Accurate determination of the permeability of crushed coal medium is the basis for the study of their permeability characteristics. To investigate the permeability characteristics of this special porous medium composed of crushed coal particles, the permeability parameters of crushed coal specimens of different initial porosities were measured by designing a lateral-limit compression seepage test system. Parameters were determined separately for specimens of different initial porosities. (1) the Reynolds number distribution region characterising the seepage state was determined and obtained. Specimens with initial porosity distribution between 0.02 and 0.08, and seepage Reynolds number distribution in the low-permeability zone, under Darcy flow; (2) the intrinsic permeability of the crushed coal medium was obtained by using the Ergun equation. The complex inverse proportional relationship between the drag coefficient and Reynolds number was derived; (3) Through the determination of the permeability of the crushed coal medium, the mean value of βK value was obtained to be about 45.7, and the analysis of the permeability of porous medium can determine its critical permeability. The relationship between the Forchheimer number Fo and critical Reynolds number was measured. The results indicate that it conforms to a linear distribution. In-depth analysis of these two parameters can be used to explore the flow transition process between laminar, transition, and turbulent flow. This study provides insight into the permeability characteristics of the media in fractured coal bodies.
Water content is an important factor in the deformation-destruction process of coal bodies. To analyze the influence of water on the creep acoustic emission (AE) characteristics of coal rock surrounding a borehole, we conducted graded loading creep AE tests of single-hole specimens with different water contents (0%, 4%, 8% and water-saturation) under uniaxial loading. The findings include the following: the water content affects the creep mechanical properties of the coal body around a borehole. The creep transient strain and steady-state strain increased exponentially with rising water content; the saturated specimen showed the highest increase, reaching 44.5% and 28.6%, respectively. The specimen water content affected the cumulative ringing count (CRC) and the axial strain during creep. The axial strain increased with rising water content, the CRC increased linearly with rising axial strain. The higher the water content, the greater the CRC rise. At different stress levels, the CRC in the 4%, 8% and saturated water content specimens changed by 43%, 53% and 74%, respectively. The AE ringing rate showed a pattern of grow–decline–stabilize at each creep stage. The AEs decreased significantly with the rising water content and the creep curve lagged behind the AE data. This paper provides guidelines for gas extraction, borehole maintenance and AE detection.
In view of the problem that it is difficult to measure the gas extraction of each coal seam alone by traditional device in the process of coal seam group gas combined extraction (CSGGCE), the calculation model of the mixing gas ratio is established and solved by using mathematical method, based on the definition of isotope value in gas isotope geochemistry and the ideal gas state equation. At the same time, the field test is selected in the Fengchun Coal Mine in Songzao Mining Area of China. The spatial distribution characteristics and differences of coal seam group are obtained, and the law of the CSGGCE mixing ratio is revealed with the variation of the extraction time, according to the discrete data of the separate and combined extraction gas components and isotope values. The results show that the coal seam gas has a thermal origin and a significant mantle-derived contribution. Compared with the traditional method, the CSGGCE mixing ratio obtained based on the isotope principle is dynamic. The mixing ratio is mainly controlled by the geochemical characteristics of the gas isotope in the initial stage of extraction and affected by gas deposit difference and physical space limitation as the extraction time increases, which is more in line with the actual situation of the CSGGCE in coal mines. KeywordsCoal seam group • Gas extraction • Component contents • Carbon isotope • Rare gas isotope Nomenclature K Relative abundance ratio, % x Relative abundance (mole fractions) n Amount of substance, mol p Pressure, Pa V Volume, m 3 T Thermodynamic temperature, K R Ideal gas constant w Component content, % a Proportion, Greek symbols δ Isotope value, Subscripts sta Standard sample tot Total substances mix m-Group mixed gas i i-Group gas * Jiaokun Wu
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