New insights into drill string impact on wellbore instability in hot dry rock drilling under THM coupling

ABSTRACT: Wellbore instability is a major challenge in deep drilling, with drill string impacts often overlooked despite their critical role. Existing research has not fully investigated the impact mechanisms between the drill string and wellbore, especially under rock heterogeneity, initial THM (Thermo-Hydro-Mechanical) coupled damage, and random impact trajectories. This study develops an impact model that integrates heterogeneous rock properties, THM coupling, and drill string dynamics, using Mohr-Coulomb and Drucker-Prager criteria to capture wellbore failure under both static and dynamic loading conditions. The effects of initial THM damage and wellbore roughness on wellbore damage, collapse, crack propagation, and stress distribution are analyzed. Results reveal that focusing solely on impact effects can overestimate forces, while ignoring THM coupling may underestimate instability risks. Increased friction coefficient leads to more and deeper cracks, with intergranular crack propagation becoming more pronounced. Friction impacts both stress distribution and crack propagation risk. Wellbore collapse is driven by radial displacement and normal forces, with friction affecting crack propagation paths. Overall, the friction coefficient exhibits nonlinear effects on wellbore instability, impacting stability under heterogeneous conditions and THM coupling. These insights enhance understanding of drill string impact mechanisms and guide drilling practice optimization.