Mechanism of “hole-protecting and pressure relief” drilling in soft coal seam

There is high drilling drag and low drilling efficiency for the drilling using casing-pipe drilling tool in soft coal seam. A new drilling technology with dual functions of hole-protecting and pressure relief was proposed in this study. The mechanical models of rotation drag torque and axial drag were established con-sidering confining pressure pi surrounding borehole. In addition, the hole-protecting numerical model was established. The defects of casing-pipe drilling technology were analyzed,and the principles of pressure relief and drag reduction were studied on new drilling technology. Result shows that when the casing-pipe drill pipe is applied to the drilling operation in soft coal seam,the rotation drag torque and axial drag are more than the drill rig rating for the range of the shallower drilling depth,then the drill pipe can’t rotate and retreat,and the drilling will be terminated. By designing hole or space structure on the casing-pipe drill pipe surface to form a new hole-protecting drill pipe,it provides an advantage in deducing the contact area with borehole wall. A “pressure-released area” is formed on the surface of the drill pipe,which can effectively release the expansion stress of the coal around the drill pipe,and the rotating drill pipe continuously removes the expansion coal,thereby reducing the rotation drag of the drill pipe. The hole-protecting area ratio Sk is presented in this paper,and used to indicate the hole or space structure area on the drill pipe surface. The rotation drag torque and axial drag decrease by as much as 50% with the reduction of hole-protecting area ratio Sk. It is equivalent to reducing the power loss of the drilling rig and effectively increasing the drilling depth of the soft coal seam. Based on the research result,a spiral hole-protecting drill pipe model and porous structure hole-protecting drill pipe model were constructed.In practical applications,combined with the mechanical parameters of coal and rock at the construction site,a reasonable design of the hole-protecting area ratio Sk is the key to the structural design of the hole-protecting drill pipe.