A novel technology for high-efficiency borehole-enlarging to enhance gas drainage in coal seam by mechanical cutting assisted by waterjet

Abstract

ABSTRACT Although the effect of pressure relief and permeability increasing achieved by the technology of traditional hydraulic flushing (enlarging hole by high-pressure water jet) is great significance, it has some shortcomings, such as low efficiency, large consumption of water resources, and being difficult to guarantee the sizes of holes. The limitations of water jet are known as the main factors that would restrict the effective pressure relief in soft and low-permeability coal seam. For the purpose of solving the problems existing in the traditional technology, a new technology of mechanical cutting assisted by waterjet has been developed. Then, a numerical simulation method was employed to illustrate the flow field and the force characteristics of equipment equipped with new technology. Finally, the new technology and its supporting system were tested in the Xinjing Coal Mine, Shanxi Province, China. The test results obtained are as follows: the efficiency of mechanical cutting is much higher than that of hydraulic flushing; the maximum equivalent to enlarge hole radius is increased by 83.3% (t = 30 min), and the efficiency of the new technology is 3 times more than that of the traditional hydraulic flushing; the time used for gas drainage is increased by 19.3%, and the drainage efficiency is increased by 28%; the cost spent on gas control in one working face decreases by more than 14.7%. It is concluded that the new technology can efficiently enlarge borehole so as to enhance gas drainage in coal seam and decrease the cost of coal mining.