Engineering lesson
Annular capacity versus annular volume
Annular capacity is volume per unit length. Annular volume is capacity multiplied by the measured length of the interval with that geometry.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Separate the cross section from the interval
An 8.5 in hole around 5 in casing has a fixed area difference. In the site's rounded field convention its annular capacity is 0.0459 bbl/ft. Multiplying by 1,000 ft gives 45.9 bbl; multiplying by 2,000 ft gives 91.8 bbl.
The capacity does not double when length doubles. It describes the cross section, while volume describes how much of that cross section extends along the well.
Use the right two diameters
For an open-hole annulus use hole diameter and casing OD. Inside a larger casing use that casing's ID and the smaller string's OD. Nominal pipe size does not automatically supply either required dimension.
If geometry changes, calculate a new capacity for the next interval. Adding lengths before checking diameters can silently assign the wrong cross section to part of the well.
Keep the volume basis visible
The calculator returns barrels using a rounded field coefficient. The SI expression is area difference times length with all dimensions in consistent units. Small differences from an exact geometric conversion should be identified as coefficient rounding.
Capacity is a geometric result. It does not include slurry excess, displacement efficiency, losses or a shoe track unless those volumes are explicitly added.
Explore the example
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Measured length (ft) = 1000. Annular volume (bbl): 45.9; Capacity (bbl/ft): 0.0459.
- Annular volume (bbl)
Worked example
Capacity multiplied by measured length
8.5 in hole around 5 in casing; the rounded library coefficient gives 0.0459 bbl/ft capacity.
Relation and unit basis
Va = 0.7854(Dh² − Do²)L/808.5
Diameters in inches, measured length in feet, volume in petroleum barrels. Capacity is Va/L.
1. Define the baseline
Measured length (ft) = 1000
8.5 in hole around 5 in casing; the rounded library coefficient gives 0.0459 bbl/ft capacity.
2. Select the relation
Va = 0.7854(Dh² − Do²)L/808.5
Diameters in inches, measured length in feet, volume in petroleum barrels. Capacity is Va/L.
3. Substitute the values
0.7854 × (8.5² − 5²) × 1,000/808.5 = 45.9 bbl
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Annular volume (bbl) = 45.9; Capacity (bbl/ft) = 0.0459
Length changes volume, while capacity is unchanged at fixed diameters.
Result and interpretation
Annular volume (bbl) = 45.9; Capacity (bbl/ft) = 0.0459
Length changes volume, while capacity is unchanged at fixed diameters.
The length sweep keeps diameters fixed. Volume increases linearly and capacity stays constant.
Exact scenario data
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Measured length (ft) | Annular volume (bbl) | Capacity (bbl/ft) |
|---|---|---|
| 250 | 11.475 | 0.0459 |
| 500 | 22.95 | 0.0459 |
| 1,000 | 45.9 | 0.0459 |
| 1,500 | 68.85 | 0.0459 |
| 2,000 | 91.8 | 0.0459 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. How much volume is represented by 0.0459 bbl/ft over 750 ft?
Show the worked answer
34.425 bbl. The capacity is still 0.0459 bbl/ft.
2. Reproduce the baseline Annular volume (bbl) without rounding intermediate values.
Show the worked answer
0.7854 × (8.5² − 5²) × 1,000/808.5 = 45.9 bbl. Annular volume (bbl) = 45.9. Length changes volume, while capacity is unchanged at fixed diameters.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Measured length (ft) changes from 250 to 2000. Other controls remain as stated in the premise. Annular volume (bbl) changes from 11.475 to 91.8.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. Diameters in inches, measured length in feet, volume in petroleum barrels. Capacity is Va/L. Length changes volume, while capacity is unchanged at fixed diameters.
Before using the result
- Use outer boundary ID and inner string OD.
- Multiply capacity by measured interval length.
- Add excess after calculating geometry.
Length changes volume, while capacity is unchanged at fixed diameters.
Continue learning
Start with the broader workflow guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.