Influence of Coal Rock Hardness and Confining Pressure on load Fluctuation Characteristics of PDC bit during Gas Extraction Borehole in Soft Coal Seam

Load fluctuation characteristics have a great influence on drilling performance. so, study the influence of coal rock hardness and confining pressure on the load fluctuation characteristics of PDC bitbecome more and more important. Gas drainage drilling experiment table was built and Gas drainage drilling experiment was carried out under different coal rock hardness and confining pressure.The results indicate that drilling torque and feed resistance are decreased with the increase ofcoal rock hardness. Vibration amplitude of drilling torque and feed resistance increase with increase of coal rock hardness when the value of coal rock hardnessis larger than 1.3. Vibration amplitude of feed resistance is maximumwhen the value of coal rock hardness is 1.03 and Vibration amplitude of feed resistance is minimum when the value of coal rock hardness is 1.03. Drilling torque and feed resistance are increased with the increase of confining pressure. Vibration amplitude of feed resistance decreases from 3MPa to 7MPa then increases. Vibration amplitude of drilling torque and feed resistance are all minimum when the value of confining pressure is 7MPa. It provides a good basis for setting the appropriate feeding speed and rotation speed in actual work.