ABSTRACT
The rapid development of the economy leads to the high demand for deep coal resources, which further poses the potential problem of deep gas (or methane) emissions. The clarification of deep gas occurrence law for coal mines provides theoretical and data support for methane emission predictions, and assists industrial and mining enterprises in planning targeted emission reduction measures. This study defined and verified the existence of a critical depth for the deep gas occurrence in coal mines based on a multiple-scale case study of how the gas occurrence is associated with depth and stress status changes in the Pingdingshan No.8 Coal Mine. In addition, 882 sets of gas content data from 7 major mining areas in China were collected and their gas content distributions among various depths were statistically analyzed to prove the universal existence of critical depth. The results show that the critical depth of Pingdingshan No.8 Coal Mine is 509 m, and the critical depth of other Chinese areas is about 400 to 1000 m. Significant differences were observed in the pore space, surface, and gas desorption characteristics for coal samples with different depths and stress states. The pore structure in the critical depth area is relatively developed, and gas is easily accumulated. The gas occurrence of both normal and abnormal gas gradually increases with the depth's increase in areas above the critical depth, whereas the gas occurrence gradually decreases for areas below the critical depth, showing that the existence of critical depth lead to significant deviations in gas emission predictions. The results provide a fundamental reference for gas emission prediction, greenhouse effect assessment, and carbon emission factor calculation and indicate that using the traditional linear method may be misleading for evaluating deep gas occurrence and emission.
ABSTRACT
Coal structure is one of the key geological factors that affects the effect of coal reservoir stimulation. Based on the geological spatial combination characteristics, thickness, and proportion of different coal structures, the coal reservoir is divided into different coal structure combination types. The hydraulic fracturing device is used to carry out indoor fracturing experiments and dissect the crack initiation and expansion characteristics with different coal structure combinations. The results show that the coal structure combination is of the binary type (undeformed coal + granulated coal or cataclastic coal + granulated coal), and the undeformed coal (cataclastic coal) can overcome the tensile strength and minimum principal stress when it is driven by the high-pressure fluid. The undeformed coal (cataclastic coal) ruptures and forms longitudinal cracks. The increasing proportion of granulated coal inhibits crack expansion and promotes the transverse deformation of coal. The interface contact point between the undeformed coal (cataclastic coal) and granulated coal easily fractures along the cross section of the specimen. When the coal structure combination is the triplex type (undeformed coal + granulated coal + cataclastic coal or cataclastic coal + granulated coal + cataclastic coal), the undeformed coal or cataclastic coal is transformed. The forming fractures in the undeformed coal (cataclastic coal) can cut through the soft coal when the thickness of the undeformed coal (cataclastic coal) is large and the thickness of granulated coal is thin. On the contrary, it is not easy to cut through. When the coal structure combination is granulated coal + cataclastic coal + granulated coal, the cataclastic coal fails under shear stress and forms the crack along the cross section of the coal sample. The granulated coal inhibits the crack expansion at both ends. The research results have an important indicative significance for further understanding the fracture initiation and propagation mechanism of hydraulic fracturing with complex coal structures in coal reservoirs.
ABSTRACT
The research on the time-frequency characteristics and evolution law of acoustic emission (AE) signals during deformed coal failure is more conducive to understand the damage mechanism of coal. In this study, the experiments of AE monitoring during the intact and deformed coal failure were first conducted under loading axial stress and unloading confining stress conditions. Based on the evolution characteristics of volume strain and AE event rate, the damage process of coal was divided into three stages: nonfracture development stage, stable development stage of fracture, and unstable development stage of fracture. The distribution and evolution of AE waveform time-frequency properties under different damage processes were then analyzed and discussed. Besides, the evolution of the average value of different time-frequency parameters per 200 s for the intact coal and per 25 s for the deformed coal was discussed. The results show that the amplitude of most AE events stabilizes in 40-50 dB during the intact and deformed coal failure. The average amplitude of the deformed coal has an approximate positive correlation with the loading stress. The percentage of AE events with longer duration and rise time increases suddenly before the peak stress for the intact coal and after the peak stress for the deformed coal, which corresponds to the abrupt increase property of the average duration and rise time. For the frequency properties, the peak frequency and frequency centroid of the intact coal are distributed within 50-125 and 75-150 kHz, with those of the deformed coal located within 20-120 and 80-130 kHz, respectively. The average peak frequency and frequency centroid of the intact coal show an upward trend except for the initial fracture closure stage, while the average peak frequency and average frequency centroid of the deformed coal present a downward trend before the peak stress and have a smaller growth after the peak stress. According to the above-mentioned analysis, the sudden increase of the average duration and rise time, the lower average peak frequency, and the lower frequency centroid can be regarded as the precursor for the instability and failure of deformed coal. This research can provide a new idea and theoretical guidance for the early warning of outbursts.
ABSTRACT
Mine gas disasters are a major safety concern in underground coal mining. Protective layer mining is widely used in gas disaster control, but there are limited theoretical and experimental results that can provide guidance for site-specific mining circumstances. Taking the Xinji No. 1 mine as an example, gas disaster treatments were conducted in a new panel with overlying goaf located 85 m above the coal mine and adjacent goaf located at 30 m intervals. This study involved a comprehensive investigation, which included four steps: the selection of the first mining face, gas control and prevention, tracking and investigation, and effect analysis and assessment. The safety strategy focused on gas control planning in new mining areas or panels. The distribution and evolution characteristics of the stress, the gas permeability coefficient and the deformation volume within the protected layer were determined by numerical simulation. The coal deformation, gas emission and extraction effect were analyzed by field observation. The deformation and gas permeability of the coal seam were consistent with the stress evolution, for which the maximum compressional and expansional deformation of 6-1 coal were 18‱ and 28‱, respectively. Gas disaster control and prevention treatment of the mining face produced a significant protective effect on the underlying No. 6-1 coal seam. This work is beneficial for the planning of gas control in successive panels.
