**Buoyancy Factor** is the factor that is used to compensate loss of weight due to immersion in drilling fluid.

Before explaining any further, I will explain you about the basic concept of Buoyancy. Buoyancy is the upward force that keeps things afloat. The net upward buoyancy force equals to the amount of the weight of fluid displaced by the body volume. This force will make objects lighter when it immerses in fluid. For example, we feel ourselves lighter when we are in swimming pool because this is the effect of buoyancy.

In drilling operation, we need to know how much weight of string of drill pipe, completion string, etc in drilling fluid. Therefore, Buoyancy Factor is value that we need to know and be able to calculate this value. Please follow the formulas below to calculate Buoyancy Factor in different mud weight units, ppg and lb/ft^{3}.

**Buoyancy Factor using mud weight in ppg**

**Buoyancy Factor (**BF) = (65.5 – mud weight in ppg) ÷65.5

**Note:** 65.5 ppg is density of steel.

Example: Determine the buoyancy factor for a 13.0 ppg fluid:

BF = (65.5 – 13.0) ÷ 65.5

BF = 0.8015

**Buoyancy Factor using mud weight in lb/ft**^{3}

**Buoyancy Factor (**BF) = (489 – mud weight in lb/ft^{3}) ÷489

**Note:** 489 lb/ft^{3} is density of steel.

Example: Determine the buoyancy factor for a 100 lb/ft^{3} fluid:

BF = (489 – 100) ÷489

BF = 0.7955

**How to use the Buoyancy Factor**

In order to figure out the actual weight of drilling string in mud, the air weight of drilling string times the buoyancy factor equal to actual weight in mud, called buoyed weight.

**Example: **Determine the string weight in 13.0 ppg mud. Air weight of string is 350 klb.

The buoyancy factor for a 13.0 ppg fluid:

BF = (65.5 – 13.0) ÷ 65.5

BF = 0.8015

**Buoyed weight of drill string = String weight in the air x BF**

The buoyed weight of drill string in 13.0 ppg mud = 350 x 0.8015 = 280.5 Klb.

**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.