Abstract

Coiled Tubing (Cr) is being used increasingly to service highly deviated wells. Among the mechanical problems encountered in running coiled tubing into a highly deviated well is helical buckling of the tubing which leads to lockup. Furthermore, contact between the Cr and casing can restrict the weight that can be applied to a downhole tool, thereby limiting the placement of tools in deviated sections.

A tubing forces model has been developed that calculates the resulting stresses and deformation of the coiled tubing as the coiled tubing is run into and pulled out of the well, the points at which helical buckling occurs and an indication of whether lockup has occurred.

An inconsistency in current tubing forces models is that they utilize two differing coefficients of friction for running in hole and pulling out of hole. This criterion suggests some technical deficiency in the formulation of the models. Results from the tubing forces model presented in this paper account for the residual bend due to coiled tubing being wound on the reel and gooseneck. A consequence of this is that it is unnecessary to use two differing friction coefficients for running in hole and pulling out of hole.

Results are presented in the form of the surface weight indicator and compared to field data. It is observed that the surface weight indicator predictions are in good agreement with field data.

This work suggests that accounting for residual bend, the tubing forces model accurately provides the state of stress, operating forces and deformations during coiled tubing placement. This allows for accurate job design prior to running the tubing in the hole at the well site.

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