Coiled Tubing Unit Definition / Meaning
A Coiled Tubing Unit (CTU) is the surface equipment package that runs continuous, spoolable steel pipe into a live well for well intervention, drilling and completion work. Unlike jointed pipe, coiled tubing is one unbroken length wound on a reel, so it can be run in and out of a well quickly and, critically, under pressure without killing the well first. That combination makes the CTU the workhorse of low-cost, low-risk intervention onshore and offshore.
Overview
A CTU is built around three assemblies: the power pack that supplies hydraulic power, the control cabin from which the operator runs the job, and the reel and injector head that handle the pipe itself. Because no connections are made up or broken out, tripping speed is measured in hundreds of feet per minute rather than stands per hour, and the pipe can be worked while fluid is pumped through it continuously.
Key Components
| Component | Function |
|---|---|
| Reel | Stores and spools the tubing, typically 1 to 3-1/2 inch outside diameter, in lengths of several thousand metres. |
| Injector head | Grips the pipe between opposed chain drives and provides the force to push it into the well against pressure and to pull it back out. |
| Gooseneck | The curved guide arch above the injector that bends the pipe from the reel into vertical alignment at a controlled radius. |
| Control cabin | Houses the operator controls, gauges, and monitoring and recording equipment for weight, depth, pressure and pump rate. |
| Power pack | Diesel-hydraulic unit driving the injector, reel drive and auxiliary functions. |
| Pressure control equipment | Stripper or packoff to seal around moving pipe, plus a coiled tubing BOP stack for well control. |
Typical Applications
- Wellbore cleanouts: circulating out sand, scale and fill, often with nitrogen or gelled fluid to lift solids.
- Nitrogen lifting: unloading a well that will not flow by lightening the fluid column.
- Acid and chemical stimulation: spotting treatment accurately across a target interval.
- Milling and drilling: removing plugs, scale or cement with a downhole motor, including plug drill-outs after multistage fracturing.
- Logging and perforating: conveying tools into highly deviated or horizontal wells where wireline cannot reach.
- Fishing and tool work: setting and retrieving downhole equipment.
Operating Limits
Coiled tubing is worked well beyond its elastic limit every time it crosses the reel and gooseneck, so the governing constraint is low-cycle fatigue. Every bend cycle is tracked against a running fatigue model, and strings are retired on accumulated damage rather than on visible condition. Pipe is also subject to ballooning under internal pressure and to diameter growth over life.
The second constraint is lock-up. Pushing pipe into a deviated or horizontal well eventually causes it to buckle helically against the wall, at which point additional surface force produces no further depth. Reach is extended with tractors, friction reducers, tapered strings or heavier wall pipe, but every horizontal well has a practical limit.
Usage Example
“They ran the CTU to drill out fourteen composite plugs in the lateral and lost weight transfer at the eleventh. Rather than push into lock-up, the crew tripped out, added a friction reducer and completed the drill-out on the second run.”
Comparison with Other Intervention Methods
Coiled tubing occupies the middle ground between wireline and a workover rig. Wireline is cheaper and faster but cannot circulate fluid or push against significant resistance. A workover rig can handle any load and pull the completion, but it is expensive, needs the well killed, and takes days to rig up. The CTU circulates, pushes, and works under live pressure, which is why it dominates routine intervention. Its limits are pipe fatigue, tensile capacity and the reach constraint above.