Design of Flexible Drill Pipe with Ultra-Short Radius in Sidetrack Horizontal Well Based on the Snake Bone Directional Variability

A snake-bone flexible drill pipe was designed based on bionics to relieve the overlarge deflecting curvature radius of the sidetrack horizontal well. Specifically, flexible design is carried out for coiled tubing in combination with morphology and mechanism bionics. The ball head+external casing+torque pin structure was used to decompose the drill pipe, and the torque pin was used to transfer torque. Each section can reach a torsion angle of 4.1°, which can alleviate the buckling of the drill pipe in the casing pipe. A maximum bending angle of 5.12° can be generated between sections, and the deflection curvature radius is shortened to 1.8 m. To overcome the directional uncertainty between flexible drill pipe sections of small curvature radius during dispersion, a highly flexible pipe was added at the angle in the flexible drill pipe, which guaranteed a high discharging rate. Based on the multi-body dynamics finite element analysis software ADAMS and the statics simulation software ANSYS, the multi-body linkage dynamics and statics simulation analysis were carried out for the flexible drill pipe as a whole, which confirmed the effectiveness of the described deflecting method. Field experiments were carried out in Well PU 2-365, and the borehole trajectory was calculated based on the data derived from the inclinometer. The deflection section curvature radius could reach 1.8 m, which completed the design expectation.