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Coiled tubing (CT) well intervention and drilling operations require that the continuous-length tube be subjected to repeated deployment and retrieval cycles during its working life. The tubing stored on a service reel is deployed into the wellbore to the designated depth and then retrieved back onto the service reel. All of the aforementioned items act on the tube body to some degree during any CT service and contribute to the eventual mechanical failure of the tubing. To ensure safe and reliable well intervention and drilling operations, the user must understand the unique behavior of CT to minimize the possibility of tubing failure. Numerous decisions must be made throughout the working life of a CT string to maximize the remaining life. From this approach, the decision to retire the tubing must be made on the basis of current tube conditions, service history, and the anticipated service loading. Fatigue is generally considered to be the single major factor in determining the working life of CT. The deployment and retrieval of the continuous-length tubing string require that the tube be subjected to repeated bending and straightening events, commonly referred to as "bend-cycling." The amount of strain imposed upon the tube body during the bend-cycling process is considered to be enormous, in many cases on the order of 2 to 3%. When subjecting the CT to this type of fatigue cycling, the stress and/or strain fluctuations to failure may be estimated using conventional axial fatigue life prediction approaches.
The coiled tubing (CT) injector is the equipment component used to grip the continuous-length tubing and provide the forces needed for deployment and retrieval of the tube into and out of the wellbore. Figure 1 illustrates a typical rig-up of a CT injector and well-control stack on a wellhead. There are several types of counter-rotating, chaindrive injectors working within the industry, and the manner in which the gripper blocks are loaded onto the tubing varies depending on design. These types of injectors manipulate the continuous tubing string using two opposed sprocketdrive traction chains, which are powered by counter-rotating hydraulic motors. Figure 1--CT injector and typical well-control stack rig-up (courtesy of SAS Industries Inc.).
Coiled-tubing drilling (CTD) can be very effective in certain situations. Its application is growing as experience defines what it takes to be successful. Coiled-tubing drilling (CTD) has a rather extensive history and received a large amount of press and hype from the 1990s to date, a significant amount being less than positive. There have been numerous highly successful applications of CTD technology in such regions as Alaska and the United Arab Emirates, yet CTD is still considered an immature new technology. One example of exaggerated expectations is CTD's reputation for offering certain advantages, including small footprint, high mobility, and quick operations. However, when more complex CTD services are planned, including directional drilling and cased completions, these advantages may no longer apply. These materials are typically not required for conventional CT services. When including the additional separators and nitrogen-pumping equipment required for underbalanced drilling (UBD), the advantages related to small footprint and high mobility may no longer be the case. Numerous truckloads of equipment can take days to rig up in preparation to drill with CT. Figure 1 shows a purpose-built CTD rig working in Oman.
Coiled tubing (CT) is an electric-welded tube manufactured with one longitudinal seam formed by high-frequency induction welding without the addition of filler metal. Coiled tubing can be used in well intervention, and more recently, in drilling operations. The first step in the typical CT manufacturing process involves the acquisition of steel stock supplied in 40- to 48-in.-wide As a result, the lengths of sheet steel will vary depending upon the wall thickness. When the diameter of the CT is selected, the sheet steel on the master coil is "slit" into a continuous strip of a specific width to form the circumference of the specified tube.
Over the years, attempts have been made to track the working history of coiled tubing (CT) strings in service to maximize the service utility of the tube while minimizing fatigue failures. As a result, three commonly used methodologies for predicting the fatigue condition of the CT were developed. A relatively simplistic approach used to predict the working life of coil tubing is commonly described as the "running-feet" method, in which the footage of tubing deployed into a wellbore is recorded for each job performed. This deployed footage is then added to the existing record of footage deployed in service for any given string. Depending upon the service environment, type of commonly performed services, and local field history, the CT string is retired when the total number of running feet reaches a predetermined amount.
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. At present, the predominant equipment design for CT well-intervention and drilling services incorporates the vertically mounted, counter-rotating chaindrive type of injector. 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. A simplified illustration of a CT unit is shown in Figure 1.
The service reel serves as the coiled tubing(CT) storage apparatus during transport, and as the spooling device during CT well-intervention and drilling operations. The inboard end of the CT may be connected either to the hollow segment of the reel shaft (spoke and axle design), or to a high-pressure piping segment (concave flange plates), both of which are then connected to a high-pressure rotating swivel. This high-pressure fluid swivel is secured to a stationary piping manifold, which provides connection to the treatment-fluid pumping system. As a result, continuous pumping and circulation can be maintained throughout the job. A high-pressure shutoff valve should be installed between the CT and reel shaft swivel for emergency use in isolating the tubing from the surface pump lines.
There are several connections used in coiled tubing (CT) services for the purpose of isolating pressure and transferring tension, compression, and torsional loads from tools and bottomhole assemblies onto the tube. These connections are typically designed to be field installed and reusable. The most common CT connections are discussed in this article. The following connections have the capability of securing loads and pressure to the end of the CT in a manner that, during makeup, does not result in yielding of the tube body. The external slip-type connection requires the use of a slip or grapple-type load ferrule placed on the outside diameter (OD) of the tube body.
The productive section in a high-pressure, high-temperature (HP/HT) geothermal Field A in the Philippines features shallow and deep reservoirs separated by a low-permeability formation. However, recent years have seen a reduction in production levels. To activate and enhance well production, coiled tubing (CT) nitrogen-lift operations were required. CT simulations were combined with simulations from the geothermal reservoir to overcome modeling limitations.