Theory and Application of Buoyancy in Wells

The paper presents a generalized buoyancy model for application in wells, called the generalized Archimedes’ principle. There exists confusion in the oil industry about buoyancy effects as related to the force-area method (also known as piston force method) or the Archimedes’ principle, the stability force in buckling and axial forces during well intervention operations. This paper will present a general model called the generalized Archimedes’ principle, which is valid for all cases. The paper presents examples of application to ordinary drilling operations, deviated wells and different fluid densities inside and outside the pipe. For well intervention scenarios, the effects of wellhead shut-in pressures on buoyancy are demonstrated. This simple model, not requiring use of a “stability force”, can also analyze buckling. Two field cases are presented to demonstrate the use of the generalized Archimedes’ principle in torque and drag analysis. When the drill string is run in hole having a float valve installed, the buoyancy factor changes depending on how much air is in the drill string. The generalized Archimedes’ principle may simply be applied to the torque and drag model for accurate lowering force calculations. Using the generalized Archimedes’ principle, a correct assessment of the real axial load of the pipe is obtained, a critical factor for tubular work in petroleum wells. The model is also significant as it is simple and can be applied to all well operations.