The Defining Series: Introduction to Coiled Tubing

A Wide Range of Applications

Coiled tubing technology is frequently used to deploy tools and materials through production tubing or casing while remedial work is performed on producing wells. Coiled tubing fulfills three key requirements for downhole operations on live wells by providing a dynamic seal between the formation pressure and the surface, a continuous conduit for fluid conveyance and a method for running this conduit in and out of a pressurized well.

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Coiled tubing strength and rigidity, combined with its capability to circulate treatment fluids, offer distinct advantages over wireline techniques in workover operations. In addition to drilling and completion operations, oil and gas companies are using CT to help fish for lost equipment and for conveying well logging tools. It has been used to push or pull equipment through highly deviated or horizontal wellbores and past restrictions or to push obstructions beyond a zone of interest. Well logging is typically per-formed with tools that store data in memory; however, some logging operations use an optional cable to provide surface power and readouts when running tools downhole on CT. Operators also employ coiled tubing to convey and place bridge plugs and mechanical, hydraulic or inflatable packers to establish zonal isolation.

One of the most common applications for CT is the cleanout and removal of fill materials that restrict flow through tubing or casing (at right). Fill material can impede production by blocking the flow of oil or gas. It also may prevent the opening or closing of downhole control devices such as sleeves and valves. Common sources of fill are sand or fine material produced from the reservoir, proppant materials used during hydraulic fracturing operations, debris from workovers and organic scale. Fill removal typically involves circulating a cleanout fluid, such as water or brine, through a jet nozzle run on the end of the CT. The circulating fluids carry the debris back to the surface through the annulus between the CT string and the completion tubing.

Coiled tubing technology also extends to well perforating operations - shooting holes in casing to initiate production in a well. In many wells, perforating guns are run downhole on wireline; however, because wireline tools depend on gravity to reach the target zone, they may not reach target depth in horizontal or highly deviated wells. One alternative is to convey the guns downhole at the end of the CT, which allows for substantially longer gun strings and higher-angle deployments than are possible on wireline. These operations can even be performed with tubing in place.

Its capacity to circulate or inject fluids makes CT especially suited to initiating production in a well. When drilling or workover fluids exert hydrostatic pressures that exceed formation pressure, reservoir fluids are prevented from entering the wellbore. Pumping nitrogen gas through the CT string and into the fluid column is a common method for reducing hydro-static pressure within the wellbore to initiate production. The CT string is run to its target depth, and the nitrogen is pumped through the string to reduce the density of the hydrostatic column. When the hydrostatic pressure of the fluid column drops below reservoir pressure, the well can flow.

Operators frequently utilize coiled tubing as a conduit for accurate placement of cement downhole. Cement is used for sealing perforations or casing leaks, for primary or secondary zonal isolation and for plugs used in kickoff or abandonment operations. A cement squeeze enables the operator to plug casing leaks or existing perforations by pumping cement slurry under pressure into these openings. The cement fills openings between the formation and the casing, forming a seal. Setting a cement plug involves circulating a cement slurry into position using CT then withdrawing the CT string to a point above the top of cement. A slight squeeze pressure is applied if necessary, any cement remaining in the tubing is displaced by a tail slurry then the CT is pulled out of the hole.

Well treatment programs may use CT to convey stimulation fluids that boost production by restoring or improving reservoir permeability. In a matrix treatment, fluids are pumped into a reservoir at a pressure that is greater than reservoir pressure but less than the formation fracture threshold. This technique pushes the fluids through the formation pore spaces without initiating fractures. A similar operation, fracture acidizing, pumps fluids at a pressure that intentionally initiates fractures.

CT can facilitate the installation of production tubing and associated completion equipment. In certain wells, a string or section of CT may be left in the borehole as a permanent part of the completion. CT completions often provide a low-cost approach for prolonging the life of old wells. Typical installations include velocity strings, tubing patches and through-tubing gravel packs.

There are several coiled-tubing (CT) equipment manufacturers presently marketing various designs of CT injectors, service tubing reels, and related well-control equipment in the industry today. The injector designs available within the industry include:

• The opposed counter-rotating design
• Chaindrive system
• Arched-chain roller drive
• Single-chain opposed-gripper-drive system
• The sheavedrive system

Equipment design for CT

At present, the predominant equipment design for CT well-intervention and drilling services incorporates the vertically mounted, counter-rotating chaindrive type of injector. For purposes of practical demonstration, the following descriptions of CT equipment focus on the specific unit components supporting the vertical, counter-rotating chaindrive type of injector.

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The CT unit is a portable, hydraulically powered service system that is designed to inject and retrieve a continuous string of tubing concentric to larger-inside diameter (ID) production tubing or casing strings. At the present time, CT manufactured for well intervention and drilling application is available in sizes ranging from 0.750 to 3.500 in. outside diameter (OD). A simplified illustration of a CT unit is shown in Fig. 1.

• Fig. 1&#;Mechanical elements of a hydraulic CT unit (courtesy of SAS Industries Inc.).

The basic components of a CT unit are:

References

Noteworthy papers in OnePetro

R.E. Frishmuth and J.C. Pursell . Design, Construction, and Use of a Coiled Tubing Drilling Structure for Onshore and Offshore Operations, Offshore Technology Conference, 6 May-9 May . -MS. http://dx.doi.org/10./10./-MS

Robert Ewen, Kevin Mackay, and Frank Shepard . Development of Computer-Controlled Hybrid CT Drilling Units, SPE/ICoTA Coiled Tubing Roundtable, 7-8 March . -MS. http://dx.doi.org/10./-MS

See also

PEH:Coiled-Tubing_Well_Intervention_and_Drilling_Operations

Category

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