drilling · casing design
Annular/Hole Capacity
Capacity = (π/4)(Dh² − Dp²)
click formula to derive ↑
Inputs
in
in
Description
Computes the volumetric capacity of the annular space between a hole (or outer casing) and an inner string, per unit length. This is the foundational geometric calculation behind cement volume, hole-cleaning velocity, and displacement calculations.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Cap | bbl/ft | Annular Capacity |
| Dh | in | Inside diameter of the open hole or the previous casing string forming the outer boundary of the annulus (222.3 mm = 8.75 in, a common intermediate hole size). |
| Dp | in | Outside diameter of the pipe or casing run inside the hole (127 mm = 5 in, a common drillpipe OD). |
Assumptions
- Both pipe and hole/casing are circular and concentric (no significant standoff or washout)
- Hole ID is either a gauge open-hole diameter or a casing drift/ID value, as appropriate to the section being evaluated
Limitations
- Open-hole capacity calculated from bit size ignores washout — actual hole volume in soft formations can be significantly larger than gauge size implies
- Does not account for pipe eccentricity in a deviated or horizontal wellbore, which changes local annular velocity distribution even though it does not change total capacity
- For multi-diameter annuli (e.g., a tapered string or a hole with multiple bit sizes), this calculation must be applied separately to each interval
Use Cases
- → Cement volume calculation: Provide the per-unit-length annular volume needed as a direct input to cement slurry volume calculations.
- → Hole cleaning / annular velocity: Combine with pump rate to compute annular velocity for hole-cleaning and cuttings-transport evaluation.
- → Displacement volume planning: Determine the volume of fluid that must be pumped to fully displace the annulus during a cementing or completion fluid change.
Related Calculations
Region Notes
Permian Basin
An 8.75 in hole around 5 in drillpipe gives an annular capacity of about 0.0459 m³/m (0.0292 bbl/ft) — a typical intermediate-section value used for cement and hydraulics planning.
Global
Always use the casing drift diameter (not nominal OD) when computing capacity inside a previous casing string, since drift ID accounts for coupling protrusions and is the true restriction.
References
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