Numerical investigation and optimization of structural and operating parameters for cuttings transport in a bidirectional eccentric micro-underreamer

Cuttings transport behavior plays a crucial role in downhole safety, drilling efficiency, and wellbore cleaning during underreaming operations. Because of the complex local flow passage and eccentric structural characteristics of a bidirectional eccentric micro-underreamer, cuttings migration and discharge are highly sensitive to downhole operating conditions. In this study, a three-dimensional solid–liquid two-phase flow model was established for a bidirectional eccentric micro-underreamer, and an Eulerian–Lagrangian approach was employed to describe the two-way coupling between the drilling fluid and cuttings particles. After the blade structural parameters were optimized, the effects of drill string rotational speed, weight on bit, drilling fluid density, and drilling fluid viscosity on cuttings transport behavior were systematically investigated. The results indicate that drill string rotational speed and weight on bit both exert non-monotonic effects on cuttings discharge efficiency, and relatively favorable cuttings-carrying performance was achieved at 50 r/min and 100 kN, respectively. By contrast, increasing drilling fluid density weakened cuttings transport performance, whereas increasing drilling fluid viscosity enhanced the drag and suspension capacities of the fluid for cuttings particles. Within the investigated range, a drilling fluid density of 1.6 g/cm 3 and a viscosity of 50 mPa·s were associated with relatively favorable cuttings-carrying performance. These findings provide a theoretical basis for the optimization of operating parameters and the engineering application of bidirectional eccentric micro-underreamers.