Research of weight on bit transmission law of ultra-long directional borehole along coal seam

In order to further increase directional borehole depth along coal seam and improve drilling efficiency of ultra-long directional borehole, influencing factors of weight on bit(WOB) transmission of ultra-long directional borehole along coal seam were analyzed, including borehole trajectory, borehole depth, sediment in borehole and drilling strata. Based on engineering background of 2 570 m ultra-long directional borehole along coal seam in Baode Coal Mine, law of WOB transmission under compound directional drilling was researched. The research results show that with the increase of borehole depth, feed pressure of sliding directional drilling increases exponentially, and finally reaches rated feed pressure of drilling rig, which leads to decrease of transmission efficiency of WOB. Under condition of compound drilling, feed pressure of compound drilling keeps at a low level with the increase of borehole depth, which is far lower than rated feed pressure of drilling rig. Rotary pressure of drilling rig almost increases linearly with the increase of borehole depth, but it is lower than rated rotary pressure of drilling rig, which leads to high transmission efficiency of WOB. The higher the pump pressure and pump volume of mud pump are, the higher the slag discharge efficiency is, the lower the friction resistance of drilling tool is, and the higher the transmission efficiency of WOB is. Conversely, the lower the transmission efficiency of WOB is.