为解决常规套管钻具应用软煤层钻进存在着钻进阻力大、效率低的技术难题，提出“护孔卸压”钻进新观点，建立护孔钻进旋转阻力矩模型、轴向阻力模型及护孔数值模型，分析常规套管钻具应用于软煤层钻进存在的技术缺陷，探讨“护孔卸压”钻进工艺方法实现卸压、降阻的力学机制。结果表明:受软弱煤岩层钻孔收缩产生的围压影响，应用常规套管钻具钻进，在较短的钻孔深度范围内其旋转阻力矩和轴向阻力超过了钻机的额定扭矩及提拔力，钻杆将无法旋转及退钻，致使钻进终止;通过在套管钻具表面设置间隙或孔结构，减小护孔钻杆与钻孔壁的接触面积，为钻孔孔壁留设变形空间，钻杆表面形成的“卸压区”释放了钻杆周围煤体膨胀应力，并且旋转的钻杆不断切除膨胀的煤体，降低了软煤钻孔膨胀变形对钻杆产生的摩擦阻力;提出护孔面积比S_k概念，用来表征钻杆表面孔隙留设面积大小，伴随S_k减小，其旋转阻力矩和轴向钻进阻力呈逐渐降低趋势，降低幅度可达50%以上，从而降低了钻机的动力损耗，有效提高松软煤层的钻孔深度。基于“护孔卸压”原理，构建了螺旋护孔钻杆模型和多孔结构护孔钻杆模型，在实际应用中，结合施工地点的煤岩力学参数，合理确定护孔面积比S_k是护孔卸压钻具结构优化设计的关键。

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.