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Abstract Almost all the coal seams in China are characterized by high gas content, extremely low permeability, complicated geological structure, thus, natural disasters such as gas explosive and outburst easily occur. At present, gas extraction is an effective approach for preventing such disasters. In this paper, in order to improve gas extraction efficiency, the Multi-discharge CO 2 fracturing system (Multi CO 2 Frac) was proposed and tested in Changping coal mine. Through analysis of the waveforms from the explosions of 0.5 and 1 kg TNT dynamites and 1 kg liquid CO 2 , it can be obtained that the TNT equivalence of 1 kg CO 2 is about 400–430 g, that is, the explosion of 1 kg liquid CO 2 has same fracturing ability as 400–430 g TNT dynamites. Besides, field test shows that the effective drainage radius caused by Multi-CO2-Frac technique is about 12.5 m, regardless of the number CO 2 discharging sets. However, the gas extraction concentration increases with the number of CO 2 discharges. This phenomenon indicates that more cracks will be created along the axial direction in coal seam drilling boreholes by Multi CO 2 Frac techniques. By comparing gas concentration before and after fracturing, it can be concluded that the Multi CO 2 Frac techniques can change and maintain the uptrend of gas concentration at a higher level in a long period of time. Therefore, in order to extend the duration of high efficiency gas extraction, Multi CO 2 Frac techniques can be used periodically at different position of coal seam. Meanwhile, after gas extraction, the maximum gas emission of coal seam was reduced from 5.5 m3/min to 3.48 m3/min and the driving footage was enhanced from 2.4 m/d to 5.7 m/d. In summary, Multi CO 2 Frac techniques can effectively improve the coal seam permeability and further enhance the gas drainage efficiency, which can meet the requirement of gas extraction in high gas content thick and soft coal seam. Therefore, this technique has a promising application prospect because of its advantages of safety, environmental protection and economy. Highlights • Multi-discharge CO 2 Fracturing System was proposed to improve coal seam permeability and enhance gas extraction efficiency in high gas content thick coal seam. • The explosion 1 Kg liquid CO 2 has same fracturing ability as 400-430 g TNT dynamites. • The effective influence radius of gas drainage by Multi-CO 2 -Frac technique is about 12.5 m. [ABSTRACT FROM AUTHOR]

Directed at creating fractures and extracting methane in low permeability, high gas-outburst coal seams, this paper presents the results of a comprehensive integration trial of under panel cross strata drilling with a new coal seam stimulation technique: the Cardox system in a colliery located in a major high gas content and outburst prone coal field in the central part of China. On the basis of the coal seam parametric measurements, the precise designation of under panel cross strata borehole patterns and the arrangement of the fracturing and monitoring procedures, the advantages of the Cardox system in creating fractures and improving gas drainage efficiency of low permeability coal seams are: (1) Changing the trend of gas concentration and/or pure gas quantity over time; (2) Maintaining the uptrend of gas concentration and/or pure gas quantity in a relatively long time period, which, in the current case, is ninety days and counting; and (3) Elevating gas drainage to a higher level. Stimulation mechanisms of the highly pressurized gas system have been proposed, and based on these mechanisms, the crushing and fracturing radii created by the highly pressurized gas have been calculated and corroborated well with the field monitoring data. [ABSTRACT FROM AUTHOR]

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]