Foundation, Concrete and Earthquake Engineering

Buoyancy Factor Calculation and Its Application

Buoyancy Factor is the factor that is used to compensate loss of weight due to immersion in drilling fluid. Before explaining any further, it is very important to explain 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 instant, we feel ourselves lighter when we are in a swimming pool because this is the effect of Buoyancy.

How to calculate the Buoyancy Factor.
The formulas below demonstrates how to determine this factor.
Buoyancy Factor (BF) = (65.5 - m ud weight density in pound per gallon (ppg)) ÷ 65.5
For example, if the drilling fluid weight is 13.0 pound per gallon (ppg), as per the equation above, the factor can be calculated by simply inputting mud weight density into the equation.

BF = (65.5 - 13.0) ÷ 65.5 
BF = 0.8015

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 drill string weight, called buoyed weight, in drilling fluid. The simple equation shows the relationship of actual weight in drilling fluid.

Actual Weight = Air Weight of Drill String x BF 

If weight in the air of a drilling string is 350 kilo pound (Klb), we can use this relationship to determine the string weight in 13.0 ppg drilling fluid. The buoyed weight of drill string in 13.0 ppg mud is equal to 350 x 0.8015 (280.5 Klb).

According to the calculation, when a drill string is immersed into drilling fluid, its weight almost decreases by 20%.Therefore, it is very important that every personnel working at drilling sites should be able to calculate the buoyancy factor in order to figure out how mud weight effects the total string weight in a wellbore.

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