Research On Secondary Gas Extraction Technology In Dynamic Pressure Area Of Prominent Coal Seam Under Dual Stress Disturbance

With abundant coal seam gas resources reserves,strengthening coal seam gas extraction can ensure safe coal mining and also use the extracted gas as clean reserve energy to reduce greenhouse gas emissions,which is important for energy development and carbon capture.Based on the floor rock roadway for hydraulic punching can promote the gas pre-extraction to meet the standard and cover the coal roadway excavation,however,the mining desorption gas and coal fall gas gushing out from the working face of the outburst coal seam during the mining process will still cause the working space and corner gas concentration to exceed the limit.Therefore,the enhanced gas extraction of the coal in the area disturbed by the mining stress in front of the working face also becomes the key to ensure the safety of mining.In view of the specific environment of high gas and low permeability prominent coal seam in Shoushan No.1 Mine,the author proposes the secondary enhanced gas extraction technology in the dynamic pressure area based on the floor rock roadway.By using the methods of theoretical analysis,experimental investigation,numerical simulation and field practice,the research is conducted on the coal seam gas extraction in the dynamic pressure area(i.e.stress loading and unloading area)in front of the working face,to reveal the stress distribution,seepage characteristics of the coal seam and the mechanism of unloading gas flow under the influence of coal seam mining and hydraulic punching,and to realize the high efficiency gas extraction before and during mining of outburst coal seam through the implementation of hydraulic punching+dynamic pressure area secondary enhanced extraction in the floor rock roadway.The main conclusions are as follows:(1)Forming an analysis system for the layout of floor rock roadway combining"energy control,roadway stabilization,geological analysis,and anti-outburst utilization".With the decrease of horizontal and vertical distance between the floor rock roadway and the upper pre-set coal roadway,the pressure relief effect of the coal in the upper pre-set coal roadway is obvious,and the elastic strain energy accumulated in the unit volume of coal is smaller.Considering the influence of coal seam mining on the stability of the floor rock roadway,it should be at least 15 m away from the floor of the upper coal seam.The floor rock roadway is less affected by coal seam mining when it is arranged directly below the coal roadway.(2)During the mining process,the difference between the permeability of the coal body away from the punching cavity and the permeability of the coal body at the surface of the punching cavity wall decreases,i.e.,the permeability of the coal body at the area less affected by hydraulic punching also increases under the influence of coal seam mining.The punching cavity will promote the occurrence of plastic damage of the coal within about 50 m in front of the working face,and will also provide enough expansion space for the coal damage extension.Especially,the coal within 20 m in front of the workings is disturbed by the influence of punching and mining to release a large amount of gas,which is conducive to the formation of gas extraction advantageous channel.(3)According to the results of permeability test and COMSOL gas extraction simulation,when not affected by coal seam mining,the influence of punching disturbance of coal in the central area of punching cavity is small,and the gas pressure drops slowly.Under the influence of dual disturbance,the coal in a certain area in front of the workings undergo plastic damage,and the closer the distance to the cut-hole,the more serious the damage state of the coal,and the faster the gas pressure drops during the extraction process.Punching interval also affects the rate of gas pressure drop of coal,the larger the punching interval is,the slower the gas pressure reaches the standard.(4)The focus of gas control route is to change from pre-extraction to unloading extraction,to delay the recovery time of gas extraction pipeline in the floor rock roadway,and to carry out secondary enhanced extraction after gas pre-extraction in the dynamic pressure area in front of the working face through hydraulic punching to improve the effect of gas extraction during the mining period.The coal mining face can be divided into high efficiency,effective and original gas extraction areas according to the gas extraction concentration of dynamic pressure area on the coal seam mining direction.The average gas concentration in the corner and the middle of the wind roadway is lower than 0.5%.The monthly gas extraction volume in the dynamic pressure area of the middle extraction road during the monitoring period is 89,700,000m~3,accounting for 51.10%of the total monthly extraction volume in the middle extraction road at the highest,and the enhanced extraction rate of dynamic pressure gas in the promotion area can reach 55.31%,which can achieve a larger extraction effect with a smaller extraction borehole ratio.The thesis contains 49 figures,10 tables,and 131 references.