Drilling Process Dynamics of Flexible Drill String in Short-Spacing Rescue Well

To extract remaining reserves beneath the plugged sections of an abandoned well, a casing-outer sidetracking technique is proposed to connect a rescue well to the original well. Due to the short spacing and high curvature, an ultra-short-radius drilling method with an articulated double-layer flexible drill string is adopted. However, random contact during drilling introduces nonlinear mechanical challenges. To address this, a transient dynamic modeling strategy is developed: the drill string is discretized into spatial beam elements, with articulation modeled by universal joints and contact described using gap elements. A transient dynamic model is established, along with criteria for contact state identification and convergence. An iterative solution procedure for multiple contact scenarios is proposed. Validation using a single-layer case confirms model reliability. Results show that the outer casing provides guidance and experiences increasing fluctuating contact forces in curved sections, while the inner flexible drill string transmits weight on bit (WOB), producing high contact forces near the bit that increase with WOB. The maximum deformation of the flexible drill string reaches 2.12[Formula: see text]m, with an average WOB loss of 2[Formula: see text]kN. This paper offers theoretical guidance for safe and efficient sidetracking operations.