Dynamic Buckling of Coiled Tubing During a Completion/Workover Application

Abstract

The purpose of this paper is to present how hand-calculations and a general finite element analysis program can be utilized to determine under what conditions coiled tubing (CT) could buckle helically in a vertical well. This is especially useful when determining loads on the tubing in a completion or work-over application. Two situations are considered as described below. • A Quasi-Static case when either: –Weight is slacked off from the surface. or –A bottom force that may be generated by reservoir pressure for instance. • A Dynamic case, which occurs when: –Weight is slacked off too quickly and the tubing impinges upon an object causing an impulse to be generated into the tubing. This paper presents the theory for the hand calculations and shows how the hand calculations may be utilized for a field example. In addition, the same field example is considered in the general finite element analysis program and the limitations of the hand calculations are discussed as well as the capability of the dynamic finite element model simulation. Hand calculations restrict us to the quasi-static scenario and only allow an assessment of the deformation of the coiled tubing deformation in the linear-elastic range. In contrast, the general purpose finite element analysis software allows for large deformation, and non-linear material behavior. In addition, the general purpose finite element analysis software allows for tubing evaluation locally after lockup of the tubing occurs. Generally hand calculations or quasi-static simulators have been used to assess the deformation of tubing during an impact event; these have proven to be inadequate because they cannot capture the transient event and do not model the inertial forces correctly. The general purpose finite element code provides better estimates of the deformation and stress state of the tubing. When assessing the dynamic behavior of tubing, it would be prudent to utilize the capability of general purpose finite element software to obtain realistic and an accurate assessment of the deformation and stress state of the tubing.