Wellbore instability in hot dry rock drilling under multi-field coupling---new insights from drilling string impact

Wellbore instability is a major challenge in hot dry rock (HDR) drilling, with drill string impacts often overlooked despite their critical role. Existing research has not fully explored the impact mechanisms between the drill string and wellbore, particularly under rock heterogeneity, initial thermo-hydro-mechanical (THM) coupled damage, and random impact trajectories. This study develops an impact model incorporating heterogeneous rock properties, THM coupling, and drill string dynamics, using Mohr-Coulomb and Drucker-Prager criteria to capture wellbore failure under static and dynamic loading conditions. Analyzed the effects of drill string impacts on wellbore instability and crack propagation. Results show that focusing solely on impact effects can overestimate forces, while neglecting THM coupling may underestimate instability risks. Increased friction coefficient leads to deeper cracks and pronounced intergranular crack propagation. Rotational speed affects wellbore damage nonlinearly: low speeds exhibit prevalent intergranular and transgranular cracks, while higher speeds intensify intragranular and intergranular cracks through stress concentration. Rotational speed and friction affect the frequency and force of impacts but have minimal influence on crack paths in areas with initial THM damage. Wellbore collapse is driven by radial displacement and normal forces, with friction influencing crack propagation paths. These insights enhance understanding of drill string impacts and guide safe HDR extraction.