ABSTRACT
The complexity of the evolution of the permeability of coal is determined by the reservoir structure. Further, there exists an interaction between the fracture matrix, which further complicates changes in permeability. When the actual mining conditions of a coal mine are considered, a permeability model that considered the combined effects of stress, gas adsorption, and temperature was proposed. Subsequently, the model is verified by published test data. Based on the analysis of permeability, a calculation model of the slip coefficient that considered the combined effects of stress, gas adsorption, and temperature is proposed. With respect to this, any change in the slippage coefficient is only determined by the width of the fracture channel, which affected the flow of coal gas. In the process of a temperature increase, the slip coefficient tends to increase and the larger effective stress corresponds to a larger slip coefficient. In addition, under constant-temperature conditions, we also discuss the evolution of coal permeability and the variation of the coal gas slippage factor under different boundary conditions through the proposed model. This study aims to further the understanding of the seepage characteristics and slippage effects of coalbed methane, which would have a positive impact on the mining of coal.
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.
