evolution Law Of The Dominant Channel Of Pressure Relief Gas In The Overburden Cracks In The Mining Area

The pressure-relief gas generated during the mining of underground coal seams flows to the working face,which seriously affects the safe production of coal mines.Gas drainage can not only effectively prevent the occurrence of gas disasters in underground coal mines,but also turn waste into treasure,alleviate the greenhouse effect and increase the application of coalbed methane.This paper uses a combination of theoretical analysis,physical experiment and field measurement to study the evolution law of the dominant channel of pressure relief gas in the overburden cracks in the mining area,and based on the above research results,the best location and horizon for surface drilling are proposed.The main research results are as follows:Through theoretical analysis and similar simulation tests,the overburden fracture,crack development and stress distribution law during mining are studied.The collapse zone,transition zone(lower part of the fracture zone)and vertical fracture zone in the trapezoidal fracture structure of the overlying strata are the advantageous migration channels for pressure relief gas.Gas migration in the compaction zone is blocked,and the separated fracture zone is gas storage space.In the horizontal direction,the overburden fissures show a distribution pattern of few in the middle and more on both sides.In the vertical direction,as the distance from the stope decreases,the number of fissures increases as a whole,and there are more abundant fissure shapes.During the mining process,stress concentration will occur in the overlying rock above the working face,and the stress concentration will be more obvious with the increase of the buried depth of the rock layer.The stress relief phenomenon occurs at the contact part of the working face and the goaf,and the stress recovery area,is behind the pressure relief zone,where the stress gradually recovers to the original rock stress.The overburden stress above the coal pillar at the cut-off hole gradually increases with the advancement of the working face,and it will increase suddenly when the pressure is first applied and the key layer is fully compressed.The supporting stress of the coal pillar overlying rock at the stop line will go through three stages as the working face advances: the original rock stress stage,the stress slow increase stage,and the stress rapid increase stage.The supporting stress is delayed in the longitudinal transmission,and the longer the distance from the coal seam,the more obvious the delay.The gas flow in the stress concentration area is blocked,the gas flow in the original rock stress area is not affected by mining,and the gas flow in the stress relief area is smooth,which is the dominant channel for gas migration.The evolution and distribution of overburden rock displacement caused by mining is studied.In the whole process of advancing the working face,the displacement from large to small is divided into red,yellow,green and blue.It is found that there are five types of displacement distribution: spoon type,peak type,trapezoid type,two-sided type,and frustum type.And divide the stope overlying rock into collapse failure zone,compaction zone,separated layer fissure zone and boundary penetration zone,among which the collapse failure zone and boundary penetration zone are the dominant gas migration channels,and the separated layer fissure zone is gas storage aisle.When arranging the surface well,it should pass through the blue-level displacement zone and enter the boundary penetration zone,so as to ensure that the drilling is not easy to be damaged,and to maintain the efficient drainage of the drilling.Research on the extraction parameters of a mining shaft in a mine in Shanxi,and divide the life cycle of the mining shaft into three stages: short-range efficient extraction?mediumdistance effective extraction?long-distance ineffective extraction.According to the on-site drainage data combined with the previous analysis,the optimal well position and horizon of the production well should be selected: the horizon of the production well should be arranged within 15.2 m?27.4 m from the bottom of the gob area.The location of the mining well on the dip is 25 m?60 m away from the air return lane.And in the strike direction,the first surface well is arranged within the range of 30 m?59 m from the cut hole,and the spacing in the strike is 270 m.