Coiled tubing is a continuous length of flexible steel pipe, manufactured without joints and wound onto a large reel for transport and deployment. It’s a workhorse of the oil and gas industry, used to perform maintenance, cleanouts, and even drilling inside wells. Unlike traditional pipe that must be screwed together one section at a time, coiled tubing unspools in one unbroken strand, sometimes stretching over 30,000 feet, and can be run into a well continuously without stopping to make connections.
How Coiled Tubing Is Built
The tubing starts as flat steel strip that gets continuously milled and shaped into a round tube during manufacturing, then spooled directly onto a take-up reel. The result is a single, jointless pipe that can flex enough to wrap around a drum yet remain strong enough to withstand the pressures found thousands of feet underground.
Standard outer diameters range from 0.75 inches to about 4 inches, with wall thicknesses typically between 0.125 and 0.276 inches depending on the application. A common configuration is around 1.25 to 1.5 inches on the outside and roughly 1.25 inches on the inside. Most reels in the field carry about 20,000 feet of tubing, though single-reel lengths exceeding 30,000 feet have been commercially produced.
The steel used is specifically engineered for repeated bending. Every time the tubing unspools off the reel, passes over a guide arch, and enters the well, it undergoes a cycle of bending and straightening. Industry-standard grades (designated CT70 through CT110 under API Specification 5ST) have yield strengths ranging from 55,000 to 120,000 psi, giving operators a range of options to match well conditions. Higher-strength grades resist fatigue better but cost more, so the choice depends on how deep the well is, what pressures are expected, and how many times the tubing will be cycled.
The Four Parts of a Coiled Tubing Unit
A complete coiled tubing unit has four core components that work together:
- The reel: A large spool, often mounted on a truck or trailer, that stores and transports the tubing string. It rotates to pay out or take up tubing during operations.
- The injector head: This sits directly over the wellhead and does the heavy lifting. It uses hydraulically driven chain assemblies with specially shaped gripping blocks to grab the tubing and push it into the well or pull it back out. The injector provides the force needed to overcome wellbore pressure that would otherwise push the tubing back up.
- The power pack: A self-contained unit (usually diesel-driven) that generates the hydraulic and pneumatic power for the injector, reel, and other equipment.
- The control cabin: Where the operator sits, monitoring tubing depth, weight, pressure, and pump rates while controlling the speed and direction of the tubing.
What Coiled Tubing Is Used For
The biggest advantage of coiled tubing is speed. Because there are no pipe connections to make or break, the tubing moves in and out of the well continuously. That saves hours or even days compared to conventional jointed pipe on operations where trip time matters. It also means the well can stay under pressure the entire time, since the injector head and wellhead equipment form a sealed system.
Common jobs include:
- Wellbore cleanout: Pumping fluid through the tubing to wash out sand, scale, or debris that has accumulated at the bottom of a well and is restricting production.
- Stimulation: Delivering acid or other treatment fluids directly to a specific zone in the well to improve flow from the reservoir.
- Nitrogen kickoff: Injecting nitrogen gas through the tubing to lighten the fluid column in a well and help it start flowing again.
- Logging and perforating: Running measurement tools or perforation guns on the end of the tubing to evaluate or open up producing zones.
- Cementing: Placing cement plugs at precise depths for well isolation or abandonment.
- Milling: Attaching a cutting tool to the end of the tubing to grind away obstructions like plugs, collapsed casing, or hardened deposits inside the well.
Coiled Tubing Drilling
Beyond intervention work, coiled tubing can also be used to drill new wells or extend existing ones. Coiled tubing drilling is particularly attractive in mature fields where wells are shallower and conventional rigs would be overkill. Studies comparing the two approaches have found coiled tubing drilling delivers average cost savings of about 37% over conventional drilling, with the range falling between 22% and 48% depending on conditions.
One technique called underbalanced coiled tubing drilling keeps the pressure inside the wellbore lower than the reservoir pressure, which prevents drilling fluids from damaging the producing formation. This approach has been shown to improve ultimate oil recovery by an average of 17% compared to standard overbalanced drilling, and accessing depleted reservoirs this way can add roughly 35% to total recovery. The trade-off is risk: drilling underbalanced raises the chance of wellbore collapse and pipe sticking, which can wipe out cost savings if things go wrong.
Real-Time Monitoring With Smart Coiled Tubing
Traditional coiled tubing operations rely on surface readings to infer what’s happening thousands of feet below. Smart coiled tubing changes that by embedding either an insulated electrical conductor or a fiber-optic cable inside the tubing wall. These internal wires transmit real-time pressure, temperature, and depth data from sensors at the bottom of the string back to the surface control cabin.
Electrical conductor systems send power down the wire to run the sensors, while fiber-optic systems require batteries in the downhole tool package. Fiber optics offer an additional capability called distributed temperature sensing, which measures temperature along the entire length of the tubing rather than just at the bottom. This lets operators see exactly where fluids are entering or exiting the wellbore during stimulation jobs, so they can adjust chemical placement on the fly instead of running a separate logging trip afterward. Offshore Mexico, this combination of coiled tubing with internal fiber optics and distributed temperature surveys has been used to selectively place treatment fluids in target zones during a single trip into the well, eliminating what previously required multiple runs and radioactive tracers.
Limitations Worth Knowing
Coiled tubing isn’t a replacement for conventional pipe in every situation. The tubing has a finite fatigue life because each trip in and out of the well bends and straightens it, gradually weakening the steel. Operators track bending cycles carefully and retire tubing strings before they reach their fatigue limit.
The tubing also can’t rotate easily the way jointed drill pipe can, which limits its ability to transfer torque for certain drilling or milling operations. And because the tubing is a fixed diameter from top to bottom, there’s no way to increase stiffness or flow capacity partway down the string. In deep or high-pressure wells, the weight of the tubing itself and the friction against the wellbore can consume most of the available push force from the injector, limiting how far the tubing can reach. For these reasons, coiled tubing tends to complement rather than replace conventional rigs, excelling in lighter-duty, speed-sensitive operations where its continuous deployment and live-well capability give it a clear edge.