Introduction to Coiled Tubing
History of Coiled Tubing
The development of coiled tubing as we know it today dates back to the early 1960's, and it has become an integral component of many well service and workover applications. While well service/workover applications still account for more than 75% of CT use, technical advancements have increased the utilization of CT in both drilling and completion applications.
The ability to perform remedial work on a live well was the key driver associated with the development of CT. To accomplish this feat, three technical challenges had to be overcome:
- A continuous conduit capable of being inserted into the wellbore (CT string).
- A means of running and retrieving the CT string into or out of the wellbore while under pressure (injector head).
- A device capable of providing a dynamic seal around the tubing string
(stripper or packoff device).
Prior to the Allied invasion in 1944, British engineers developed and produced very long, continuous pipelines for transporting fuel from England to the European Continent to supply the Allied armies. The project was named operation "PLUTO", an acronym for "Pipe Lines Under The Ocean", and involved the fabrication and laying of several pipelines across the English Channel. The successful fabrication and spooling of continuous flexible pipeline provided the foundation for additional technical developments that eventually led to the tubing strings used today by the CT industry.
In 1962, the California Oil Company and Bowen Tools developed the first fully functional CT unit, for the purpose of washing out sand bridges in wells.
You can download the PLUTO video in MPEG format or order a CD from ICoTA.
Early CT Equipment
The first injector heads operated on the principle of two vertical, contra-rotating chains. This design is still used in the majority of CT units today. The stripper was a simple, annular-type sealing device that could be hydraulically activated to seal around the tubing at relatively low wellhead pressures. The tubing string used for the initial trials was fabricated by butt-welding 50 ft. sections of 1 3/8 in. OD pipe into a 15,000 ft. string and spooling it onto a reel with a 9 ft. diameter core.
Evolution of CT Equipment
Throughout the late 1960's and into the 1970's, both Bowen Tools and Brown Oil Tools continued to improve their designs to accommodate CT up to 1 in. OD. By the mid-1970's, more than 200 of the original-design CT units were in service. By the late 1970's, several new equipment manufacturing companies (Uni-Flex Inc., Otis Engineering, and Hydra Rig Inc.) also started influencing improved injector head design.
CT strings were also undergoing significant improvements during this period. Through the late 1960's, CT services were dominated by tubing sizes of 1 in. and less, and relatively short string lengths. Tubing diameter and length were limited by the tubing mechanical properties and currently available manufacturing processes.
Early CT operations suffered many failures due to the inconsistent quality of the tubing and the numerous butt welds required to produce a suitable string length. However, by the late 1960's, tubing strings were being milled in much longer lengths with fewer butt welds per string. During this time, steel properties also improved. These changes and the associated improvement in CT string reliability contributed greatly to the continued growth of the CT industry.
Today it's common for CT strings to be constructed from continuously milled tubing that can be manufactured with no butt welds. In addition, CT diameters have continued to grow to keep pace with the strength requirements associated with new market applications. It's not unusual for CT diameters of up to 2 7/8 in. to be readily available for routine use.