Abstract
ABSTRACT Low gas permeability of coal seams is a key factor restricting effective prevention and control of gas disasters and high-efficient exploitation of gas resources. Hydraulic slotting technology can enhance the permeability of a low-permeability coal seam with high gas content and significantly improve the efficiency of gas drainage. A gas–solid coupling model of gas flow in a coal seam was established based on changes of permeability caused by variation of gas pressures, and gas flow laws in coal subjected to hydraulic slotting under various working conditions were quantitatively elaborated. The research results demonstrated that slots increased the exposed area of a slotted borehole and allowed more gas to flow to the borehole per unit of time, so that the efficiency of gas drainage from the slotted borehole significantly improved. With the increase of slot spacing and width, enhancement index of gas drainage and cumulative gas drainage volume rose. After 90 d of gas drainage, cumulative gas drainage volume ratio after hydraulic slotting was 1.75 times that before hydraulic slotting on average. The enhancement index of gas drainage decreased with the enlargement of the permeability. When the permeability increases from 0.001 to 1, cumulative gas drainage volume ratio after hydraulic slotting drops from 3.0 to 1.1. The increase of gas pressure in the coal seam reduced the enhancement index of gas drainage. In addition, a prediction model for the ratio of cumulative gas drainage volume after slotting to that before slotting based on back-propagation neural network was constructed. The model realized the continuous and accurate prediction of enhancement effects of slotting on gas drainage under different working conditions. The research results could provide theoretical supports for revealing the permeability-enhancing mechanism of hydraulic slotting and properly determining slotting parameters in the field.
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