Influencing Factors of the Wellbore Cleaning Efficiency in Extended Reach Wells Based on Seawater Drilling Fluid

Under the construction of extended reach wells, cutting beds are very likely to form due to inadequate wellbore cleaning. The inorganic salts in seawater aggravate this problem by affecting the rheology of drilling fluids. By developing a cutting transport dynamic simulation platform, the reasons and laws of cutting beds formation, damage and removal were revealed. The results demonstrated that cutting beds are most likely to form at well inclination angles ranging from 30° to 45°. The larger the cuttings are, the higher the possibility of cutting bed formation. Properly increasing the viscosity and flow rate may reduce the formation probability of cutting beds. Furthermore, the novel concept of the critical flow velocity window for cutting bed migration was proposed. The velocity window narrowed with increasing cutting particle size, well inclination angle, ultimate high shear viscosity and Carson dynamic shear force. The efficiency of cutting bed removal by the drilling fluid was positively correlated with the well inclination angle, Carson dynamic shear force and ultimate high shear viscosity and negatively correlated with the cutting particle size. The migration mode varied with the particle size. Small-particle cutting beds were damaged by the drilling fluid into many blocks, which generated several failure points, and all the blocks were then separately transported. However, the migration mode of large-particle cutting beds was surface migration, which implies that the cuttings on the surface were always propelled by the drilling fluid first. Migration mode variation could be the likely reason for the change in the velocity window.