Design and application of pump-free double channel mechanical hole enlarging bit for gas penetration in middle hard coal seam

In borehole enlarge drilling, it is necessary to adjust the flow of mud pump to switch drilling and borehole enlarging state, and the control effect is not ideal. To improve the precision of the borehole enlarging technology while drilling machinery control, a pump-free double channel mechanical borehole enlarging drill bit was developed, which equiped with dual channel to form a complete set of double channel drill pipe and double channel water device. Two sets of individual waterways were designed, annulus water way for cooling bit and carry out rock powder, and center water way is used to control the opening and closing of blade, so as to improve the precision of the opening and closing. At the same time, there is no need for matching mud pump, which improves the application range of borehole enlarging technology while drilling. Through the bit structure design, rack and pinion calculation and waterway structure optimization, the reliability of the bit was increased. The field test was carried out with the designed drill bit. A total of 101 boreholes with diameter of 120 mm were completed. The total construction footage was 2 861.5 m; the average drilling depth was 28.3 m; the coal output was 0.20-0.69 t/m; and the average coal output per meter was 0.22 t. The double channel cavitation technology can enlarge the diameter of borehole from ø 120 mm to 4.1 times. The field test proved that the design of double channel mechanical borehole enlarging bit, can meet the needs of non-pump mechanical borehole enlarging, greatly improving the construction efficiency. This study provides drilling tool support for anti-reflection improvement of gas drainage in low permeability coal seam and provides technical scheme for similar hole cutting drilling construction in this industry.