适用于特定地层的PDC钻头个性化设计

破碎性地层的井下掉块常常导致常规设计的PDC钻头出现卡钻现象,严重的甚至会造成钻头掉齿、断刀翼等井下事故,严重影响了PDC钻头的推广使用。为此,对Φ215.9 mm六刀翼PDC钻头进行了全面改进设计：冠部形状为“直线-圆弧-直线”,相邻刀翼之间夹角约为60°;主切削齿直径为16 mm且按照“中密度”方式布置;将刀翼尾部的保径块整体上移一定距离,且4个保径块围绕钻头轴线成90°。之后采用PDC钻头数字化钻进仿真系统和CFD软件对设计方案进行了力平衡和井底流场优化分析：优化布齿参数的钻头横向力约为钻压的3.9%;高的流速分布在1号、5号刀翼工作面鼻部位置及2号、3号、4号、6号刀翼背面,有利于岩屑及时清除出PDC齿表面。改进后的新型Φ215.9 mmPDC钻头在吐哈油田破碎性地层中应用总进尺1 339 m,与邻井同井段相比,节约了5次起下钻工作量和作业时间,钻头出井后,仅两颗齿损坏,其余PDC齿磨损均衡,冠部冲蚀情况与钻头的井底流场分析的结果完全吻合,说明PDC钻头的个性化设计能够为此类地层钻井提速提供切实可行的技术思路。

An individualized PDC bit design for specific formations

Downhole falling masses in broken formations often cause the sticking of conventionally designed PDC bits. In more serious cases, they even cause cutter loss, blade snapping and similar downhole accidents which severely impact on the popularization of PDC bits. Therefore, theΦ215.9 mm six blade PDC bit was entirely modified in this paper. The crown is a "linear arc linear" shape and its included angle between adjacent blades is about 60°. Those16 mm main cutters are arranged in a moderately spaced way, and gauge pads at the end of the blades are shifted upwards as a whole so that all four gauge pads form 90° angles around the axial line of the bit. After that, an analysis of the equilibrium of forces and flow field optimization was conducted for the design using the CFD software and the drilling digital simulation system for PDC bits. Among the optimized cutter arrangement parameters, the transverse force of the bit was about 3.9% of the WOB (Weight on Bit). Higher flow rates were distributed on the nose parts of the working surface of 1^# and 5^# blades and the back faces of 2^#, 3^#, 4^# and 6^# blades to facilitate timely removal of rock cuttings from the surface of the cutters. In the application of this new modifiedΦ215.9 mm PDC bit to the broken formation drilling in the Tuha Oilfield, the total drilling footage reached up to 1339 m. Compared with the same sections of adjacent wells, this new modified bit helped reduce the workload and time by 5 tripping operations. After the bit was tripped out, only two PDC cutters were found to be broken and all the other cutters were evenly worn. The crown erosion corresponded completely with the findings of the downhole flow field analysis of the bit. This study proves that the individualized PDC bit design provides a practicable approach to enhancing the drilling speed in broken formations.