Experimental Study on the Mechanical Properties and Acoustic Emission Characteristics of Different Bedding High-Rank Coals.

Bedding has an important influence on the macroscopic and microscopic mechanical properties of coal, and the mechanical properties of coal and rock mass and acoustic emission characteristics are very important for rock burst monitoring and warning. In order to explore the influence of different beddings on the mechanical properties and acoustic emission characteristics of high-rank coal, using the RMT-150B electrohydraulic servo rock mechanics test system and the DS5 acoustic emission analyzer, the uniaxial compression and acoustic emission characteristics of high-rank coals with different beddings (parallel bedding 0°, oblique bedding 30, 45, 60°, and vertical bedding 90°) were investigated. The results show that the uniaxial compressive strength and deformation modulus of vertical stratified coal samples are the largest, which are 28.924 MPa and 2.95 GPa, while the average values of uniaxial compressive strength and deformation modulus of oblique stratified coal samples are the smallest, which are 10.91 MPa and 1.776 GPa, respectively. With the increase of the bedding angle, the uniaxial compressive strength of high-rank coal decreases first and then increases. The stress-strain process of coal varies greatly with different high stratification grades (parallel bedding 0°, oblique bedding 30°, 45°, 60° and vertical bedding 90°). The loading times of parallel, oblique, and vertical beddings are 700, 450, 370, 550, and 600 s, and the acoustic emission mutation point values are 495, 449, 350, 300, and 410 s. The mutation point value can be used as precursor information to judge the failure of high-rank coal in different beddings. Research results to find the high-rank coal destruction instability prediction method and the index provide a basis; further damage to the acoustic emission testing high order coal provides reference significance; and the use of acoustic emission in the monitoring and early warning of percussive ground pressure, the bedding surface of coal, and the actual stress on site should be considered.

Experimental Study on the Variation Mechanism of Permeability and Seepage Characteristics of High-Rank Coal with Different Bedding.

In order to understand the variation mechanism of permeability and seepage characteristics of high-rank coal with different bedding, we prepared cylindrical raw coal samples according to the bedding angles of 0, 30, 45, 60, and 90° and conducted permeability tests under two stress paths (stress path 1, unloading confining pressure under constant axial pressure; stress path 2, simultaneous loading axial pressure and unloading confining pressure). The results show that the relationship between the permeability and effective stress of high-rank coal with different bedding in the two stress paths conforms to an exponential function, and the permeability increases gradually with an increase in differential stress. Under the two stress paths, the initial permeability of different bedding under the loading axial pressure and confining pressure shows a pattern of a maximum for parallel bedding coal samples, followed by oblique bedding coal samples, and a minimum for vertical bedding coal samples. Under path 1, the increase in the permeability of the oblique bedding is 21.4 times that of the vertical bedding and 14.94 times that of the parallel bedding, and under path 2, the increase in the permeability of the oblique bedding is 26.45 times that of the vertical bedding and 142.11 times that of the parallel bedding; the coal samples of the oblique bedding suffer the greatest damage. The increase in the permeability of parallel bedding coal samples, oblique bedding coal samples, and vertical bedding coal samples under path 2 is 1.47 times, 13.96 times, and 11.3 times the increase in the permeability of the corresponding coal samples under path 1, respectively, and the damage produced by coal samples under path 2 is greater than that under path 1.

Low Nuclear Magnetic Resonance Experimental Study on Gas Adsorption of High-Rank Coals with Different Beddings.

In order to deeply study the influence of the coal bedding structure on coal gas adsorption, low nuclear magnetic resonance (LNMR) and a confining pressure loading system were used to carry out the LNMR experiment of gas adsorption of high-rank coals with different beddings under different confining pressures. The results showed that the amount of gas adsorption of high-rank coals with different beddings increases with time and decreases with the increase of confining pressure. In the process from low confining pressure to high confining pressure, the coal sample with oblique bedding (bedding angles 30°, 45°, and 60°) has the largest average increment of gas adsorption, followed by the coal sample with vertical bedding (bedding angle 90°), and the coal sample with parallel bedding has the smallest increment of gas adsorption (bedding angle 0°). The linear function relation between the different-bedding high-rank coal gas adsorption state and the confining pressure is y = a - bx. The relation between the free peak area and the confining pressure conforms to the exponential function y = a + bexp(cx). Different-bedding high-rank coal adsorption peaks and the peak area decrease with the increase of confining pressure, and the free peak continues to move to the left; that is, the large pores gradually shrink. With the increase of angle and bedding, the area of the adsorption peak increases first and then decreases, presenting an "inverted V" shape on the whole. The area of the free peak decreases first and then increases, presenting a "V" shape on the whole.

Anisotropic Flow-Solid Coupling Model for Gas Extraction from Cis-Layer Boreholes and Its Application.

To investigate the effect of anisotropy of coal body on the gas extraction effect of cis-borehole, the anisotropy permeability model of coal based on structural anisotropy ratio and flow-solid coupling model were established at a working face of Zhongmacun mine Henan Province, China, as the research object, and COMSOL numerical simulation software was used. The results show that considering coal anisotropy, the gas pressure decreases more faster than that without coal anisotropy, and the farther away from the borehole, the smaller the difference between them. The extraction time was a logarithmic function of the effective extraction radius, the negative extraction pressure was an exponential function of the effective extraction radius, and the borehole diameter satisfies a power function relationship with the effective extraction radius. The variation of gas pressure with extraction time in different stratigraphic directions was analyzed, and gas pressure decreases faster in parallel stratigraphic directions and slower in vertical stratigraphic directions. Considering the complexity and safety of gas extraction at the working face, a 30% redundancy factor is added to determine the maximum magnitude and range of gas pressure drop when the spacing of cascade drill holes in a working face of Zhongmacun mine Henan Province, China, is 6 m, which can avoid the superposition of "blank zone" and ineffective extraction.