CFD Modeling of Cuttings Transport Efficiency in Wellbore Annuli: Effects of Inclination Angle and Drilling Fluid Density

Hole cleaning ensures drilling safety and efficiency. Well inclination angle and drilling fluid density are important parameters affecting cuttings transport. To reveal their coupled interaction mechanism, this study employs the Euler–Euler multiphase flow model to conduct CFD simulations of cuttings transport in a 3D eccentric annulus with an eccentricity of 0.6 under various inclination angles (30°, 45°, 60°, 75°) and drilling fluid densities (1200~1800 kg/m3). Using the cuttings transport ratio (CTR), annulus cuttings volume concentration (CVT), outlet cuttings volume fraction, and annulus pressure drop as evaluation indicators, the influence mechanism of these parameters on hole cleaning efficiency is systematically analyzed. The results show that the effect of drilling fluid density on the CTR is regulated by inclination angle, with 45° being the critical angle for the extreme value of the CTR. Increasing density can significantly reduce cuttings deposition in the annulus, with a more pronounced improvement effect in high-inclination sections. Effective cuttings transport can be achieved by increasing the density to 1500, 1650, 1800, and 1800 kg/m3 for inclination angles of 30°, 45°, 60°, and 75°, respectively. The annulus pressure drop increases approximately linearly with density, and first rises then falls as the inclination angle increases from 30° to 75°, with 45° being the critical angle for peak pressure drop. This study clarifies the coupled regulation law of inclination angle and drilling fluid density, and determines the critical drilling fluid density under different inclinations, providing a numerical basis for optimizing hydraulic parameters and improving hole cleaning efficiency in directional drilling.