Failure mechanism analysis of the threaded connections of the stabilizer in bottom hole assemblies equipped with a positive displacement motor and a rotary steerable system

Drilling practices in Tarim Oilfield of China have shown that the combined use of a Positive Displacement Motor (PDM) and a Power Drive System can significantly enhance wellbore trajectory control and drilling efficiency, but it is likely to cause drill tools failure. Preliminary studies once attributed this failure to High-Frequency Torsional Oscillation (HFTO), but the actual failure morphology does not match HFTO induced failure features. This paper, based on finite element model (FEM) of drill string dynamics, investigates the mechanical mechanism behind the failure of the stabilizer’s threaded joint in a Bottom Hole Assembly (BHA) equipped with a PDM and a Power Drive System. The results show that when stick-slip vibration occurs, the drill tool at the fracture position experiences severe lateral vibration with a frequency as high as 211.4 Hz, and the maximum dynamic bending stress reaches 148.6 MPa in which the maximum additional bending stress caused by collision reaches 126.1 MPa when the closest rub-impact point is 2.49 m above the upper end of the stabilizer. This indicates that it is precisely these high-frequency lateral vibrations (HFLV) that induce high-frequency alternating stress within the threaded joint, thereby causing fatigue damage and ultimately leading to drill tool fracture.