**Annular capacity ** is volume of fluid between two diameter of cylindrical objects per length or length per volume. This article demonstrates you how to calculate annular capacity between casing or hole and drill pipe, tubing, or casing. There are several formulas as shown below to calculate annular capacity depending on unit of annular capacity required.

Note: Dh is bigger ID and Dp is smaller OD. The examples below will show the Dh as hole size and Dp is drill pipe OD

## Oilfield Unit

## a) Calculate annular capacity in bbl/ft

Annular capacity in bbl/ft = (Dh^{2}– Dp^{2}) ÷1029.4

Where;

Dh in inch

Dp in inch

**Example: **Hole size (Dh) = 6-1/8 in.

Drill pipe OD (Dp) = 3.5 in.

Annular capacity in bbl/ft = (6.125^{2} – 3.5 ^{2}) ÷1029.4

Annular capacity = 0.0245 bbl/ft

## b) Calculate annular capacity in gal/ft

Annular capacity in gal/ft = (Dh^{2}– Dp^{2}) ÷ 24.51

Where;

Dh in inch

Dp in inch

**Example:**

Hole size (Dh) = 6-1/8 in.

Drill pipe OD (Dp) = 3.5 in.

Annular capacity in gal/ft = (6.125 ^{2} – 3.5^{2}) ÷24.51

Annular capacity = 1.031 gal/ft

## Metric Unit

## c) Calculate annular capacity in cu-meter/meter (m^{3}/m)

Annular capacity inm^{3}/m= (Dh^{2}– Dp^{2}) ÷1,273,240Where;

Dh in mm

Dp in mm

**Example:
**Hole size (Dh) = 155.56 mm.

Drill pipe OD (Dp) = 88.90 mm.

Annular capacity in m

^{3}/m = (155.56

^{2}– 88.90

^{2}) ÷1,273,240

Annular capacity = 0.0128 m

^{3}/m

## d) Calculate annular capacity in litre/meter (l/m)

Annular capacity inm^{3}/m= (Dh^{2}– Dp^{2}) ÷1,273.24Where;

Dh in mm

Dp in mm

**Example:
**Hole size (Dh) = 155.56 mm.

Drill pipe OD (Dp) = 88.90 mm.

Annular capacity in m

^{3}/mt = (155.56

^{2}– 88.90

^{2}) ÷1,273.24

Annular capacity =12.8 l/m

**Annular volume can be determined by this following formula;**

## Oilfield Unit

Annular volume in bbl = annular capacity (bbl/ft) x length of annulus (ft)

**Example:**

Annular capacity = 0.0245 bbl/ft

Length of annulus = 1000 ft

Annular volume = 1000 x 0.0245 = 24.5 bbl.

## Metric Unit

Annular volume in m^{3 }= annular capacity (m^{3}/m) x length of annulus (m)

**Example:**

Annular capacity = 0.0128 m^{3}/m

Length of annulus = 330 m

Annular volume = 330 x 0.0128 = 4.224 m^{3}.

** Please remember that if you have several annular profile, you must calculate volume based on each annular profile in order to get total annular volume.

**Download Free Excel Spread Sheet**

This is 2020 update with Metric Unit – Please find the Excel sheet for calculating annular capacity.

**Ref books: **

Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.