Study On The Law Of Gas Migration And Enrichment In Goaf By Upper Slicing Mining On Inclined Extra-Thick Coal Seam

The coal seams in western China generally have the characteristics of large inclination angle and large thickness.In the process of upper slicing mining in inclined extra-thick coal seams,the gas in the goaf mainly comes from the gas emission from the bottom of the goaf.The study on the law of gas migration and enrichment in the goaf of upper slicing mining in inclined extra-thick coal seam is of great significance to distinguish the gas enrichment area in the goaf,realize the efficient gas drainage in the goaf and ensure the safe production of the working face.The law of gas migration and enrichment in the upper slicing mining goaf mainly depends on the following three main factors:the air leakage condition of the goaf,the distribution and dynamic evolution of voidage in the goaf with the advance of the working face,and the distribution and change of gas emission from the bottom of the goaf.Among them,the ventilation pressure distribution of the working face,the air leakage passage at the interface between the mining face space and the goaf,and the source and sink of air leakage in the goaf determine the air leakage and distribution in the goaf.The distribution and evolution of voidage in goaf are determined by the caving and separation movement of overburden rock and the development of fractures in overburden rock.The gas content and pressure in the lower delamination,the characteristics of gas adsorption and desorption in coal,and the permeability distribution and evolution law of coal in the lower delamination are closely related to the gas emission from the floor of the goaf.The paper takes the 9-15（06）working face of Liuhuanggou coal mine in Xinjiang as the research background.With the advance of the upper slicing mining of the inclined extra-thick coal seam,the changing process of the caving movement,fracture field and displacement field of the overlying strata are studied,and the distribution and evolution law of voidage in the goaf with the advance of the upper slicing are obtained.Through the study of the load evolution process of the lower delamination coal with the upper slicing advancing,the stress distribution and variation law of the lower delamination coal are obtained.The stress loading and unloading experiments of the lower delamination coal samples collected in the field are carried out according to the coal body stress path,and the variation law of the lower delamination coal permeability with the advance of the upper slicing is obtained.Based on the numerical simulation of the gas emission process of the lower delamination coal with the upper slicing advancing,the distribution and change law of the gas emitted from the lower delamination coal to the goaf are obtained.On the basis of the above research,considering the air leakage condition and ventilation pressure distribution of the working face,the spatial gas distribution and change characteristics of the goaf in the process of upper slicing mining on the inclined extra-thick coal seam are obtained by numerical simulation,and the distribution range and evolution law of gas enrichment area are clarified.According to the distribution of gas enrichment area,the layout of gas drainage boreholes in working face is designed,and a good extraction effect is obtained.The main research results are as follows:（1）The changes of overburden rock caving,crack field and displacement field in the process of upper slicing advance on inclined extra-thick coal seam are simulated by 3DEC.The trapezoidal platform evolution law of mining fracture in overburden is obtained,and the distribution range and evolution law of caving zone and fracture zone are determined according to the distribution characteristics of trapezoidal platform.Through the fractal geometry theory,it is concluded that the fractal dimension of overburden rock fractures changes in stages of the time dimension,which quantitatively reveals the law of the generation and development of overburden rock fractures.and reveals that the fractal dimension has the characteristic of bimodal distribution in the spatial dimension,and the area where the peak value of fractal dimension is divided into the distribution range of gas migration dominant channel.On the basis of considering the swelling characteristics of caving zone and the subsidence characteristics of overlying rock in fracture zone,the three-dimensional voidage distribution function of goaf is put forward,and the voidage distribution characteristics of upper layered goaf are obtained:On the same height plane of the goaf,the voidage gradually decreases from shallow to deep along coal seam strike direction,along coal seam inclined direction the voidage gradually decreases from both ends of the goaf to the middle and lower part of the goaf.Due to the fact that under the action of gravity,the caving rock in the middle and lower part of the goaf is significantly compacted under the load of uncollapsed overlying rock and the compression of upper falling rock,and the voidage is relatively small.（2）The stress transfer model of goaf is constructed by FLAC^3D,and the stress transfer of uncollapsed strata above the goaf-caving rock-goaf floor is realized.On this basis,the changing process of the load in lower slicing of the floor during the upper slicing propulsion is simulated,the stress distribution and evolution law of the lower slicing coal are obtained,and the stress path of the coal sample in the lower slicing permeability experiment is determined.Then the stress loading and unloading experiments of lower slicing coal samples are carried out,and the permeability evolution law and distribution characteristics of lower slicing coal are obtained.The results show that with the advance of upper slicing,the permeability of the lower slicing coal has gone through five stages:Ⅰ-initial permeability,Ⅱ-permeability decreased,Ⅲ-permeability increased,Ⅳ-permeability suddenly increased,Ⅴ-permeability decreased to stable.The permeability distribution of the lower slicing coal under the same depth plane below the floor of the goaf shows that the permeability of the coal body along coal seam strike direction is symmetrically distributed along the middle of the goaf,and along coal seam inclined direction,the permeability of the coal body decreases gradually from both ends of the goaf to the middle and lower part.This is due to the large compressive stress of the caving rock on the floor in the middle and lower part of the goaf,so the permeability of the coal body in the middle and lower part of the goaf is relatively low.（3）On the basis of considering the occurrence of lower slicing gas,the adsorption and desorption characteristics of coal gas,and the evolution process of coal permeability and porosity,the gas emission process of lower slicing coal is simulated.And the gas pressure and content distribution and dynamic evolution law of lower slicing coal are obtained,and the distribution and evolution of gas emission from the floor of goaf are revealed.The results show that with the advance of the upper slicing,the residual gas pressure and content of the coal body near the edge of the goaf are higher than those of the coal body inside the goaf.In the interior of the goaf,along coal seam strike direction,from the cutting hole to the working face,and along coal seam inclined direction,from both ends of the goaf to the upper part of the goaf,the pressure and content of residual gas in coal gradually increase.Coal gas emission time and pressure relief range affect the change of coal gas pressure and content.The distribution of gas emitted from the lower delamination coal body to the goaf on the floor shows:the amount of gas emission at the edge of the goaf is always higher than that inside the goaf,and innside the goaf,along coal seam strike direction,from the cutting hole to the working face,along coal seam inclined direction from the two ends of the goaf to the middle and lower part of the inclination,the gas emission from the floor shows an asymmetric"basin"distribution.（4）According to the distribution and evolution of goaf voidage,the basic parameters such as goaf permeability are determined,and the distribution and evolution of gas emission from the bottom of goaf determined the amount of gas emission from goaf.On this basis,considering the ventilation condition of the working face,the process of gas migration in the goaf is simulated with the advance of the upper slicing,the characteristics of gas distribution in goaf space are obtained,and the distribution range and evolution law of gas enrichment area are defined.The results show that in the process of advancing of working face from 80m to 400m,the gas enrichment area is distributed in the shape of trapezoidal platform.Along coal seam strike direction,the long edge of the trapezoidal bottom increases from 62 m to 384 m behind the working face,and the long edge of the upper bottom increases from 43 m to 352 m behind the working face.Along coal seam inclined direction,the short side of the bottom of the trapezoidal platform increases from 7m to 12m from the lower side of the return roadway,and the short side of the upper and bottom increased from 24m to 35m at the lower side of the return roadway.At vertical backplane direction,the height of the trapezoidal platform increases from28～36 m to 25.2～75.5 m above the bottom plate.（5）According to the distribution range and evolution law of gas enrichment area,the design of drainage borehole in 9-15（06）working face of sulfur ditch mine is guided,and the gas drainage effect of borehole is investigated on the spot.The results show that,when the drill hole enters the goaf and the extraction concentration reaches a stable level,the gas drainage concentration of the end hole located in the gas enrichment area is 29.28%,the gas drainage concentration of the end hole located in the middle and lower part of the caving zone is 1.97%,the gas drainage concentration of the end hole at the top of the caving zone and above the rock strata is 23.57%.The borehole in the gas enrichment area has a good extraction effect.During the extraction of borehole,the average gas concentration in upper corner,return air flow and return air roadway is 0.21%,0.23%and 0.28%,respectively,it ensures the safe production of the working face.The research results provide a reference basis for efficient gas extraction in upper slicing goaf on the same type of inclined extra-thick coal seam.There are 125 pictures,44 tables and178 references in this paper...