Helical Buckling and Lock-Up Conditions for Coiled Tubing in Curved Wells
- Xiaojun He (Rogaland Research) | Age Kyllingstad (Rogaland Research)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1995
- Document Type
- Journal Paper
- 10 - 15
- 1995. Society of Petroleum Engineers
- 3.1.6 Gas Lift, 1.6 Drilling Operations, 3 Production and Well Operations, 1.14 Casing and Cementing, 4.3.4 Scale
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An improved formula for critical buckling forces has been derived. This formula, which takes the well curvature into account, has been verified in small scale laboratory tests. The theory has been applied to survey data from a real horizontal well and it predicts that the well curvature substantially affects the critical force for helical buckling, and thereby also the maximum run-in length of coiled tubing. Criteria for operational limits, such as lock-up and tubing failure, are also discussed in the paper.
Coiled tubing has numerous applications in well technology. Coiled tubing has been found useful for logging, well clean outs, well stimulation, gas lift and cementing. Encouraging attempts at drilling with coiled tubing have recently been carried out. Coiled tubing is also used as a stiff wireline for a number of tool operations in highly deviated wells.
One of the main limitations associated with coiled tubing is assumed to be helical buckling and the additional wall friction force generated by buckling. When axial compression forces over a critical value are applied on coiled tubing, the coiled tubing will buckle. The coiled tubing will first buckle into a sinusoidal wave shape. As the compression force increases further, it will ultimately deform into a helix. Confined to the wellbore, the helically buckled coiled tubing will be forced against the wall of wellbore and additional contacting forces developed.
The force needed to push coiled tubing into a well increases dramatically once the coiled tubing is forced into a helix. The frictional drag developed as coiled tubing is forced against the hole or casing wall will ultimately overcome the pushing forces. This phenomena is called lock-up.
The critical buckling force in inclined well sections is currently determined by the Dawson formula for sinusoidal buckling and the Chen et al formula for helical buckling. These formula are currently used as the operational criteria for coiled tubing. In practice, it is often found that the operational force can be significantly larger than the theoretical buckling force, and the operations still be successful. There is considerable field evidence that using the critical buckling force as operational limit is too conservative.
Two major shortcomings exist for these formulas and the way buckling analysis is performed:
- The well curvature effects on the critical buckling force are not considered in these formulas.
- The previously published analysis method is restricted to subcritical forces and no post-buckling is considered. It is incorrectly assumed that operations are not feasible if the axial compression force anywhere exceeds the critical buckling force.
The above two points have been addressed in a recent publication, but no detailed information is given.
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