Coiled tubing is an old resource industry tool that continues to pay new dividends. The concept is simple: feed a continuous length of steel tubing into a wellbore, then use that access to insert anything from fluids to specialized tools to stimulate production. 

Coiled tubing’s advantage over jointed tubing is the ability to work on a live well without first killing production. Coiled tubing operations also offer increased speeds in delivering payloads with fewer personnel. And, while wirelines rely on gravity to deliver their payloads, the rigidity of coiled tubing allows it to be pushed into the wellbore, even in horizontal drilling applications. 

The concept dates back to WWII when the Allies established project PLUTO (Pipe-Lines Under The Ocean), which used coiled tubing to feed fuel across the English Channel to France, replacing vulnerable ocean tankers. 

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Today, coiled tubing with diameters between 1.25 and 3.5 inches delivers a slate of services including acid/solvent stimulations, cement squeezing/plugging, fluid unloading, scale removal, velocity string installation, wellbore clean-outs, tool introduction or retrieval, and coiled tubing fracturing. 

Multiple variables

“The limitations of the technology are based on how much tubing can be carried on a single reel,” says Saad Hamid, technical specialist, Coiled Tubing and Nitrogen Services, Client Solutions with Sanjel Corporation. “The second variable is the power of the injector head, which uses a set of continuous chains to feed the tubing into the wellbore. The deeper the well, however, the larger the diameter of the tube we would want to insert.” 

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Each project begins with an examination of the conditions of the wellbore. Engineers examine reservoir information and look at the history of production to determine why the well is not producing to client expectations. The job is then designed to suit the project. A sand clean-out operation, for example, can be achieved by pumping fluid down the hole. If the problem is caused by a tool jammed into the wellbore a fishing operation is devised, using a latch tool. 

A computer simulation is then run to determine the maximum reach of tubing in a range of diameters. Coiled tubing can be inserted to a length of up to 18,000 feet, Hamid says. 

“Once we determine the maximum diameter and length of the tubing, it’s delivered to the site in coils,” he says. “The tubing is straightened out through a guide arch and then inserted by the injector. Friction isn’t really a factor until the resistance exceeds the snubbing force of the injector. However, we typically only see this in horizontal applications where friction is greater. At that point we can extend the reach of the tubing through mechanical or fluid-activated tools. The most likely cause of any kinking or deformation in the tube is typically junk that was left behind in the wellbore.” 

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A fluid-activated tool known as an indexer is used to induce the tubing to bend in the direction desired by the operator. 

New methods and technology

Suretech Completions, Sanjel’s sister company, is using coiled tubing to develop methods of extending the reach and efficiency of shale gas operations. A new technology, for example, allows operators to more easily retrieve fracturing sleeves. 

Coiled tubing also continues to maintain a niche in the drilling industry. It’s typically used in “sidetracking” — drilling in new directions from a wellbore’s existing production tubing to exploit additional production possibilities. Hybrid drill rigs can also use rotary drills to begin a traditional wellbore, then use coiled tubing technology to branch out and achieve completion. 

Hamid says that coiled tubing operations require a much smaller footprint than traditional drilling. “It’s nothing like the mini-city that springs up around a drill rig,” he says. “Depending on the operation, coiled tubing requires from three to seven people on a very small footprint. Mobilizing and demobilizing the operation is extremely quick and efficient.” 

After 10 years in the industry, Hamid says he enjoys an engineering discipline where every project provides unique challenges. “We haven’t reached the limit of coiled tubing yet,” he says. “I think it’s still the most promising technology in the industry.”

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