Oil-Based Drilling Fluid's Cuttings Bed Removal Properties for Deviated Wellbores

Results from cuttings transport tests in the laboratory using different field-applied oil-based drilling fluids with similar weight and varying viscosities are presented in this paper. The fluids are designed for highly deviated wells, and the cuttings transport performance at relevant wellbore inclinations was investigated. The experiments have been performed in a flow loop that consists of a 10-m-long test section with 50.4 mm (2″) diameter freely rotating steel drill string inside a 100-mm (≈4″) diameter wellbore made of cement. Sand particles were injected while circulating the drilling fluid through the test section. Experiments were performed at three wellbore inclinations: 48, 60, and 90 deg from vertical. The applied flow loop dimensions are designed so that the results are scalable to field applications; especially for the 12 ¼″and 8 ½″ sections. The selected setup provides correct shear rate ranges and similar Reynolds numbers to the field application when the same fluids are applied. Results show that hole cleaning abilities of the tested fluids vary significantly with well angle, drill string rotation, and flowrate. Results support field experience showing that low viscous fluids are more efficient than viscous fluids at higher flowrates and low drill string rotation. As well as per field experience, more viscous fluids are efficient in combination with high drill string rotation rates. The results show the effect of cuttings transport efficiency as a function of hydraulic frictional pressure drop, demonstrating methods to achieve a more optimal hydraulic design in the tested conditions. The key findings have direct relevance to drilling operations.