**Annular capacity , **which is one of basic values that you really need to understand, 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

## a) Calculate annular capacity in bbl/ft

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

**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 ft/bbl

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

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

Drill pipe OD (Dp) = 3.5 in.

Annular capacity in ft/bbl = 1029.4 ÷ (6.125

^{2}– 3.5

^{2})

Annular capacity = 40.743 ft/bbl

## c) Calculate annular capacity in gal/ft

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

**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

## d) Calculate annular capacity in ft/gal

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

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

Drill pipe OD (Dp) = 3.5 in.

Annular capacity in ft/gal = 24.51 ÷ (6.125

^{2}– 3.5

^{2})

Annular capacity in ft/gal = 0.97 ft/gal

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

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

Note: annular volume can be expressed in several unit depending on unit that you use in the calculation.

**Example:**

Annular capacity = 0.0245 bbl/ft

Length of annulus = 1000 ft

Annular volume = 1000 x 0.0245 = 24.5 bbl.

** 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.

**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.