coiled tubing string

A coiled tubing string (also called a CT string or CT pipe) is the continuous, unjointed steel tube wound on a surface reel and deployed into oil and gas wellbores to convey downhole tools and fluids for well intervention, stimulation, completion, and drilling operations without threaded connections between individual pipe joints; CT strings are manufactured from high-strength low-alloy steel strip that is cold-formed into a tube, longitudinally seam-welded, heat-treated to achieve the target yield strength grade, and wound onto a shipping reel in continuous sections of 1,200 to 3,000 m per mill run that are butt-welded end-to-end at the factory to produce a single-string total length of 4,000 to 9,000 m depending on the CT OD and the application. In the Western Canada Sedimentary Basin, CT strings are deployed across a wide range of services and well environments: Montney and Duvernay horizontal plug-and-perf completions and plug drill-out (2-3/8" to 2-7/8" OD QT-800 at 1,500 to 4,000 m depth), SAGD well interventions at Athabasca oil sands (1-3/4" to 2-3/8" OD for sand cleanout and scale removal in SAGD producer liners, sour-service grades for H2S in steam condensate), Cardium and Viking gas well nitrogen unloading (low-temperature CT alloys rated to minus 40 degrees Celsius), and Foothills deep sour gas operations (QT-900 or QT-1000 NACE MR0175-compliant CT at 3,000 to 6,000 m with H2S requiring sulfide stress cracking resistant metallurgy). The CT string's fatigue life is its most critical and life-limiting property: unlike drill pipe or production tubing that experience purely axial or pressure loads, CT undergoes cyclic plastic bending every time it spools off the reel drum, bends over the gooseneck at the injector head, straightens through the injector, and re-bends on retrieval, accumulating damage that is tracked by CT management software and terminates the string's usable life when the calculated consumed fraction exceeds the operator-specified retirement threshold.

• CT string manufacturing process, seam welding, butt welding, and quality inspection for WCSB operations: CT string manufacturing begins with hot-rolled low-alloy steel strip (typically HSLA grades with manganese, chromium, molybdenum, and vanadium microalloying) that is cold-formed progressively through a series of rolls into a circular tube profile and welded along the longitudinal seam by high-frequency induction (HFI) or laser welding processes that produce a narrow (0.5 to 2 mm) fusion zone with minimal heat-affected zone width to preserve the strip's base mechanical properties; the welded tube is then heat-treated in a continuous furnace to the target quench-and-temper grade (QT-700 through QT-1000, corresponding to minimum yield strengths of 700 to 1,000 MPa or 70,000 to 100,000 psi) that determines the CT's axial load capacity, burst pressure rating, and collapse resistance. Factory butt welds joining individual 1,200 to 3,000 m mill-run sections are the most mechanically critical locations in the finished CT string because the butt-weld HAZ is a local strength-reduction zone that must be post-weld heat-treated (PWHT) to restore ductility before the string is placed in service; CT manufacturers perform 100 percent ultrasonic testing of each butt weld to verify wall integrity and absence of lack-of-fusion defects that would initiate fatigue cracking at the stress concentration of the weld notch during WCSB field operations. WCSB CT service companies require mill certificates documenting yield strength, elongation, Charpy impact at minus 40 degrees C (min 27 J per API 11IW for N2 service), and HIC test results before acceptance for sour-service applications.
• CT string fatigue mechanics, bending cycle accumulation, and fatigue tracking software in WCSB programs: CT fatigue is a plastic-strain-controlled low-cycle phenomenon: each pass over the reel drum (radius 1.5 to 3 m) and gooseneck (radius 0.5 to 1.5 m) subjects the CT wall to a compressive strain on the inner face and tensile strain on the outer face, with peak strain amplitude of 0.3 to 1.5 percent depending on CT OD relative to the bend radius, well within the plastic deformation regime for high-strength steel and therefore accumulating irreversible damage with each cycle. CT management software packages (including NOV CoilScan, SLB CTES WellCat, and Halliburton CoilCADE used by WCSB CT service companies) integrate depth-tagged operational data from each run (total meters in-hole, in-hole pressures, wellhead pressure history, and elapsed time at each depth) against a manufacturer-supplied S-N (stress versus cycles to failure) curve for the specific CT OD, grade, and wall thickness to calculate a remaining fatigue life percentage that decrements with every job; most WCSB CT operators set a retirement threshold of 70 to 80 percent consumed fatigue life as the point at which the CT string is removed from high-risk applications (sour wells, deep HPHT, large-bore stimulation) and re-assigned to shallow low-pressure jobs, and retired entirely at 90 to 95 percent consumption regardless of visual inspection results. The inner wraps on the reel drum (the oldest, deepest portion of the CT) accumulate fatigue cycles faster than the outer wraps because they experience the smallest drum bend radius; WCSB operators tracking 3,000 m Montney laterals find that the inner 500 m of CT (first on and last off the reel) may consume fatigue life at 1.5 to 2 times the rate of the outer 500 m section, requiring the CT tracking software to maintain a depth-resolved fatigue map rather than a single average life estimate.
• CT string grade selection, wall thickness, and pressure rating for WCSB well programs: WCSB CT grade selection balances burst pressure, collapse pressure, tensile capacity, and fatigue life against the wellbore depth, pressure, and temperature of the target operation; the most common CT grade in WCSB Montney and Duvernay plug-and-perf is QT-800 (minimum yield 800 MPa, 116,000 psi) in 2-3/8" OD with 5.15 mm (0.203") wall, providing burst pressure of approximately 105 MPa (15,200 psi), collapse pressure of 95 MPa (13,800 psi), and tensile capacity of 615 kN (138,000 lb force), sufficient for WCSB Montney frac plug-and-perf at surface injection pressures of 50 to 80 MPa and depths of 1,500 to 3,500 m. WCSB Foothills and Deep Basin deep sour gas CT operations at 3,000 to 6,000 m depth with bottomhole pressures of 40 to 100 MPa require QT-900 or QT-1000 grade CT (yield 900 to 1,000 MPa) with wall thickness of 5.54 to 6.35 mm (0.218" to 0.250") to achieve the higher burst and tensile ratings required, but these high-strength grades have reduced ductility that makes them more susceptible to sulfide stress cracking in H2S environments; WCSB sour-service CT must additionally meet NACE MR0175/ISO 15156 hardness limits (maximum Rockwell C22 or Brinell HB 237 at the pipe ID surface measured after full heat treatment) to prevent SSC initiation at the CT bore where H2S-saturated wellbore fluid contacts the steel surface during injection operations.
• Nitrogen-service CT string requirements for WCSB gas well unloading and underbalanced CTD: Standard QT-grade CT steel becomes brittle below minus 10 to minus 20 degrees Celsius (the DBTT of QT-700 to QT-1000 grades), above the minus 40 degrees Celsius that liquid nitrogen reaches as it expands through cryogenic hoses into the CT bore for WCSB gas well unloading or underbalanced CTD. Using standard CT below the DBTT drops Charpy impact from 60-80 J at 20 degrees C to below 10 J at minus 40 degrees C, increasing brittle fracture risk at stress concentrations (butt weld HAZ, corrosion pits) during the cryogenic nitrogen portion of the job. WCSB nitrogen-service CT strings are manufactured from special low-carbon, high-nickel or high-manganese alloys (typically 2.5 to 3.5 percent nickel addition to the base HSLA composition) that depress the DBTT to below minus 60 degrees Celsius and are certified by the manufacturer with Charpy impact testing at minus 40 degrees Celsius per API 11IW Appendix A, with a minimum impact energy requirement of 27 J (20 ft-lb) at the test temperature; WCSB CT service operators using nitrogen unloading services must verify the reel tag certifying N2 service suitability before beginning any nitrogen pumping operation.
• CT string end-of-life criteria, electromagnetic inspection, and field splice procedures in WCSB operations: WCSB CT string end-of-life is determined by three independent criteria, any one of which triggers mandatory retirement: fatigue life consumed past the operator threshold (typically 80 to 90 percent of the S-N calculated total); localized wall loss exceeding 20 percent of the nominal wall thickness at any cross-section measured by electromagnetic (EM) flux leakage inspection; or confirmed H2S exposure above the NACE MR0175 threshold in a sour-service WCSB well where the CT hardness cannot be re-verified (H2S exposure can cause SSC cracks in CT that are not detectable by visual inspection or conventional OD caliper). Electromagnetic inspection (EC tools or magnetic flux leakage scanners traversing the CT at the wellsite or at a CT yard) detects both internal and external wall anomalies including corrosion pits, abrasion grooves, fatigue cracks, and weld defects as flux leakage signals proportional to the volume of missing metal; WCSB CT operators perform EM inspection annually on strings used in sour or corrosive wells and before return to service following any stuck-string, over-pull, or pressure exceedance event. Field splicing (cutting out a damaged CT section and butt-welding the ends together in the field) is permitted in WCSB CT operations only with AER-recognized welder certification for API 11IW CT welding, a field PWHT unit capable of achieving and soaking the weld HAZ at 580 to 620 degrees Celsius for 30 minutes minimum, and a field-portable UT inspection of the completed weld; most WCSB CT service operators prohibit field splicing on nitrogen-service or sour-service strings and retire the entire reel rather than attempt a field repair that cannot replicate factory metallurgical conditions.

CT String Fatigue Management Extending WCSB Montney Drill-Out Program

A northeast British Columbia Montney operator managed a 2-7/8" QT-800 CT string (0.203" wall, 3,600 m total length) through a 12-well plug drill-out program (28 to 34 plugs per well, 2,200 to 2,800 m measured depth laterals). CT fatigue tracking software logged each trip depth and surface pressure, calculating consumed fatigue life after each well. After 7 wells (average 31 plugs per well, average 2,450 m measured depth), the fatigue tracking software reported 68 percent consumed fatigue life on the inner 400 m of the reel. The operator downgraded the string from Montney deep lateral service to shallow Cardium cleanout service (maximum 1,200 m depth) and reassigned a fresh reel to the remaining 5 Montney wells. EM inspection of the retired string after the Cardium cleanout program (total 81 percent consumed fatigue) detected a 22 percent wall thinning anomaly at a factory butt weld at 185 m from the reel core, confirming the retirement decision before the anomaly could propagate to a through-wall fatigue crack downhole.

Related Terms

Coiled tubing is the technology platform the CT string enables; the continuous unjointed design eliminates connections and allows live-well entry, making plug-and-perf and SAGD interventions 40-70% faster than jointed workstring in WCSB operations. Coiled tubing unit (CTU) is the surface equipment package that deploys the CT string; reel drum diameter must maintain at least a 48:1 drum-to-CT-OD ratio to limit plastic strain per bending cycle in WCSB programs. Fatigue is the life-limiting failure mode; cyclic plastic bending over reel and gooseneck accumulates irreversible damage tracked by S-N curve in CT management software; inner reel wraps fail first. Sulfide stress cracking (SSC) limits CT grade selection in WCSB sour-gas wells; NACE MR0175 requires RC22 maximum hardness at the CT bore to prevent H2S cracking in QT-900/1000 grades. Electromagnetic inspection detects CT wall anomalies (corrosion pits, fatigue cracks, weld defects) as flux leakage signals; mandatory annually for sour-service WCSB CT and after any over-pull or pressure exceedance event.