Coiled tubing has been used as a well intervention tool since the early 1960's. Since its introduction, the diameter of coiled tubing available has steadily increased, driven by demands for higher flow rates as well as larger push and pull forces. However, bigger coiled tubing sizes require larger equipment and in many cases, the simple physical size of the equipment becomes unworkable, particularly for offshore applications.
Some of the most common reasons for not using coiled tubing are:-
The platform and/or platform crane is not big enough for coil tubing size thought to be required.
The available or manageable coiled tubing size cannot access the bottom of the well.
Insufficient pull or push is available using the available or manageable coiled tubing size.
The low fluid rates achievable through the available coiled tubing make the job time too great.
This paper addresses each of the above arguments put forward against the use of coiled tubing. The paper shows how advances in technology have, to a large extent, removed these perceived barriers, most notably for the most common coiled tubing activities, these being fill cleanouts, gas lifting and acidizing/stimulation. Several technologies have emerged recently that enable small, light coiled tubing units to be used where previously larger units were thought necessary.
The strength of coiled tubing lies in the fact that it is a continuous length of pipe, with no joints and no upsets. Continuous pipe is easy to strip in and out of live wells, even against very high wellhead pressures. It is also fast and safe as no manual operations are required at the wellhead while tripping.
At the same time, the fact that coiled tubing is one continuous length can be its own down fall. Two limitations arise from having no such joints: -
The coiled tubing generally has to be picked up in one single lift. This can be a very significant lift, in excess of what many offshore cranes can achieve.
Pump rates can be limited as the coiled tubing diameter is relatively small and there may be a significant length of coiled tubing remaining on the drum, when the end of the coiled tubing is in the well. This pipe on the drum is a source of significant fluid friction, so limiting pump rates.
The advantages of continuous pipe outweigh the disadvantages generally speaking when the size of coiled tubing used is small (11/2" outside diameter or smaller). Even moderately long strings of 11/2" coiled tubing can be picked up by many offshore platform cranes. (e.g. 14,000ft of 11/2", 0.109" wall coiled tubing on a lightweight metal drum weighs about 12 tonnes). The equipment required to run such small sizes is also generally manageable.
When the coiled tubing size exceeds 11/2", the weight of the coiled tubing and the associated equipment can become more than can be accommodated, particularly in a marine environment (e.g. 12,000ft of 23/8", 0.156" wall coiled tubing on a steel drum weighs about 24 tonnes).
An obvious solution is then to only use small diameter pipe. Historically, this often cannot be done due to the reasons listed above. However, new technologies have greatly extended the capabilities of small coiled tubing. Coiled tubing operations, most notably offshore rigless applications, can now be conducted, where they may not have been possible a short time ago. The following sections examine in further detail why smaller coiled tubing was previously unsuitable, and shows how the new technologies have now enabled its use.