Annular Velocity
Annular Velocity (AV) is the velocity of drilling fluid in the annulus. It is an important element for achieving effective hole cleaning. Flow rate and annular cross-sectional area are used for calculating average annular velocity. The average annular velocity is a theoretical calculation considering a gauge hole. If there are some over-gauge areas in the well due to hole instability or caving, the risk of cuttings getting lodged in those enlarged sections should be carefully evaluated.
Cutting accumulation is severe in high-angle wells as the cuttings tend to accumulate on the low side of the well. The combined effect of mud rheology, annular velocity, and pipe rotation helps clean the hole by removing drilled and suspended solids from the well.
In general, a minimum average annular velocity of 150 ft/min is recommended for achieving effective cutting cleaning in a high-angle well. However, higher annular velocity also increases Equivalent Circulation Density (ECD). ECD higher than a certain value could cause losses. Hence annular velocity is optimized based on well configuration, well profile, and limits of open formations.