Introduction to Coiled Tubing (CT) in Oil and Gas

Development of the coiled tubing like we know it today started in early 1960’s. Now it is an important component of various workover and service applications. While still the use of Coil Tubing is about 75% in workover/service applications, the technical advancements have resulted in an increase in utilization of Coil tubing in drilling as well as completion applications.

Ability of performing remedial work on live well was an important driver associated with development of Coil Tubing. In order to achieve this feat, there was a need to overcome three different technical challenges: These were:

  • Continuous conduit that is capable of being inserted in to wellbore (the CT string).
  • Means of running the CT string and retrieving it in to or out of wellbore when under pressure (the injector head).
  • Device that is capable of giving a dynamic seal around tubing string (packoff or stripper device)
Coiled Tubing Unit

Coiled Tubing Unit (Credit: ShutterStock)

The Origin of Coiled Tubing

Before the invasion of allies in the year 1944, very long and continuous pipelines were developed and produced by the British Engineers for transporting the fuel from England to European Continent in order to supply Allied armies. This project was named as operation “Pluto” which meant “Pipe Lines under the Ocean”. This operation involved fabrication and laying of a number of pipelines throughout the English Channel. Successful fabrication and the spooling of the continuous flexible pipeline gave the foundation for more technical developments which eventually resulted in the tubing strings that are used today by CT industry.

In the year 1962, Bowen Tools and California Oil Company developed first fully functional Coil Tubing unit. The purpose of this unit was washing the out sand bridges in the wells.

Early Coiled Tubing Equipment

Earlier injector heads worked on principle of 2 vertical chains having contra-rotating chains. Even today this design is used in majority of Coil Tubing units today. Striper was simple sealing device of annular-type which could be activated hydraulically to seal around tubing at relatively lower wellhead pressures. Tubing string which was used for initial trials was fabricated by the butt-welding 50 feet sections of 1 3/8 in. OD pipe in to 15,000 feet string and spooling it on to a reel with core of 9 feet diameter.

Evolution of Coiled Tubing Equipment:

During late 1960’s and in to 1970’s, both Brown Oil Tools and Bowen Tools continued improving their designs in order to accommodate the CT up to 1 in. OD. By mid-1970’s, more than two hundred of original-design Coil Tubing units were in service. By late 1970’s, a number of different new manufacturing companies (Otis Engineering, Hydra Rig Inc., and Uni-Flex Inc.) also started influencing the improved design of injector heads.

The CT strings also went through significant improvements in this period. Through late 1960’s, the CT services were dominated by the tubing sizes of 1 in. and less. And the string lengths were relatively short. The tubing length and diameter were limited by tubing mechanical properties and the available manufacturing processes of that time.

The early CT operations suffered a number of failures because of inconsistent quality of tubing and various butt welds required for producing suitable string length. However, by late 1960’s, the tubing strings were being milled in longer lengths and the butt welds were fewer for each string. The properties of steel also saw major improvements during the time. These changes and associated improvements in the reliability of CT string contributed a great deal to the continuous growth of CT industry.

Today it is common for the CT strings to be constructed from the continuously milled tubing which can be manufactured without any butt welds. Additionally, the CT diameters have continued growing to keep up the pace with requirements of strength associated with the new market applications. It is not unusual for the CT diameters of up to 2 7/8 in. to be available readily for the routine use.

Coiled Tubing Can Perform Various Tasks

Coiled Tubing Can Perform Various Tasks

Coiled Tubing Applications

Nowadays, coiled tubing is widely used in several operation and these are operations conducted by coiled tubing units.

Work String

− Fluid placement

  • Nitrogen Lift
  • Cement Plugs
  • Cement Squeeze
  • Acid Wash
  • Fracture Stimulation
  • Water Shut-off
  • Scale Squeeze

− Tool Conveyance /Tool Manipulation

  • Perforating Guns
  • Plugs
  • Production Logging
  • Video cameras
  • Fishing

− Wellbore Cleanout

  • Jetting
  • Milling
  • Scale removal

Completion String

− Velocity String

− Gas Lift String

− ESP deployment

− Rod pump deployment

− Straddle repair

− Sand Control

Drill String

− Coiled tubing drilling using mud motors

− Side track and wellbore deepening

− Under-balanced Drilling

References

Jr. Adam T. Bourgoyne, 1986. Applied Drilling Engineering (Spe Textbook Series, Vol 2). Edition. Society of Petroleum Engineers.

J.J. Azar, 2007. Drilling Engineering. Edition. PennWell Corp.

George E. King. 2009. Coiled Tubing Introduction. [ONLINE] Available at: http://gekengineering.com/Downloads/Free_Downloads/Coiled_Tubing_Surface_Equipment.pdf. [Accessed 14 August 2016].

Worl Oil , 2005. Coiled Tubing Handbook. 1st ed. Texas: World Oil.

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