Find the annular capacity between the hole (or casing ID) and the pipe OD, and the total annular volume over a given length.
The annulus is the space between the pipe and the hole (or the casing it runs through). Its volume tells you how many barrels sit in that space — which you need for cementing (how much cement and spacer to pump), for circulating bottoms-up, for kill-sheet volumes, and for tracking returns. Get it wrong and you either under-displace or over-displace.
Annular capacity depends on the difference between the two diameters squared — the bigger the gap, the more volume per foot.
Where Dhole is the hole diameter or casing ID and Dpipe is the pipe OD, both in inches. Multiply by length (ft) for total annular volume in barrels. The inverse gives linear capacity (ft/bbl) — how many feet of annulus one barrel fills.
Assumes an in-gauge hole. Washed-out or over-gauge open hole holds more than this — use a caliper volume where accuracy matters, e.g. cement jobs.
Running 5" OD pipe in an 8.5" hole: annular capacity = (8.5² − 5²) / 1029.4 = 0.0459 bbl/ft. Over 1,000 ft that is 45.9 bbl in the annulus. One barrel fills about 1/0.0459 ≈ 21.8 ft of that annulus.
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