Distributed Nonuniform Friction and Vibration Mode Switching of Drillstring in Complex Wellbore Trajectories

Summary To investigate the influence of complex wellbore trajectories on the torsional dynamics of drillstrings, we focus this study on off-bottom conditions, establishing a distributed torsional dynamic model based on real wellbore trajectory data that considers the velocity-weakening effect. The model is decoupled using the method of characteristics, and the dynamic responses of drillstring systems in two real wells with different geometries are comparatively analyzed under various resonance frequency excitations. The results reveal that, under specific high-order resonance excitations, a drillstring system with strong nonlinearity will switch from a forced vibration state to a quasiself-excited vibration state dominated by the first-order resonance frequency and characterized by higher energy. Spatiotemporal stress analysis further confirms that this vibration mode switching leads to an increase in the internal stress amplitude of the drillstring. This finding indicates that traditional frequency-domain-based methods for vibration risk assessment can be misleading for strongly nonlinear drillstring systems. This research not only elucidates the mechanism of drillstring vibration under these specific conditions but also provides a theoretical foundation for vibration suppression through the optimization of wellbore trajectories.