A new method of drilling long boreholes in low permeability coal by improving its permeability

When drilling holes for gas pre-extraction from soft coal seams having low permeability, phenomena such as hole collapse and drill pipe sticking can occur. These problems are due to the coal's low intensity and high gas pressure, both of which severely limit the efficiency of gas extraction. Based on an analysis of borehole collapse in soft seams, and on a study of the pore-formation mechanism in stiff layers, this paper proposes a new drilling technique that uses a high-pressure pulsed water jet. This technique can be used to drill holes in stiff layers of the roof/floor until the soft layers of the coal seam are reached. This method can improve the gas extraction rate. According to damage models for rock, a theoretical analysis and numerical simulation was performed for a high-pressure pulsed water jet. This analysis examined the instantaneous dynamic loads, the damage features when using a flexible impact regime, the changes in fracture field patterns, and the high-pressure pulsed water jet's oscillatory effect upon the gas flow of coal-mass. The impact and vibration effects of a high-pressure pulsed water jet could effectively contribute to crushing of the coal, to improvement of the coal's cracking and connectivity rate and to improvement in the permeability of the coal. Additionally, the high-pressure pulsed water jet's dynamic pressure and flow rate must be calculated for breaking rock during drilling and slotting. The developed system was applied in the Sanhui mine of Sichuan Province, China. The hole drilled by the new method is 2.7 times deeper than that of the hole drilled in the coal seam. In the Yutianbao mine, the average length was 148 m when drilling between the roof and the seam, and this value was more than 2.4 times deeper than that previously drilled in the coal seam. The gas flow was about 1.06 m3/min per meter, which is more than 5 times that of a traditional hole.

Research highlights
► Based on the theoretical analysis of slotting pressure and flow of high-pressure pulse water jet when slotting coal seam, a novel technology used to drill long borehole and improve coal seam permeability in soft and low permeability seam was developed. Besides, the system devices which were applicable to the novel technology were developed. The application results showed that the system could guarantee the length of the borehole and improve the permeability of the coal seam. And the most important was that the devices were maneuverable.
► Through numerical simulation, the mechanism of high-pressure pulse water jet improving coal seam permeability was revealed. That was, the pulse of the water jet could prompt the generation and connection of the cracks, and then, the channels of the gas extraction were improved.