drilling · hydraulics
Annular Velocity
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Inputs
gpm
bbl/ft
Description
Computes the average velocity of drilling fluid moving up the annulus from flow rate and annular cross-sectional area — the fluid-side half of the hole-cleaning comparison against cuttings Slip Velocity. Same underlying geometry Reynolds Number and Lag Time / Bottoms Up already use, exposed here as its own standalone, hole-cleaning-framed result.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Va | ft/s | Annular Velocity |
| Q | gpm | Circulating flow rate, from Pump Output or a measured pump rate. |
| Cap | bbl/ft | Capacity of the open hole or casing around the pipe, from Annular Capacity. Numerically equal to the annular cross-sectional area (m³/m ≡ m²), which is what this calculation actually needs. |
Assumptions
- Flow is evenly distributed across the annular cross-section — a concentric annulus with no significant pipe eccentricity
- A single uniform annular geometry over the interval evaluated — a washout or diameter change requires evaluating each interval separately
- The flow rate supplied is the actual circulating rate, not an average over a variable-rate interval
Limitations
- Does not account for pipe eccentricity (drillpipe resting against the low side of the hole in a deviated well), which concentrates flow on the wide side and can leave the narrow side effectively understocked despite an adequate bulk average velocity
- A bulk average velocity only — real annular flow has a velocity profile (fastest near the center of the annular gap, slowest at the walls), not a uniform plug flow
- Does not itself determine whether hole cleaning is adequate — that requires comparing this value against cuttings Slip Velocity via Cuttings Transport Ratio
Use Cases
- → Hole cleaning screening input: Provide the fluid-side velocity for Cuttings Transport Ratio, alongside cuttings Slip Velocity, to evaluate whether current circulating parameters adequately clean the hole.
- → Flow rate planning: Check what annular velocity a candidate flow rate produces in a given hole/pipe geometry before finalizing a hydraulics program.
- → Erosion / washout screening: Flag excessively high annular velocities that risk washing out unconsolidated or weak formations in open hole.
Related Calculations
Region Notes
Global
100 ft/min (0.51 m/s) is a widely used rule-of-thumb minimum annular velocity for adequate vertical-well hole cleaning with typical water-based mud — treat it as a starting screening value, not a substitute for an actual transport-ratio check.
References
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