Multi-Objective Optimization of Annular Flow Structure for Coring Drilling Tools in Ultra-Deep Wells

Abstract

Conventional coring drilling tools suffer from suboptimal annular flow design, leading to excessive pressure loss and hydraulic torque in ultra-deep well coring operations. To address this issue, we propose a systematic multi-objective optimization framework for the annular flow structure of single-acting double-tube coring tools, with the dual objectives of minimizing annular pressure loss and local circumferential hydrodynamic torque exerted on the outer pipe surface. Four key design variables, including the inner diameter and angle of the flow distribution sub, the number of water outlets on the centralizer, and the distance between the slip seat and the inner step surface of the drill bit, were selected. 81 sample points were generated via L81 orthogonal experiments combined with 3D CFD simulations. High-precision XGBoost surrogate models were established, and PAWN global sensitivity analysis was performed to identify dominant factors. The improved non-dominated sorting whale optimization algorithm was used to obtain the Pareto optimal solution set. The results show that the number of water outlets on the centralizer is the most significant factor affecting annular pressure loss, while the distance between the slip seat and the drill bit’s inner step dominates hydraulic torque. Compared with the original design, the optimized structure reduces annular pressure loss by 32.36% and local hydraulic torque by 4.02%, an absolute decrease of approximately 9.46 N·m. This study provides a reusable optimization method and clear parameter direction for ultra-deep well coring tools.