Buoyancy Force on a Plain or Perforated Portion of a Pipe

Torque and drag models have been used for many decades to calculate tensions and torques along drill-strings, casing strings and liner strings. However, when applied to sand-screens, it is important to check that all the initial hypotheses used for torque and drag calculations are still valid. In particular, it should be checked whether the buoyancy force on a perforated tube may differ from the one applied to a plain tube. The buoyancy force applied on a pipe, contributes to the sum of efforts at the contact between the pipe and the borehole and therefore influences torque and drag calculations. This contact force is local and should account for localized effects as well as the material internal forces, torques and moments on each side of the contact. As the buoyancy force is the result of the gravitational component of the pressure gradient on the surface of the pipe that is in contact with the fluid, the presence of holes in the pipe also influences the buoyancy force. When applied to a portion of a pipe, buoyancy does not have contributions at the end caps of that portion of the drill-stem since these end caps are not in contact with the fluid, except at positions with a change of diameter. Therefore, one shall be cautious when calculating the local buoyancy force either on a plain or a perforated tube. The paper describes how to calculate the local buoyancy force on a portion of a drill-stem by application of the Gauss theorem accounting for the necessary corrections arising from the end caps not being exposed to the fluid. An experimental setup has been built to verify that the tension inside a pipe subject to buoyancy does follow the derived mathematical calculations. With complex well construction operations, for instance during extended reach drilling or when drilling very shallow wells with high kick-off rates, the slightest error in torques and drag calculations may end up in jeopardizing the chance of success of the drilling operation. It is therefore important to check that all initial calculation hypotheses are still valid in those contexts and that for instance, sand-screens may be run in hole safely after a successful drilling operation.