A tubing coil is a continuous, lengthy pipe that is milled and wound onto a reel during production. The weight, grade, and size of the tubing coil determine its maximum weight and length. Factors such as road restrictions for truck transport, crane lifting requirements, and the tubing’s weight and grade influence the maximum manufacturable length. Common coiled tubing steels have yield strengths between 55,000 and 120,000 psi and are produced in diameters ranging from 0.75″ to 4″. The maximum lengths can reach up to 30,000 feet, with most reels containing around 20,000 feet of tubing and weighing approximately 30 tons.
To reduce the reel’s weight, tubing coils are produced using sections with varying wall thicknesses. The thickest section is positioned at the inner end of the reel, where tensile forces are highest. Tubing coils can be made with up to six different wall thickness sections, creating a “tapered” string known as a 5-step taper.
By eliminating the need for making or breaking connections, as required in sectioned tubular operations like Hydraulic Workover Operations, tubing coils can be deployed and retrieved more rapidly, thus reducing the time needed for specialized services. Depending on wellbore conditions, running speeds can reach up to 250 feet per minute. The development of higher yield-strength tubing coil grades has led to increased pressure service limits. To counteract frictional and gravitational forces during deployment, tubing coil tractors may be employed to assist in positioning the tubing coil.
Tubing coil services utilize specialized equipment to manage wellbore pressures while allowing tubing to enter the well. Unlike sectioned tubular workover operations, tubing coil well control involves unspooling the tubing from a reel and feeding it through a hydraulic-drive injector assembly, which includes a gooseneck and motorized drive chains. This process straightens the tubing and forces it into the well. The tubing then passes through a stripper assembly, a quad ram pass (a small set of BOPs), a flow tee, well valves, and finally into the well. These operations are powered by a separate power plant that combines prime movers, pumps, and valves to deliver pressurized hydraulic fluid through high-pressure hoses..
Tubing coil is used in a variety of application such as:
Nitrogen lifting wells where HP prevent well flow.
Velocity string installation/retrievals that include installing small bore strings inside the completion to aid wellbore flows in older wells.
Fill clean-up operations after workovers, etc.
Spotting acid at the perforations (either to clean the perforations or remove formation skin damage).
Fishing operations.
Cleaning out sumps, rat-holes.
Spotting cement plugs.
Through-tubing services such as straddle packers and bridge plugs.
Use of stiff wireline (Tubing coil with logging cable inside).
Side tracking and drilling small diameter holes.
Drilling/milling operations inside pipe.
Opening/closing sliding sleeves, etc.
ESP deployment.
Memory PLT logging.
Fracturing.
High-pressure Abatement.
Underbalanced formation drilling.
Sand jetting perforations.
Sand cutting tubulars.
Scale/paraffin removal.
Specialized services also feature tubing coils equipped internally with electric logging cables and/or small-bore stainless-steel lines, similar to DHSV control lines. This configuration allows for the independent control of specialized downhole tools. Additionally, multiple strings within the tubing coil can be utilized to pump different fluids separately through the same string.
