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Introduction to Coiled Tubing


The Tubing
The manufacture of CT involves multiple steps, and the following contains an overview of the key components involved in the manufacturing process.

  • Raw Material for CT
  • CT Manufacturing
  • CT Mechanical Performance
  • CT String Design
  • CT Inspection Tools
  • Repairs and Splicing
  • Alternatives to Carbon Steel CT

Raw Material for CT
Virtually all CT in use today begins as large coils of low-alloy carbon-steel sheet. The coils can be up to 55 in. wide and weigh over 24 tons. The length of sheet in each coil depends upon the sheet thickness and ranges from 3,500 ft. for 0.087 in. gauge to 1,000 ft. for 0.250 in. gauge.

CT Manufacturing
At the end of 2003, two companies supplied all of the steel CT used by the petroleum industry. Quality Tubing and Precision Tube Technology (PTT) each have manufacturing facilities in Houston, TX. The first step in tube making is to slice flat strips from the roll of sheet steel, and this step is usually performed by a company specializing in this operation. The strip's thickness establishes the CT wall thickness and the strip's width determines the OD of the finished CT.

The steel strips are then shipped to a CT mill for the next step in the manufacturing process. The mill utilizes bias welds to splice the flat strips together to form a single continuous strip of the desired CT string length. The mechanical properties of the bias strip welds almost match the parent strip in the as-welded condition, and the profile of the weld evenly distributes stresses over a greater length of the CT. The CT mill then utilizes a series of rollers to gradually form the flat strip into a round tube. The final set of rollers forces the two edges of the strip together inside a high frequency induction welding machine that fuses the edges with a continuous longitudinal seam. This welding process does not use any filler material, but leaves behind a small bead of steel (weld flash) on both sides of the strip.

The mill removes the external bead with a scarfing tool to provide a smooth OD. The weld seam is then normalized using highly localized induction heating. Next, the weld seam is allowed to cool prior to water cooling. Full tube eddy current or weld seam ultrasonic inspection may also be performed, depending upon the mill setup. The tubing then passes through sizing rollers that reduce the tube OD slightly to maintain the specified manufacturing diameter tolerances. A full body stress relief treatment is then performed to impart the desired mechanical properties to the steel. Subsequent to the CT being wound on a shipping reel, the mill flushes any loose material from the finished CT string.

CT Mechanical Performance
The mechanical performance of CT is fundamentally different from all other tubular products used in the petroleum industry because CT is plastically deformed with normal use. Plastic deformation of material imparts fatigue on the CT string, and fatigue continues to accumulate over the life of the CT string, until such time as fatigue cracks develop, resulting in a CT string failure. [Plastic deformation can be described as deformation that remains after the load causing it is removed. Fatigue can be defined as failure under a repeated or otherwise varying load, which never reaches a level sufficient to cause failure in a single application.]

For standard CT operations, the tube is plastically deformed as the tube is straightened coming off the reel at point 1 as shown below in Figure 9 below. It is then bent at point 2 as it moves onto the guide arch, and is straightened again at point 3 as it travels to the injector and enters the wellbore. The CT string is then plastically deformed at the same three points during retrieval from the well.

CT service providers utilize sophisticated CT fatigue modeling software and field data acquisition systems to track the operating history of the CT string as it is utilized in the field. This operating history allows the CT string life to be monitored, and the string replaced prior to failure. Figure 15 contains a sample screen capture from a fatigue modeling software program, depicting the amount of CT life that has been spent during two downhole operations.

CT String Design
The length of CT on a reel varies depending on diameter. For comparison, a small reel may only be able to hold 4,000 ft. of 2 7/8 in. tubing, but may have a 15,000 ft. capacity if 1 1/2 in. tubing is spooled on it. A properly sized CT string must have the following attributes for the planned operation:

  • Enough mechanical strength to safely withstand the combination of forces imposed by the job
  • Adequate stiffness to RIH to the required depth and/or push with the required force
  • Light weight to reduce logistics problems and total cost
  • Maximum possible working life

Optimizing the design of a CT string to simultaneously meet the criteria shown above for a given CT operation requires a sophisticated CT simulator and numerous iterations with proposed string designs. CT strings designed in this manner usually will contain multiple sections of differing wall thickness. Often called "tapered strings", the wall thickness does not necessarily taper smoothly from thick to thin (top to bottom). Instead, the wall thickness will vary according to the position in the string. However, the OD of the string will remain constant.

The simplest method of designing a CT string considers only the wall thickness necessary at a given location for the required mechanical strength and the total weight of the string. This method assumes the open-ended CT string is hanging vertically in a fluid with the buoyed weight of the tubing being the only force acting on the string. Starting at the bottom of the string and working up, the designer selects the wall thickness at the top of each section that provides sufficient tensile force at that location.

CT Inspection Tools
In addition to the practical reason for determining whether CT can safely pass through the surface equipment and be gripped properly by the injector, real-time measurements of tubing geometry are crucial for avoiding disastrous failures. To determine the suitability of a CT string for proposed operation, one must determine if:

  1. The stresses in the wall of the tubing caused by pressure and axial forces will exceed the yield stress of the material, and
  2. The accumulated fatigue in any segment of the string will exceed a predetermined safe limit during the course of the operation.

Tubing geometry has a direct, significant effect on both issues.

Multiple tools capable of measuring external CT geometry have been used in the CT industry. These tools measure the tubing OD on several radials at a given cross section to determine the ovality and diameter of the CT. More recently, several "full-body" CT inspection tools, with the ability to detect tubing wall flaws as well as providing tubing wall thickness and geometry measurements, have been utilized. Real-time inspection systems are being used during offshore operations to assure total integrity of the coiled tubing.

Repairs & Splicing
The only acceptable method of repairing mechanical or corrosion damage to a CT string is to physically remove the bad section of tubing and rejoin the ends with a temporary or permanent splice.

A temporary splice consists of a mechanical connection that is formed with a tube-tube connector. This type of connection is typically not used for prolonged operations during a CT job, but rather as an emergency repair to allow the CT string to be pulled out of the hole.

However, connector technology continues to evolve and there are certain situations where connectors are used, such as to connect the tool string to the end of the CT. There are three general types of connectors, including the grapple, setscrew/dimple, and roll-on connector. Connector selection is based on the particular operation to be performed, as each type incorporates unique features that make it best-suited for a given application.

Only butt welds are possible for field welding repair of CT strings, with TIG welding being the preferred method for permanent repair of CT work strings. The low heat input and the slow deposition rate of this technique make it ideal for use with CT. The CT industry has three generally accepted TIG welding techniques:

  • Manually, with hand-held tools
  • Semi-automatically, with manual preparation with an automatic orbital welder
  • Fully-automatically, with a robotic orbital welder

All three methods can produce high quality welds. However, even the best repair weld has no more than 50% of the fatigue life of the virgin tubing.

Addressing Offshore Weight & Space Limitations
CT operations on many offshore platforms are constrained by the lifting capacity of the crane, as well as deck loading and space limitations. A loaded CT reel is typically the heaviest component of the CT system. Various solutions to address this issue have been successfully implemented in the field, including:

  1. Disassembling the CT equipment into the smallest, lightest lifts possible, and reassembling the equipment on the platform.
  2. Cut the CT string into sections, spool the sections onto lightweight shipping reels, lift the reels onto the platform, then reconnect the sections on the platform.
  3. Use a barge or jackup with a heavy-lift crane to hoist all of the CT equipment onto the platform
  4. Lift the CT unit, minus the CT string, onto the platform. Then spool the CT string onto the work reel from a loaded reel on a floating vessel.
  5. Install only the CT injector on the wellhead, leaving the CT reel and other CT unit components on a barge, workboat, or jackup, positioned alongside the platform.

The first four options can be applied where crane lift capacity is the controlling factor. Option 2 has been applied successfully numerous times in the North Sea, and requires high quality CT welding services to be available. Options 3-5 require more equipment and personnel versus that of typical CT operations, with an associated increase in the cost of the CT operation. Option 3 is rarely used, due to the high cost and scarcity of floating cranes.

Extending the CT Envelope-Downhole Tractors
In some applications, such as wellbores with long horizontal sections, the inherit strength of the CT may not be adequate for the intended downhole task or CT lockup may prevent the CT string from being able to reach the desired depth.

In many cases, these issues can be overcome and the CT operation can be successfully completed with the addition of a downhole tractor to the CT string. A downhole tractor can pull or push on the end of the CT string, enabling it to successfully reach the target depth and/or be able to apply the required downhole force (e.g. operate a sliding sleeve).

Tractors are deployed on the downhole end of the CT string and are powered by hydraulic motors driven by the flow of fluid through the CT. Various tractor designs provide the ability to pull or push on the downhole end of the CT string, as directed by surface computer control systems. Some tractors can supply up to 11,000 lbs. of force to pull or push the CT string, and can operate at speeds of up to 30 feet per minute.

Alternative to Carbon Steel CT
Conventional carbon steel CT is more than adequate to meet the needs of most field operations. However, some corrosive downhole environments dictate the use of improved CT materials. QT-16Cr is a relatively new corrosion resistant alloy (CRA) that was specifically developed for long term direct exposure to wet CO2 environments. QT-16Cr was commercially introduced in early 2003, and more than 30 tubing strings were in service a year later. Much of the early application was for permanent installations as a velocity string in environments containing wet CO2 and saline conditions. It has been installed to depths greater than 18,000 ft.

The commercial appeal of QT-16Cr goes beyond its favorable corrosion resistance characteristics. The material has also exhibited much improved abrasion resistance (approximately 1/4th the material loss vs. a well known 45 HRC low alloy steel) as well as demonstrating superior low cycle fatigue life when compared to it's equivalent in carbon steel. This data indicates the grade may be an excellent candidate for future CT work string applications.

To learn more about different types of coiled tubing grades, please visit www.nov.com/qualitytubing

HS-80-CRA is another CRA material being developed for use in downhole completion application in H2S and CO2 environments. This product is a lean duplex material that is laser welded. Early testing indicates it has very good corrosion characteristics in H2S and/or CO2 environments.

Another alternative to steel for manufacturing CT is a composite made of fibers embedded in a resin matrix. The fibers, usually glass and carbon, are wound around an extruded thermoplastic tube (pressure barrier) and saturated with a resin, such as epoxy. Heat or UV radiation is used to cure the resin as the tube moves along the assembly line. Composite CT can be manufactured with a wide range of performance characteristics by changing the mix of fibers, the orientation of their windings, and the resin matrix properties. The first commercial application for composite CT was three velocity strings deployed in The Netherlands in mid-1998.

The CT mills have also produced small quantities of CT made of titanium or stainless steel for highly corrosive environments, but the high cost of these materials has severely limited their use. Titanium was thoroughly explored for use in this application, but it is difficult to weld and costs approximately 10 times as much as carbon steel. As a result, only a handful of titanium strings have been manufactured.

Workover & Completion Applications
CT is routinely used as cost-effective solution for numerous workover applications. A key advantage of CT in this application is the ability to continuously circulate through the CT while utilizing CT pressure control equipment to treat a live well. This avoids potential formation damage associated with well killing operations. The ability to circulate with CT also enables the use of flow-activated or hydraulic tools.

Other key features of CT for workover applications include the inherent stiffness of the CT string. This rigidity allows access to highly deviated/horizontal wellbores, and the ability to apply significant tensile or compression forces downhole. In addition, CT permits much faster trip times as compared to jointed pipe operations.

Common CT Workover Applications
Some of the more common CT applications for workover operations are listed below.

Overview of Selected Workover Applications

Pumping Applications

  • Removing sand or fill from a wellbore
  • Fracturing/acidizing a formation
  • Unloading a well with nitrogen
  • Gravel packing
  • Cutting tubulars with fluid
  • Pumping slurry plugs
  • Zone isolation (to control flow profiles)
  • Scale removal (hydraulic)
  • Removal of wax, hydrocarbon, or hydrate plugs

Mechanical Applications

  • Setting a plug or packer
  • Fishing
  • Perforating
  • Logging
  • Scale removal (mechanical)
  • Cutting tubulars (mechanical)
  • Sliding sleeve operation
  • Running a completion
  • Straddles for zonal isolation
  • Drilling

Removing Sand or Fill from a Wellbore
The removal of sand or fill from a wellbore is the most common CT operation performed in the field. The process has several names, including sand washing, sand jetting, sand cleanout, and fill removal. The objective of this process is to remove an accumulation of solid particles in the wellbore. These materials will act to impede fluid flow and reduce well productivity. In many cases CT is the only viable means of removing fill from a wellbore. Fill includes materials such as formation sand or fines, proppant flowback or fracture operation screenout, and gravel-pack failures.

The typical procedure involved in this application is to circulate a fluid through the CT while slowly penetrating the fill with an appropriate jetting nozzle attached to the end of the CT string. This action causes the fill material to become entrained in the circulating fluid flow, and is subsequently transported out of the wellbore through the CT/production tubing annulus. Where consolidated fill is present, the procedure may require the assistance of a downhole motor and bit or impact drill.

An alternative fill removal approach is to pump down the CT/production tubing annulus and allow the returns to be transported to surface within the CT string. This procedure, called reverse circulation, can be very useful for removing large quantities of particulate, such as frac sand, from the wellbore. It may also be applied when a particular wellbore configuration precludes annular velocities sufficient to lift the fill material. Reverse circulation is suitable only for dead wells.

Unloading a Well with Nitrogen
The process of using CT to unload a well with nitrogen is a quick and cost-effective method used to regain sustained production. A typical field scenario consists of a wellbore that has developed a fluid column with sufficient hydrostatic pressure to prevent the reservoir fluid from flowing into the wellbore. Displacement of some of this wellbore fluid with nitrogen reduces the hydrostatic head, and this reduction of BHP allows the reservoir fluid to again flow naturally into the wellbore. If the wellbore conditions are suitable (pressure, fluid phase mixture and flow rate), production will continue after nitrogen pumping ceases.

There are numerous benefits associated with the use of CT for a nitrogen kickoff operation. The rate and depth of the nitrogen injection can be adjusted to fit a wide range of field conditions. The procedure also provides a ready source of uncontaminated production fluid samples (oil, formation water). And, the procedure is extremely simple from an operational standpoint, as only a small amount of equipment and a limited number of field personnel are necessary.

Fracturing / Acidizing a Formation
This CT application has experienced significant growth in recent years, and provides several advantages versus conventional formation treatment techniques. In particular, CT provides the ability to quickly move in and out of the hole (or be quickly repositioned) when fracturing multiple zones in a single well. CT also provides the ability to facture or accurately spot the treatment fluid to ensure complete coverage of the zone of interest. When used in conjunction with an appropriate diversion technique, more uniform treating of long target zones can be achieved. This is particularly important in horizontal wellbores. At the end of the formation treating operation, CT can be used to remove any sand plugs used in the treating process, and to lift the well to be placed on production.

One of the earlier concerns with CT fracturing was the erosion effects that occur when proppant is pumped during the fracturing operation and the resulting impact on CT string life. An ultrasonic thickness (UT) gauge is now used on location to measure CT thickness during the job. Data from these UT measurements can be used to adjust the CT fatigue models, and to accurately monitor remaining CT string life.

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