Coal is the most abundant fossil energy in China, the coal seams in western China are generally characterized by large dip angle and large thickness, among which the inclined thick coal seams are widely distributed. Mining of huge thick coal seams breaks the original state of coal and rock, causes destabilization of the overlying rock layer and fissure development, and is accompanied by a large outflow of gas from the coal and rock layers. The gas gushed out is migration and stored in the mining fissure network due to the pressure and concentration gradient, resulting in multi-layer accumulation of gas in the mining area and abnormal phenomenon of gas in specific locations downhole, which seriously restricts the safe and efficient mining of inclined huge thick coal seam. Achieving efficient and accurate prevention and control of gas in inclined thick coal beds has become the "neck" problem of this type of mine. It is of great significance to study the mechanism of unloading pressure gas migration and storage in integrated mining of inclined huge thick coal seam to identify the unloading pressure gas migration and storage area of inclined huge thick coal seam, realize the efficient extraction of gas in the mining area, and ensure the safe production of the working face.
In this paper, the 9-15(08) working face of Xinjiang Liuhuanggou Coal Mine is taken as the engineering research background. Theoretical analyses, physical experiments, numerical simulations and on-site measurements are used to carry out systematic research on the overburden rock collapse movement, overburden rock fissure field, displacement field, seepage field and other change processes under the conditions of inclined extremely thick coal seam (IETCS) consolidation mining. The morphology, distribution and evolution of the unloading gas migration and storage area, void ratio and permeability in the IETCS were obtained, and the mechanism of asymmetric cycle construction of the unloading gas migration and storage area was clarified, revealing the mechanism of linkage evolution of the unloading gas migration and storage double area. The precise identification method and process of the unloading gas migration and storage area are constructed, and a mathematical theoretical control model of the unloading gas migration and storage area in IETCS was established. A sub-divisional extraction technology system for unloading pressure gas in IETCS has been formed, and better extraction results have been achieved, and the research results can provide theoretical guidance for the safe and efficient mining of this type of coal seams. The main research results are as follows:
(1) Based on the gas conductivity of the fracture network in the overburden rock of the mining area, the transmission characteristics of gas in the fractured rock body are described, the fracture mechanism of the key rock blocks in the IETCS migration and storage area is analyzed, and the point energy dissipation structure of the key rock blocks is proposed. The fracture criterion of point energy dissipation structure was obtained, and it was clarified that the gas migration and storage areas experienced the cyclic construction process of "formation-destruction-re-formation", and the sub-domain determination criterion of the unpressurised gas migration and storage areas was constructed.
(2) Through the self-developed large-scale planar physical similarity simulation experiment platform for giant coal seam, a two-dimensional physical similarity simulation experiment was carried out in a high-gas inclined giant coal seam working face in Xinjiang, taking the working face as the experimental prototype. The distribution characteristics of mining fissures in the IETCS were obtained, and the "cooling tower" distribution characteristics of mining fissures are proposed. A gas migration and storage area with a width of about 100 m is formed on both sides of the “cooling tower” wall of the mining fissure. The movement characteristics of the unloaded gas along the fissure network within the tower wall on both sides during the mining process of the inclined huge thick coal seam were obtained, and the distribution characteristics of the unloaded gas migration and storage area are clarified.
(3) The 3DEC numerical simulation is used to investigate the distribution and evolution characteristics of the void ratio under the conditions of IETCS. The dynamic evolution laws of rock subsidence characteristics and overburden fissure field are obtained, and the development heights of the fallout zone and fissure zone of the IETCS were obtained, and the expression of the heights of the "two zones" under the conditions of comprehensive mining of the IETCS was optimized and summarized. Based on this, the void ratio distribution function in three-dimensional space was proposed, and the void ratio distribution characteristics of the mining hollow area in three-dimensional space are revealed by this function.
(4) The permeability distribution pattern of the mining area shows a hyperbolic "cooling tower" pattern with a wide top and bottom and a narrow middle. It is found that the unloaded gas is mainly distributed on both sides of the extraction zone, and the width of the unloaded gas migration and storage area on the left side is larger. Under the action of pressure gradient, the gas mainly accumulates at the bottom of the bubbling zone. The distribution of gas pressure in the quarry and the change of gradient law are obtained, and the permeability sub-area control equation is established, combined with the evolution law of void rate, and the strain-void rate-permeability model of the overburden rock of the IETCS was constructed.
(5) A comprehensive control model of gas seepage, uplift and diffusion in the dual zone of gas migration and storage is constructed under the conditions of integrated mining. By analyzing the formation of gas migration and storage dual zones and the distribution characteristics of void ratio in the mining area, the asymmetric mechanism of unloading gas migration and storage dual zones in IETCS was found. The method of defining the spatial location boundary of the unloaded gas migration and storage zone is formed, and the linkage evolution model of the tilted thick coal seam migration and storage double zone is constructed, which provides a certain theoretical basis for the gas extraction technology.
(6) A sub-divisional extraction technology system for mining and unloading pressure gas in inclined thick coal seams has been formed. Based on the model of unloading pressure gas migration and storage area, we propose the sub-domain extraction technology for mining and unloading pressure gas migration and storage area, optimize the arrangement parameters of the gas extraction system at low, middle and high levels, and carry out on-site measurement of the extraction effect. With the management of sub-domain gas extraction, the absolute total amount of gas gushing out averaged 34.44 m3·min-1, and the total amount of gas extracted averaged 27.2 m3·min-1. The gas concentration in the upper corner of the working face is less than 1%, and the value of gas concentration in the return airway is within the range of 0.18~0.46%. The practicality, reliability and reasonableness of the decompression gas sub-division extraction technology under the conditions of comprehensive discharge mining of IETCS have been verified, and the safe, low-carbon and high-efficiency mining has been realized.