Dynamic Characteristics of Bottom Hole Assembly Lateral Vibration: A Discrete Element Method-Based Investigation

The lateral vibration of the Bottom Hole Assembly (BHA) tend to be severe, often leading to drill string damage and hindering drilling efficiency. Therefore, gaining an in-depth understanding of BHA lateral vibration characteristics and implementing effective mitigation measures are particularly crucial. A coupled dynamic model for drill strings was developed based on the discrete element method suitable for ultra-high length-to-diameter ratio structural analysis and large-deformation dynamic analysis of drill strings. The model incorporates critical factors including mass eccentricity, damping, weight on bit (WOB), and drill string-borehole friction. Validation through comparative analysis with laboratory experiments confirmed the model’s accuracy and generalizability. Systematic investigations were conducted on the vibrational state, dynamic morphology and characterization of BHA in vertical wells under varying rotation speeds, borehole friction coefficients, and WOB. The results demonstrate that with increasing rotation speed, the drill string experiences progressive amplification of contact normal force and frictional resistance during forward whirl. This dynamic evolution subsequently transitions into chaotic collisions with the borehole, ultimately developing into backward whirling characterized by near-wall rolling contact. Correspondingly, the BHA’s deflection trajectory evolves from a quasi-planar configuration during stable forward rotation to complex undulating patterns, finally forming wall-adherent helical morphology under backward whirling. Increased friction coefficient reduces both the initiation time and critical rotation speed threshold required for backward whirling while promoting intensified helical morphological characteristics. Meanwhile, elevated WOB exacerbates BHA bending or helical deformation. This study enhances the fundamental understanding of BHA lateral motion mechanisms and provides theoretical guidance for vibration mitigation strategies in drilling operations.