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Abstract As coiled tubing services expand to meet the challenges of the oil and gas industry, limitations of steel tubulars continue to temper the potential impact of this workover service. Presently, coiled tubing used in well servicing operations is milled from modified A-606 High Strength Low Alloy (HSLA) steel, with yield strengths ranging from 70 ksi to 100 ksi. Although current coiled tubing services can performed safely and reliably with conventional steel coiled tubing, the behavior of the isotropic metal limits the yield pressure and tensile load capability of the tube. Composite Coiled Tubing (CCT) offers the potential to exceed the performance limitations of isotropic metals, thereby increasing the service life of the pipe and extending operational parameters. CCT is constructed as a continuous tube fabricated from non-metallic materials to provide high body strength and wear resistance. CCT can be tailored to exhibit unique anisotropic characteristics which optimally address burst and collapse pressures, tensile and compression loads, as well as high strains imposed by bending. This enabling capability expands the performance parameters beyond the physical limitations of steel or alternative isotropic material tubulars. In addition, the fibers and resins used in CCT construction make the tube impervious to corrosion and resistant to chemicals used in treatment of oil and gas wells. Based on the performance of prototype specimens constructed and tested to date, the service life potential of CCT is substantially longer than that of conventional HSLA steel pipe when subjected to multiple plastic deformation bending cycles with high internal pressure. In addition, CCT is expected to provide the ability to extend the vertical and horizontal reach of existing concentric workover services. This paper reviews the limitations of conventional HSLA steel coiled tubing, explores the design criteria requirements for CCT, and discusses the results of tests performed on numerous CCT prototype specimens. Introduction Coiled tubing technology was developed for the purpose of performing workover services in oil and gas wellbores concentric to existing production tubulars. The operational concept of a coiled tubing system involves the deployment of a continuous string of small diameter tubing into a wellbore to perform a specific well service procedure without disturbing the existing completion tubulars and equipment. When the service is completed, the small diameter tubing is retrieved from the wellbore and spooled onto a large reel for transport to and from work locations (Figure 1). The coiled tubing unit is designed to perform numerous workover services with or without surface pressure present on the well. The primary equipment components which most affect the performance of the tubing string include the injector head, tubing guide, and service reel. |
