CT (Coiled Tubing)

Key Takeaways

• The CT injector head is the mechanical heart of the coiled tubing unit, providing the controlled traction force that pushes the CT into the wellbore against the wellbore pressure and the friction of the wellbore wall, while the CT reel motor provides the tension management to control the spooling and unspooling of the CT at the surface: the injector uses opposing chain drives with gripper blocks that clamp the CT and move it downward (injection) or upward (withdrawal), with the clamp force and chain speed controlled by the operator at the surface control console to achieve the desired weight on tool (the compressive force applied to the downhole tool string by the CT in compression) or pull force during retrieval; the maximum push force of the injector (typically 10,000 to 80,000 lbs for common CT unit sizes) limits the depth achievable in a given wellbore geometry, because the push force must overcome the friction of the CT against the wellbore (which increases with hole inclination and with the length of CT in the well), the pressure force (wellhead pressure times the cross-sectional area of the CT, which resists injection), and the weight of the tool string hanging below the CT end; in extended-reach horizontal wells where CT friction limits depth penetration, CT tractor tools (downhole tractor devices that grip the casing and pull the CT forward) or aggressive rotation-while-advancing (using a CT connector that allows CT rotation to reduce contact friction) are used to extend CT reach beyond the passive push-force limit of the injector.
• CT wellbore cleanout is the highest-volume CT application globally, used to remove accumulated sand, proppant fallback, scale, wax, or other fill from the wellbore that restricts production or prevents downhole tool deployment: the standard CT cleanout procedure involves circulating a high-rate fluid (nitrogen foam, water-based fluid, or gel fluid) through the CT annulus between the CT and the wellbore to create a high-velocity upward flow in the wellbore annulus above the CT end that entrains and carries the fill material back to the surface; the circulating fluid velocity must exceed the terminal settling velocity of the largest and densest fill particles to be transported, which for coarse sand or proppant at the low CT-to-casing annular areas typical of production tubing may require nitrogen foam (with its lower density and higher velocity for the same volumetric pump rate) rather than straight water; the CT is typically advanced slowly into the fill pile while circulating, progressively cleaning the hole from the top of the fill downward; real-time monitoring of the return fluid at the surface (sand catch, fluid samples) and the weight-on-tool indicator confirm cleanout progress; the major risk in CT cleanout is sticking the CT in the fill if the CT is advanced too aggressively before the fill above is fully cleaned, or if circulation is lost and the fill packs around the CT end; slow, controlled advancement with continuous circulation and careful monitoring of CT weight indicators is the standard operating practice for CT cleanout in fill-prone wells.
• CT fatigue life management is the critical engineering discipline that determines the safe number of well interventions possible with a given CT string before the accumulated metal fatigue requires the string to be retired: every time the CT is run in and out of the well, the section of CT that passes through the injector, over the guide arch, and across the reel suffers a low-cycle fatigue cycle — bending in one direction as it spools off the reel, straightening through the guide arch, bending again through the injector, and then straightening as it enters the wellbore; the cumulative fatigue damage at any cross-section of the CT depends on the number of bending cycles at that section, the CT wall thickness and yield strength (thicker, higher-strength CT resists fatigue better), the reel and guide arch radii (tighter radii induce more strain per bending cycle), the internal pressure during the operation (internal pressure adds a hoop stress component that combines with the bending stress to increase the total stress range, accelerating fatigue), and the H2S content of the wellbore fluid (H2S causes hydrogen embrittlement of the steel, dramatically reducing fatigue life in sour service); CT fatigue is tracked by a CT fatigue monitoring program that records the number of cycles at each axial position on the CT string (the section that has made the most trips through the injector has the highest cycle count and the shortest remaining life) and compares the cumulative fatigue to an empirical or analytical fatigue life model to determine the safe remaining life and the next required inspection or retirement point.
• CT drilling using a positive displacement downhole motor (PDM or mud motor) on the CT end is used for re-entry drilling of new lateral branches from existing wellbores, abandonment of wells by drilling through and above the cement plug, and shallow drilling applications where the continuous CT work string provides operational advantages over jointed drill pipe: CT drilling is limited in depth and rate of penetration compared to conventional rotary drilling because the CT cannot be rotated (which limits weight-on-bit to the push force of the injector, requires a downhole motor for bit rotation, and prevents the use of rock bit designs that depend on drill string rotation for optimal performance), the CT cannot be made up with new joints of drill pipe to extend length (the total drillable depth is limited to the length of CT on the reel, typically 10,000 to 20,000 feet), and the annular hydraulics of small CT strings limit the flow rate and hence cuttings transport capacity for large-diameter borehole intervals; despite these limitations, CT drilling is used extensively for re-entry of old wells (where the cost of mobilizing a workover rig is prohibitive), for drilling short underbalanced lateral windows in naturally fractured reservoirs (where underbalanced CT drilling avoids drilling fluid damage to the fractures), and for openhole sidetrack drilling in mature fields where the existing wellbore geometry prevents use of conventional drilling equipment.
• CT for stimulation (acid stimulation, matrix acidizing, and diverter placement) exploits the precise depth control and continuous nature of the CT string to place treatment fluids at specific reservoir intervals with better control than bullheading (pumping fluids down the production tubing without depth control) or conventional jointed-pipe treatment: CT acid stimulation involves running the CT to the depth of the treatment interval (confirmed by real-time depth measurement from the injector encoder and by wireline-correlation of the downhole tool's gamma ray measurement), pumping acid through the CT while slowly pulling the CT upward to distribute acid across the entire perforated interval (a technique called a pump-while-pulling or staged acid treatment), and following with a post-flush of clean brine to remove the reacted acid and reaction products from the perforated interval; the advantage of CT acid stimulation over conventional bullhead acidizing is the ability to selectively treat specific intervals within a perforated zone (by pumping more acid volume at the depth of the intended treatment interval and less at adjacent intervals), the ability to pump at higher flow rates than production tubing of the same OD permits (because the CT interior is larger relative to its OD than production tubing fittings), and the ability to perform the stimulation under full well control pressure using the CT wellhead BOP stack without killing the well first.