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The development of pore structure in coal reservoirs determines the adsorption performance of coal gas. The pore structure of three coal samples with different degrees of metamorphism, namely long-flame coal, coking coal and anthracite, was tested by low-temperature liquid nitrogen adsorption experiments. The pore structure was quantitatively characterized based on fractal theory, and the effect of the pore structure on the methane adsorption properties of coals with different degrees of metamorphism was analyzed in depth in combination with the methane isothermal adsorption experiments. The results showed that the degree of metamorphism and the pore fractal dimension D1 showed a “bathtub” variation, and a linear negative correlation with D2; while the specific surface area and pore volume of the coal sample were positively correlated with the adsorption constant a. The larger specific surface area and pore volume, the stronger the adsorption capacity of coal. With the increase of pore fractal dimension D1, the adsorption constant a presents an approximate linear growth trend. It can be seen that the less smooth the pore structure of coal, the larger the specific surface area will be, so that the ultimate methane adsorption capacity of coal will also increase.

In order to study the influence mechanism of effective stress on permeability, the coal permeability test experiment under simultaneous loading of axial and confining pressure was carried out using the self-developed coal-rock gas adsorption-desorption-seepage platform. The mean effective stress parameter is introduced to characterize the loaded state of the coal body. At the same time, in order to eliminate the influence of the individual differences of the specimens, the effective stress sensitivity coefficient is defined, and the control mechanism and variation law of the effective stress on the permeability of the coal body are discussed in depth. The results show that when the gas pressure remains constant, the coal permeability decreases gradually with the increase of the effective stress, and its variation law conforms to the negative exponential relationship. With the increase of effective stress, the sensitivity coefficient gradually decreases, and it conforms to the power function relationship. At the same time, a mathematical model of coal sample permeability and effective stress is constructed based on the sensitivity coefficient.

The pore distribution characteristics of coal not only directly affect the gas storage and migration ability in coal seam, but also directly affect the development process of physical adsorption and chemical reaction between coal and oxygen. Therefore, in-depth study of the pore distribution characteristics of coal is conducive to the reasonable classification and evaluation of coal reservoirs, and then study and analyze the laws of gas storage and migration in coal seams and the low-temperature oxidation reaction mechanism of coal, improve the accuracy of gas content prediction and gas extraction efficiency, and describe the process of coal spontaneous combustion in detail. We have systematically summarized the current research theories and methods of pore distribution characteristics of coal through a large number of relevant literature investigations, and compared and analyzed various research theories and methods, pointed out their respective advantages and disadvantages. Finally, it is proposed that the next step can be carried out from three directions: accurate measurement of micro-pores and reasonable characterization at full scale, research on microscopic effects of coal pores, and establishment of accurate coal medium models.

Based on the systematic investigation, this paper summarizes the four important development stages of the prediction technology of mine gas emission in China from 1950s to now. It focuses on the basic principles, application scope, advantages and disadvantages of the source based prediction method, mine statistical method, gas geological mathematical model method, grey prediction method, neural network prediction method and combination prediction method. The main problems of each prediction method are analyzed in detail with the field application status. Finally, centering on the development goals of mine mechanization and intelligentization, three technological breakthroughs are proposed: improving the scientific selection of predictive parameters of source separation, promoting the modularization and integration of gas geological model prediction technology, and establishing information-based high-end fusion model, which points out the direction for future research on gas emission prediction technology.

Coal mine safety standard is the basis of ensuring coal mine safety production, mine ecological environment safety and employees’ safety and health, preventing coal mine accidents and occupational hazards, and is also an important support for the implementation of coal mine safety production supervision and the establishment of safety development concept. In order to deeply understand the current situation of China’s coal mine safety standards, 853 current effective coal mine safety standards were collected and analyzed from the perspectives of standard level and nature, standard release year, standard for risk category, etc. The problems such as weak binding force of standards in current standard work were pointed out, and suggestions such as paying attention to overall planning and top-level design were proposed for reference to improve the coal mine safety standard work in China.

To prevent and control the coal seam gas disaster affected by the reverse fault, we performed gas seepage tests, which consider stress-loading and unloading schemes, to investigate the stress change and coal permeability of the mining coal with reverse fault. The experimental results show that the mechanical behavior and permeability change of the mining coal are related to the distance between the coal and the reverse fault. The stress concentration coefficient of the coal body gradually increases. The closer is the distance between the coal and the reverse fault, the larger are the deviatoric stress peak and strain. In comparison with the coal sample M1 that is 5 m away from the reverse fault, the deviatoric stress peak and axial strain of the coal sample M3, 35 m away from the reverse fault, increase by 40.74% and 26.73%, respectively. In this stage, the permeability of M1, M2, and M3 coal samples increases by 22.1%, 28.0%, and 36.7%, respectively. In another stage, the stress concentration coefficient of coal increases to the peak and then decreases, causing the deviatoric stress peak and strain of coal to rise first and then fall. In comparison with the coal sample M4 that is 65 m away from the reverse fault, the deviatoric stress peak and axial strain of coal sample M6, 5 m away from the reverse fault, decrease by 29.48% and 5.55%, respectively. The permeability of coal samples M4, M5, and M6 increases by 23.6%, 37.2%, and 20.8%, respectively. Based on the gas seepage test results, we established the permeability model of mining-induced coal under the influence of a reverse fault, with consideration of the volume changes of coal fractures induced by adsorption and desorption. In the model, the variations of permeability in both stages of the prepeak and postpeak were deduced, which was verified with the experimental data. The verification results demonstrate that the proposed model has the capacity to predict the permeability evolution of mining coal under the influence of a reverse fault. [ABSTRACT FROM AUTHOR]

This study aimed to explore the stress distribution and variation of reverse fault-affected mined coal body. A mechanical analysis model of the coal body in the reverse fault area was first established, then the coal body stress characterization equation was derived, and the stress distribution pattern on the coal body was calculated. Subsequently, applying the Mohr–Coulomb strength criterion revealed the following relationship: the closer is the distance to the reverse fault, the worse is the stability of the coal body, and that the coal body strength influences the stress concentration of the coal body in front of the working face. Moreover, simulation with FLAC3D was carried out to verify the coal body stress calculated by the mechanical model as well as the fluctuation of the coal body stress concentration. It could be concluded that while mining the hanging wall of the reverse fault, the stress concentration of mined coal body decreases with the increase of reverse fault dip angle, but increases with the increase of reverse fault throw; the stress concentration magnitude generated during footwall mining is lesser than that during hanging-wall mining. In other words, the magnitude of coal body stress concentration can be affected by the hanging wall and footwall mining, as well as parameters of the reverse fault. Finally, intrinsically safe GZY25 borehole stress sensors were used to monitor the coal body stresses in the reverse fault area under the influence of mining in Xinchun Coal Mine and ZuoQiuka Coal Mine. It was found that the coal body stress concentration in front of the working face either increased gradually or increased first before decreasing. It can be concluded that with the decrease of the distance between the working face and reverse fault, the vertical stress of the coal body increases, and the vertical stress of the coal body begins to increase obviously at a certain position. At this point, the vertical stress of the coal body can be generalized to 1.02–1.39 times of the initial vertical stress. Furthermore, the stress concentration coefficient of coal body is related to the distance from the reverse fault, and two changes occur: ① if the coal-bearing capacity does not exceed its strength, the coal stress in front of the working face increases gradually, and the stress concentration factor increases gradually; ② the stress concentration coefficient of mining coal body increases first, such that when the coal body bearing capacity exceeds its strength, the coal body fails and loses all its effective bearing capacity, followed by the decrease in coal body stress concentration coefficient. [ABSTRACT FROM AUTHOR]