Design and application of PDC bit for large diameter directional drilling of coal seam roof with double stage and double speed

The development of large diameter directional drilling technology has provided technical support for the extraction of gas in the upper corner of the goaf by "replacing roadway with borehole". At present, conventional large-diameter drilling mainly adopts the step-by-step hole-forming method, which solves the problem of insufficient power of large-diameter drilling equipment in underground coal mines. However, step-by-step reaming requires multiple times of lifting and lowering, long auxiliary time and intensive labor of the workers. In order to improve the drilling efficiency of large-diameter holes in the coal roof, a double stage drilling process with two speeds was developed. The process uses a screw motor and a drill to drive the first and second drill bits for reaming construction, which can greatly improve the efficiency of hole formation. For the implementation of such technology, a double stage and double speed drill bit was designed from the aspects of section shape, cutting tooth arrangement, waterway design and guide design by analyzing the characteristics of the drilling process and the lithology of the formation to be drilled. The drill bit adopts a ball head spiral guide, which makes it easier to drill along the existing drilling trajectory. The use of equal cutting teeth can improve the efficiency of rock breaking and reduce the uneven wear of the cutting teeth, thereby increasing the life of the bit and the efficiency of drilling. The designed drill bit was tested in the field. The results show that the double stage drill bit with two speeds has high rock breaking efficiency, uniform drill bit wear, and can drill along the pilot hole. It meets the requirements of the double stage and double speed process and greatly improves the efficiency of large-diameter drilling in the coal roof.