Various factors affect coiled tubing limits

Yong S. Yang
Precision Tube Technology Inc.
Houston

Safety and reliability remain the primary concerns in coiled tubing operations. Factors affecting safety and reliability include corrosion, flexural bending, internal (or external) pressure and tension (or compression), and mechanical damage due to improper use.

Such limits as coiled tubing fatigue, collapse, and buckling need to be understood to avoid disaster.

With increased use of coiled tubing, operators will gain more experience. But at the same time, with further research and development of coiled tubing, the manufacturing quality will be improved and fatigue, collapse, and buckling models will become more mature, and eventually standard specifications will be available.

Coiled tubing

Coiled tubing is gaining wide acceptance in the oil and gas industry. Coiled tubing is a high strength, low alloy carbon steel (Young's modulus ~29 million psi and yield strength ~70,000-90,000 psi) tubular product manufactured in a continuous string, typically extending 3,000-25,000 ft in length.

The continuous tubing wrapped on a spool with a core diameter of 150 in. or less is reeled in and out of wells.

Coiled tubing can be manufactured with diameters of 1.0-5.0 in. and wall thickness of 0.08-0.25 in.

Coiled tubing is manufactured in a tube mill from a flat steel strip (typically 3,000-ft length) that is formed and welded by high frequency induction without the addition of filler metal. Coiled tubing strings can be made to a desired length by welding lengths of flat strip together prior to forming the tube in the mill.

The string undergoes extensive radiographic and electromagnetic examination in both the tube and strip form. The stress induced by the mechanical deformation in the mill forming process is relieved using an in-line, full-body induction heat treatment.

To increase the optimal performance while minimizing the total weight of the coiled string, tapered strings (constant OD with varying wall thickness) are sometimes manufactured. As the tubing is milled, it is reeled onto a spool, then hydraulically tested before being shipped for use.

Some oil and gas field applications of coiled tubing include:

Well workovers and recompletions
Drilling, including re entry, drilling underbalanced, and aerating drilling fluids with nitrogen while drilling depleted reservoirs1
Velocity strings
Well clean-outs
Plug-and-abandonments
Steam injection en hanced oil recovery (Fig. 1 (126512 bytes))
Slim-hole production tubing
Live well completion or retrieval
Multiple function strings, with cables and capillary tubes
Running logging and production tools in horizontal or highly deviated wells
Onshore and subsea pipelines.

Limits

Safety and reliability are primary concerns for using coiled tubing.

Coiled tubing undergoes fatigue from mechanical plastic deformation as the tubing is bent on and off the reel and over the gooseneck while being injected into a well.2 A number of studies have been done on theoretical modeling of the fatigue life due to flexural bending under the action of internal pressure.

All of these models are still incomplete and empirical in the sense that all of them use test data to curve-fit the Manson-Coffin empirical relation.3 4 This strain-life method can only be used to predict the crack initiation.

From crack initiation to failure, a fracture mechanics method is needed to predict the crack growth, which is not explicitly considered now in the coiled tubing industry.5 When combined with a reliable inspection technique to detect the initial crack size, the strain-life and fracture mechanics methods will completely predict the fatigue life of coiled tubing.6 This prediction model may require a joint effort of both academia and industry.

Collapse is another problem that must be considered, especially in high-pressure wells. If the external pressure becomes too high, the tubing will collapse. This could not only lead to serious well-control problems, but may result in extensive fishing operations.

A reliable coiled tubing collapse-pressure safety criterion is needed. Theoretical models of collapse pressure are well developed for perfectly round coiled tubing but not for oval tubing.7-9

Coiled tubing is initially manufactured with nearly perfect roundness, sometimes having a small ovality, typically less than or equal to 0.5%.10 Perfectly round coiled tubing becomes oval when it is cycled.

As the cycling continues, the ovality usually increases. This ovality significantly decreases the collapse failure pressure as compared to perfectly round tubing.11 12

An analytical model based on elastic instability and the von Mises criterion has been developed to predict the collapse pressure of coiled tubing with small ovality, less than or equal to 3%.10 However, the model is conservative for predicting the collapse pressure of coiled tubing with large ovality.

Recently, a numerical model with finite element method13 adapted from a collapse propagation model of offshore pipelines14 is being developed to predict collapse of coiled tubing with any ovality, especially greater ovality. The modeling results show satisfactory agreement with test data.

Finally, as coiled tubing is pushed into a deviated or horizontal well, frictional drag forces may cause the tubing to buckle and sometimes lock up. Once a lockup occurs, the tubing cannot be pushed further. Studies show that before a lockup occurs, the tubing buckles first sinusoidally and then helically.15 16 This lockup condition has been extensively studied recently.17 18

One coiled tubing service company has developed software for coiled tubing operators, which includes a tubing forces model,19 to compute the degree to which the tubing will buckle in the well. This model, along with other models developed by the industry, gives operational guidance in preventing lockup. These models are generally conservative in predicting tubing reaches. Recently, new technologies for extending additional reach after lockup have been proposed.20

Acknowledgments

I would like to express my gratitude to K. Bhalla, D. Dunlap, W.B. Moss, and L.W. Smith for their help on this article.

References

1. Eide, E., et al., "Further advances in coiled-tubing drilling," JPT, May 1995.

2. Smith, L.W., "Methods of determining the operational life of individual strings of coiled tubing," SPE Workovers and Well Intervention Seminar, 1989.

3. Tipton, S.M., and Newburn, D.A. "Plasticity and fatigue damage modeling of severely loaded tubing," Advances in Fatigue Life-time Prediction Techniques, ASTM SPT 1122, 1992, Mitchell, M.R., and Langraf, R.W., editors, pp. 369-82.

4. Newburn, D.A., and Tipton, S.M., "Strain measurement and damage analysis in low-cycle multiaxial fatigue," 4th International Conference on Fatigue and Fatigue Threshold, Honolulu, July 1990.

5. Newman, K.R., and Newburn, D.A., "Coiled tubing life modeling," Paper No. SPE 22820, Fall Meeting Society of Petroleum Engineers, 1991.

6. Papadimitriou, S., and Stanley, R., "The inspection of used coiled tubing," 2nd International Conference on CT, Houston, March 1994 and Amsterdam, Holland, June 1994.

7. Yang, Y.S., "Collapse and burst pressure of coiled tubing under tension load," Bulletin of Precision Tube Technology, Vol. 1, No. 1, Houston, 1995.

8. Newman, K.R., "Coiled tubing pressure and tension limits," Paper No. SPE 23131, Offshore Europe Conference, Aberdeen, September 1991.

9. Walker, E.J., and Mason, C.M., "Collapse tests expand coiled tubing uses," OGJ, Mar. 5, 1990, p. 56.

10. Yang, Y.S., "Collapse pressure of coiled tubing," to be presented at Energy Week '96 Conference & Exhibition, Paper No. 96025, Houston, Jan. 30-Feb. 1, 1996.

11. Avakov, V., and Taliaferro, W., "Equations determined coiled tubing collapse pressure," OGJ, July 24, 1995, pp. 36-39.

12. Fowler, J.R., Klementich, E.F., and Chappell, J.F., "Analysis and testing of factors affecting collapse performance of casing," Transactions of the ASME, Journal of Energy Resources Technology, Vol. 105, December 1983, pp. 574-79.

13. Mansour, G., and Yang, Y.S., "Numerical modeling of collapse pressure of oval tubing," submitted to OTC, 1996, Houston.

14. Mansour, G., "Quasi-static and dynamic analyses of buckle arrestor in offshore pipelines," PhD thesis, Civil engineering department, University of Texas at Austin, Texas, May 1995.

15. Lubinski, A., Althouse, W.S., and Logan, J.L., Helical buckling of tubing sealed in packers, JPT, pp. 655-670, June 1962.

16. Sorenson, K.G., "Post buckling behavior of a circular rod constrained with a circular cylinder," PhD thesis, Rice University, Houston, 1984.

17. He, X., and Kyllingsad, ., "Helical buckling and lockup conditions for coiled tubing in curved wells," SPE Drilling and Completion, March 1995.

18. Wu, J., and Juvkam-Wold, H.C., "Coiled tubing buckling implication in drilling and completing horizontal wells," SPE Drilling and Completion, March 1995.

19. Bhalla, K., "Implementing residual bend in a tubing forces model," Paper No. SPE 28303, 69th SPE Annual Technical Conference & Exhibition, New Orleans, September 1994.

20. Bhalla, K., "Coiled tubing extended reach technology," Paper No. SPE 30404, SPE Offshore Europe Conference, Aberdeen, Sept. 5-8, 1995.