Hole Cleaning and Critical Transport Rate in Ultra-Deep, Oversized Wellbores

In ultra-deep and large well sections, high collapse stresses and diminished annular return velocity present significant challenges to wellbore cleaning. With increasing depth, rising temperature and pressure constrain the regulation of displacement and drilling fluid rheology, impairing the fluid’s capacity to transport cuttings effectively. A precise understanding of cuttings settlement behavior and terminal velocity is therefore essential for optimizing their removal. This study accounts for variations in wellbore temperature and pressure, incorporates non-spherical cuttings and wellbore diameter parameters, and develops accordingly a simplified model to predict terminal settlement velocity. The cuttings carrying ratio is introduced as a metric for evaluating wellbore cleanliness. Findings reveal that temperature and pressure fluctuations can alter terminal velocity by up to 3.4%. Cuttings shape plays a crucial role, with block-shaped cuttings requiring higher annular return velocity than flake-shaped ones at the same carrying ratio. As wellbore size increases, the minimum required carrying flow rate rises nonlinearly, though the rate of increase gradually declines. For a Φ444.5 mm wellbore, a carrying ratio of at least 0.6 is recommended. Terminal velocity decreases with increasing consistency coefficient, particularly in high-viscosity regimes. The proposed carrying ratio offers a more accurate and practical assessment of wellbore cleanliness.