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Since most coalfields in China are commonly characterized by high gas content and low permeability, there is an urgent need to improve coal seam permeability and further enhance coal bed methane (CBM) extraction efficiency. As an emerging fracturing technology, the CO2 gas fracturing (CGF) technology has been widely used because of its advantages of low cost, environmental protection and high fragmentation efficiency. In order to improve the fracturing ability of CGF technique and optimize the release orifices of discharge head, computational fluid dynamics model was used in this paper to simulate the flow fields of dynamic pressure of gas jet released from the orifices with different structures and other geometrical parameters. The results show that the orifice structure has a great influence on the flow field of gas jet, but little influence on the magnitude of the dynamic pressure. Besides, the maximum dynamic pressure of gas jet linearly decreases with the increase in the number of release orifices. Based on a series of simulation results, the discharge head which has single group of orifices with structure c, diameter of 24 mm can be identified as the best choice for fieldwork. Then, two field experiments were conducted in Pingdingshan and Changping coal mines to evaluate the enhanced CBM extraction efficiency by CGF. The results indicate that the CGF can effectively create a large number of cracks in a large range around the fracturing borehole in the coal seam and further significantly improve the permeability. And the CBM extraction efficiency can be improved to a higher level from a lower level and maintained for a long time. Besides, the effective influence radii caused by CGF in Pingdingshan and Changping coal mines are 15.19 m and 12.5 m, respectively. Compared with other fracturing techniques, the CGF technique has a promising application prospect. [ABSTRACT FROM AUTHOR]

Abstract: Most of the coal seams currently mined in the Southwest part of China have high gas content, are soft and prone to outbursts, which significantly influences the rate of gate road development and mining safety. It is believed that an outburst is mainly due to the internal energy stored in the coal seam, so it is necessary to release the energy to prevent the occurrences of coal and gas outbursts, which in turn improves the rate of gate road development. This paper presented herein discusses the usage of the waterjet slotting technique to prevent the occurrence of outbursts during gate road development, on the basis of the laboratory tests, numerical modeling and field work. The numerical modeling indicated that the internal effective stress levels can be reduced up to 70% and 45% for the vertical and horizontal stresses, respectively, when waterjet slotting is applied, and the field monitoring results showed that the gas concentration released from the face of excavation can be increased up to five times compared with the traditional borehole drainage method. As a result, the values S mix and K 1, which are determined by drill cuttings to describe the possibility of outburst occurrences, have been kept below the critical limits 70% of the excavation time and the rate of gate road development increases from 30m to 70m per month. [Copyright &y& Elsevier]

Abstract: Traditional methane drainage methods, including conventional ventilation and boreholes drilled in the coal seam, are widely used in modern coal mining operation in China. However, it is often noted that the gas drainage presents low efficiency in most coal mines due to low permeability of the coal seam, which not only affects the safety, but also the productivity of the coal mine. This paper describes the development and application of the waterjet cutting system, which is used to create artificial fractures and increase the permeability of coal seam for the improvement of methane drainage in underground coal mines in China. According to underground monitoring, a significant improvement of methane drainage efficiency of up to 3 to 6 times over the traditional methods has been achieved. [Copyright &y& Elsevier]