Abstract
In most mining areas of China, coal seams are characterized by low gas pressure, poor permeability, and high gas adsorption capacity, all of which have brought considerable difficulties to coal seam mining and coalbed methane (CBM) extraction. According to the multiyear scientific research and production practice of China, gas is migrated in quantity only after the coal body is directly mined, and the surrounding rocks deform and fracture under the mining influence. Thus, the key to effective control of gas migration and the coal and CBM comining technology lies in investigating the gas resolution, permeation, migration, and accumulation laws in the coal seams under the unloading confining pressure during mining. The MTS815.02 rock mechanics testing system and its supporting equipment are combined to test the permeability characteristics of coal and rock mass (postpeak fractured coal and sandstone specimens) under the loading and unloading of confining pressure using the steady method, and then, the permeation laws of the fractured coal and rock mass are obtained. Results show that after the postpeak rock crack propagation reaches a stable state, the confining pressure gradually increases, and the gas permeability presents an approximately linear reduction; in the postpeak unloading phase, the opening and coalescence degree of rock cracks gradually increase as the deformation extends. Thus, permeability reaches a peak value. The strain softening phase follows, where the cracks are closed and permeability declines to a certain extent. Moreover, the unloading step size of confining pressure has bearing on gas permeability. Specifically, as the unloading step size of confining pressure decreases, the change of gas permeability increases in stability.
Highlights
As burial depth increases, gas pressure at the coal seam progressively increases at the hydrostatic pressure gradient
The gas pressure and content increase with the coal mining depth, and the gas disaster increases in severity
The latter differs from the coal seam permeability in the San Juan Basin of America by 104–105 times, and this difference is the primary cause for the difficult gas extraction before coal mining in most mining areas of China
Summary
Gas pressure at the coal seam progressively increases at the hydrostatic pressure gradient. Huainan and Huaibei, the working seams have highly complicated structures, metamorphic degree of coal seam ranges from long flame coal to anthracitic coal, gas content reaches 10–30 m3/t, Protodyakonov coefficients of coal are mostly within 0.1–0.4, and the coal seam permeability is only 10-7–10-6 [1]. Permeability is closely related to the pore structure of coal rock, deformation and failure characteristics [7,8,9,10,11,12,13], mining ground pressure, gas pressure at coal seam, gas adsorption, and resolution characteristics. Probing the gas permeability change in postpeak coal and rock mass presents a considerable guiding significance to coalbed methane (CBM) extraction under pressure relief conditions. This study mainly explores the permeability characteristics of postpeak coal and rock specimens under loading and unloading conditions of confining pressure
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