Key technology of drilling in-seam directional borehole of 3 000 m in underground coal mine

The pre-mining gas governance mode in Baode coal mine requires that long directional borehole of more than 3 000 m can be drilled along coal seam. In view of the existing technology and equipment, there are technical problems in the construction of ultra-long directional borehole, such as the difficulty of sliding drilling, the large hydraulic pressure consumption in inlet waterway, the limited transmission distance of wireline MWD signal, and incapability of flushing liquid recycle. The technology of friction reduction in sliding drilling based on the hydraulic thrust of DHM and the torsion penetration of the ultra-long drilling tool, the technology of trajectory control with compound drilling based on angle control with rotary drilling and branch drilling are developed. The system of low-pressure water consumption, mud pulse wireless MWD and purification circulatory for flushing fluid are designed. Combined with the production needs of Baode coal mine, an ultra-long in-seam directional borehole through mining face with the depth of 3 353 m and the borehole diameter of 120 mm was completed. The drilling results show that the friction reduction technology of sliding drilling effectively reduces the sliding penetration pressure and improves the ability of deep borehole with sliding drilling markedly; the technology of trajectory control based on compound drilling can control the borehole trajectory effectively, which ensures the borehole along the coal seam, and improves the drilling capacity and efficiency; the signal transmission of mud pulse wireless MWD system is stable and reliable, which overcomes the limitation of signal long-distance transmission by wireline MWD system; the purification circulatory system for flushing fluid has good cleaning effect, which realizes the circulation of flushing fluid in directional drilling. The research results are of great significance to support the advance control of gas in large area of coal mine, the engineering of replacing tunnel with borehole, the prevention and control of water disaster and the geological exploration.