Theoretical Analysis of Cuttings Accumulation at Curvature Transition Zones in Double Build-Up Wells

Extended-reach and highly deviated wells often adopt a double build-up trajectory. This profile contains two curvature transition zones: the build-to-tangent transition and the tangent-to-build transition. The annular flow undergoes severe restructuring in these zones. They are potential bottlenecks for cuttings transport. This work uses a 215.9 mm wellbore with 127.0 mm drill pipe as the reference case. Analytical expressions for the cuttings accumulation ratio in both transition zones are developed based on the three-layer transport model. For power-law drilling fluids, a generalized Reynolds number is introduced to reformulate the Dean number. A dynamic disturbance coefficient is constructed from the along-hole Dean number gradient. This coefficient captures the contrasting behavior of secondary flow. In one transition zone, the secondary-flow decays. In the other, it suddenly emerges. The results show that when the two curvature radii are equal, the accumulation ratio in the tangent-to-build transition is roughly 1.15 times that in the build-to-tangent transition. Reducing the curvature radius from 250 m to 100 m increases the accumulation ratio by about 2.5 times. The ratio rises with the inclination angle. The disturbance coefficient increases monotonically with the build rate. After accounting for drill pipe eccentricity, the recommended minimum curvature radii are 160 m for the first build section and 220 m for the second. These findings offer a theoretical basis for trajectory design in sections with abrupt curvature changes.