Coiled tubing is a continuous, flexible steel pipe used primarily in oil and gas wells for maintenance, cleaning, drilling, and boosting production. Unlike conventional rigid pipe that must be assembled one joint at a time, coiled tubing comes wound on a large reel and can be fed into a well continuously, which makes operations faster and often cheaper. Its applications range from simple wellbore cleanouts to complex deepwater interventions thousands of feet below the ocean surface.
How Coiled Tubing Works
Coiled tubing is exactly what it sounds like: a long, continuous length of steel tubing stored on a spool. The tubing comes in outer diameters ranging from 0.75 inches up to 5 inches, with wall thicknesses between roughly 2 mm and 8.6 mm depending on the job. A specialized truck or platform-mounted unit unspools the tubing, feeding it down into the well through a device called an injector head that grips and pushes the pipe.
Because the tubing is continuous, there’s no need to stop and screw together individual pipe sections. This saves significant time. Fluids, chemicals, or nitrogen can be pumped through the tubing while it’s moving, and tools can be attached to the end for tasks like cutting, measuring, or perforating. The whole system can often be rigged up and operating in a fraction of the time a conventional workover rig would require.
Wellbore Cleanouts
The most common use of coiled tubing is cleaning out wells. Over time, sand, scale, wax, and other debris accumulate inside a wellbore and restrict or block the flow of oil or gas. Coiled tubing is lowered to the problem zone, and fluid is circulated through it to wash out the blockage and carry debris back to the surface. This is straightforward, effective work that keeps producing wells online.
A field trial in Oman’s Marmul field demonstrated how pairing a light workover rig with a coiled tubing unit for cleanout and stimulation returned wells to production within a week, compared to months using older sequential methods. That kind of turnaround matters when every day of downtime means lost revenue.
Well Stimulation and Acidizing
When a well’s production declines because the surrounding rock has become less permeable, coiled tubing can deliver acid or other chemicals directly to the problem zones. This process, called acidizing, dissolves mineral buildup and opens up flow paths in the reservoir rock, restoring or improving the well’s output.
The key advantage of using coiled tubing for this work is precision. Rather than pumping acid from the surface and hoping it reaches the right depth (a technique called bullheading), coiled tubing can place the treatment at a specific location downhole. In practice, however, this precision has limits. Geothermal wells in New Zealand that were acidized using coiled tubing showed that the denser acid mixture tended to sink to the bottom of the well rather than staying at the targeted zones. Diverting fluid to specific depths remains a technical challenge, and in some cases surface pumping achieves similar results at lower cost.
Beyond acidizing, coiled tubing delivers fracture stimulation treatments, where fluids are pumped at high pressure to crack the reservoir rock and create new pathways for oil or gas to flow.
Drilling With Coiled Tubing
Coiled tubing isn’t just for maintenance. It can also drill new sections of a well, particularly in situations where conventional drilling rigs would be impractical or too expensive. A downhole motor attached to the end of the tubing spins the drill bit, since the coiled tubing itself doesn’t rotate like traditional drill pipe.
This approach works especially well for re-entering existing wells to drill new lateral sections, reaching untapped pockets of oil or gas without the cost of drilling an entirely new well. It’s also suited for highly deviated or horizontal wells where the tubing’s flexibility is an advantage. Coiled tubing drilling can operate in underbalanced conditions, meaning the pressure in the wellbore is kept lower than the reservoir pressure. This protects the reservoir from damage and can allow the well to produce even while drilling continues.
Advances in downhole turbine motors have pushed coiled tubing drilling further, enabling rotary speeds up to 3,000 rpm. Higher speeds mean faster drilling rates with less weight on the bit, which extends bit life and improves hole quality.
Logging and Perforating
To understand what’s happening inside a well, operators need to lower measurement instruments, called logging tools, down the wellbore. In vertical wells, gravity does most of this work. But in horizontal or highly deviated wells, gravity won’t carry tools sideways. Coiled tubing solves this by physically pushing logging instruments to the zones of interest.
Systems have been developed that embed electrical conductors inside the coiled tubing itself, allowing real-time data transmission from logging tools back to the surface. This means operators can monitor wellbore conditions, temperature, pressure, and flow while maintaining full control of the well. The same push capability makes coiled tubing valuable for perforating, the process of punching holes through the steel casing and cement into the reservoir rock so oil or gas can flow into the well. Coiled tubing positions perforating charges precisely and holds them stable during detonation.
Velocity Strings for Gas Wells
Gas wells often develop a problem called liquid loading as they age. Reservoir pressure drops, and the gas no longer flows fast enough to carry liquids (water or condensate) up and out of the well. These liquids pool at the bottom, further choking off production until the well eventually dies.
One solution is installing a smaller-diameter coiled tubing string permanently inside the existing production tubing. This narrower pipe, called a velocity string, reduces the cross-sectional area the gas flows through. With less space, the gas moves faster. When velocities are high enough, the gas suspends liquid as a fine mist and carries it to the surface, preventing accumulation. For the well to stay free of liquid loading, the flow regime inside the tubing needs to remain in what engineers call annular-mist flow, where at least 95% of the stream is gas and the liquid travels as tiny suspended droplets.
Offshore and Deepwater Operations
Coiled tubing has become increasingly important offshore, where the cost of mobilizing a full drilling rig can run into millions of dollars per day. Subsea coiled tubing services can handle stimulation, chemical injection, and flowline cleanouts without a rig, deploying in water depths up to 10,000 feet. These systems operate at pressures up to 15,000 psi while pumping fluids at rates exceeding 10 barrels per minute, all with real-time monitoring.
The industry is also moving toward umbilical-less intervention, where coiled tubing systems operate subsea without a permanent control line connecting them to the surface. This represents a shift toward lighter, more flexible offshore well maintenance that avoids tying up expensive drilling rigs for routine work.
Limitations and Fatigue Life
Coiled tubing has a finite lifespan, and that lifespan is governed largely by fatigue. Every time the tubing is unspooled from the reel, straightened, bent over the guide arch, and then reversed back onto the reel, it undergoes a bending cycle. Steel can only tolerate so many of these cycles before it develops cracks and fails.
For ultra-deepwater systems, fatigue analysis shows that a single deployment-and-recovery cycle consumes roughly 1.3% of the tubing’s total fatigue life, giving approximately 78 allowable operations before the tubing must be retired. Operators track bend cycles meticulously, and each section of tubing has a fatigue life log that determines when it needs to be cut back or replaced. High-cycle fatigue from wave and current loading in subsea environments adds to the damage, making fatigue management one of the most critical aspects of coiled tubing operations.
The tubing also can’t transmit rotational torque like conventional drill pipe, which limits its drilling capability to whatever a downhole motor can provide. And while coiled tubing excels at speed and mobility, it has lower load capacity than rigid pipe, restricting its use in very deep or very heavy operations.