Research on pipe-following hole protection drilling technology in broken soft coal seam of the isolated island working face

It is easy to get stuck and collapse in the borehole of high stress and broken soft coal seam in the isolated island working face. This leads to great difficulty in borehole formation and poor gas extraction effect. In order to solve this problem, this paper puts forward a pipe-following borehole protection drilling technology. The 3206 isolated island working face of Wangpo Coal Mine is selected as the test site. It is analyzed that the working face needs to use high-torque and high-speed drilling rig to enhance the slag removal effect of the drilling tool and the capability to deal with accidents in the borehole. At the same time, it is necessary to consider the process of drilling wall protection in the high-stress section and the efficient slag removal process in the borehole. It is proposed to adopt the pipe-following borehole protection drilling technology in the high stress zone to achieve the effect of wall protection. The spiral drilling nitrogen-assisted slag removal process is adopted to enhance slag removal capacity and reduce the risk of coal spontaneous combustion during drilling. After the borehole passes through the high-stress area, the drilling depth of the borehole in the broken soft coal seam is further improved by optimizing the drilling tool assembly. The field test results show that the average hole depth is increased by 149% when using the second-stage hole protection drilling than when using rotary drilling directly. The average hole depth is increased by 114% when using the third-stage hole protection drilling. It shows that the pipe-following hole protection drilling is more suitable for the drilling construction of broken soft coal seam in 3206 island working face than the rotary drilling construction technology. The hole-forming rate of the plug-type screw drill pipe is higher than that of the screw thread-type screw drill pipe. The hole-forming depth of nitrogen assisted slag removal process for screw drilling is significantly greater than that of the dry screw slag removal process. ${\text{ϕ}} $100/63.5-28 mm plug-in sealed spiral drill pipe and nitrogen assisted slag removal process are most suitable for gas pre-extraction drilling construction in 3206 isolated island working face. The average hole depth is 100.6 m, and the hole formation rate is 80%. The gas extraction effect is better than other drilling tools and drilling slag removal technology.