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Liquid nitrogen (LN2) is an important means to improve the permeability of coal reservoirs. The LN2 soaking method has a wide range of application prospects. However, not enough studies exist to highlight the effect of increasing the permeability of different coal samples using LN2. Hence, this investigation was carried out to examine the effect of cold soaking in LN2 using different coal ranks such as fat coal, lean coal and anthracite coal, on their pore structures and fractal characteristics. Frenkel-Halsey-Hill (FHH) fractal theory was used to understand the fractal dimension characteristics of pores in these different coals before and after cold soaking in LN2. Furthermore, the relationship between the fractal dimensions and pore parameters was also studied. The results show that low-temperature nitrogen adsorption curves of fat coal, lean coal and anthracite before and after cold soaking in LN2 are “S” type. An increase in the total pore volume and pore surface area of coal after LN2 cold soaking is noted. LN2 cold soaking is found to transform the internal pore structure of different coals effectively. Moreover, the heterogeneity of the porous structure of coal is enhanced after LN2 cold soaking along with an increase in the fractal dimension. The growth also decreases with an increase in the metamorphism. The uneven shrinkage of coal occurred during LN2 cold soaking, and the thermal stress generated is greater than the tensile strength of coal, which promotes the development of the porous structure of coal. These results are beneficial in revealing the macroscopic and microscopic pore fissure space expansion and connectivity law of coal seam during the process of LN2 cracking. [ABSTRACT FROM AUTHOR]

In order to study the gas occurrence regularity and the main controlling factors of Panxi Mine, gas parameters of the mine were determined through the combination of field survey and experiment, and the distribution regularity of gas content in coal seam strike and dip was analyzed. Besides, the fault structure was quantitatively researched based on the fractal theory. The experimental results show that the fractal dimension of the mine mostly ranges from 0.7 to 1.6. The fault structure becomes more complex with the increase of fractal dimension, and the gas content in the region with a larger fractal dimension is higher. The fractal dimension of fault can reflect complexity of geological structure of the mine. The research is targeted to the prevention and control of gas accidents in Panxi Mine, and it has important theoretical and practical significance for promoting safety production and gas development and utilization in coal mines. [ABSTRACT FROM AUTHOR]

The rapid construction of a shaft is very important for shortening the shaft construction period and improving the mine economic benefits. In addition to the effects of hydrogeological conditions, the shaft construction speed also affects the selection of operation mode and the rationality of mechanical equipment. Considering the specific geological conditions of thick alluvium and large aquifers on the surface of auxiliary shaft in Fucheng mine, the main factors which affect the construction speed of the large-section vertical shaft in the thick alluvium are analyzed in this study. Based on the field investigations and hydrogeological conditions of the auxiliary shaft, the freezing method is determined as a rapid construction scheme for the auxiliary shaft as well as designing the arrangement of freezing holes and optimization of freezing parameters. The dynamic adjustment of freezing parameters is identified by studying the information provided by freezing construction technology. After shaft excavation, the deformation of the shaft lining in the alluvium section of topsoil is analyzed. It is concluded that the deformation of the shaft lining increases with the depth of the topsoil layer, however, the maximum deformation is within the allowable range of design. The short section excavation and masonry mixed operation mode is selected, and “five major and one deep” rapid mechanized operation lines are matched. The scientific operation cycle chart is compiled. And operation results fulfilled the actual needs. These results will provide guidance and reference significance for mine construction under similar geological conditions. [ABSTRACT FROM AUTHOR]