Image source
Within the standards of engineering and design it is always important to analyze the strength of materials, especially considering the safety aspect of equipment and personnel, in the case of drilling wells, the tubing string both drilling and casing have a high weight that could even exceed 200 tons. Such a weight can cause stress or strain to the strength of the surface equipment supporting the weight of the drill string or casing string.
However, being able to calculate the float factor can give us an estimate of the actual weight of the string and thus be able to evaluate if the surface equipment can really support the weight of the tubing string.
For the calculation of the buoyancy factor it is essential to rely on Archimedes' principle, since when the drill pipe is submerged in the well, it is actually going into a volumetric space that is filled with drilling fluid (drilling mud) which is an aqueous medium, which causes the pipe to experience an upward thrust motion, this thrust force in magnitude is equal to the force of the weight of the drilling fluid that is displaced when the pipe is pulled into the well, this thrust force is what actually reduces the weight of the tubing string.
How do you calculate the weight of the pipe going into the hole?
This is true for any tubing string that is run into the hole, be it drill pipe, casing, tubing, production tubing, among others. To calculate the real weight of the pipe, simply multiply the weight of the pipe in air by the buoyancy factor, this makes us think that the buoyancy factor will be governed mostly by the density value of the drilling mud, since the drilling mud will function as the liquid phase in which the pipe will be submerged, which means that the higher the density of the mud, the lower the weight of the drill string will be.
In conclusion, the concrete importance of calculating the buoyancy factor is that it allows us in drilling operations to know how much weight the drill pipe or casing has when it is in the hole when it is filled with drilling mud, so I will explain how to calculate the buoyancy factor and how to use it to calculate the weight of the string inside the hole:
- Since drilling fluid density is measured in the oil industry and especially in well drilling in gallons/minute, then the mathematical equation to calculate the buoyancy factor is as follows:
Example: Calculate the actual weight of a 150,000 lb tubing string that is submerged in a hole with a 13 lb/gallon density drilling fluid.
The first thing we do is calculate the buoyancy factor (BF).
We then multiply the weight of the string by the float factor:
We can conclude that the 150 thousand pounds weight of the pipe decreases to 122 thousand pounds weight when it is introduced into the hole, this occurs because of the drilling fluid and its density, and that the higher the density of the drilling mud, the lower the weight of the pipe in the hole filled with drilling mud.
References
Formulas and Calculations for Drilling, Production and Workover / Lapeyrouse, 2002.
Drilling Fluids Engineering Manual, M-I SWACO, Version 2.0, 2001.
Applied Drilling Engineering / SPE Textbook Series, Volume 2, 1986.