Stainless Steel Coil Tubing for Horizontal Drilling Applications
Product Overview
The Stainless Steel Coil Tubing (Coiltube) is a high-performance, continuous-length tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd., specifically engineered for horizontal drilling operations in the petroleum, gas, and geothermal industries. Unlike conventional jointed pipe strings, coil tubing is supplied in long continuous lengths wound on spools, enabling rapid deployment without the need for connections or couplings. This eliminates potential leak points and significantly reduces rig-up time, making it an indispensable tool for complex wellbore geometries, particularly in horizontal and directional drilling environments.
The product designation Coiltube encompasses a family of stainless steel tubulars that combine the corrosion resistance of austenitic stainless steels with the mechanical integrity required for downhole pressure containment, fluid circulation, and intervention operations. The coil configuration allows for deployment lengths exceeding 10,000 feet (3,048 meters) in a single run, a critical advantage in extended-reach horizontal wells.
Material Composition & Mechanical Properties
Base Alloy Grades
- AISI 304 (UNS S30400) — Austenitic stainless steel with 18% Cr, 8% Ni; general-purpose corrosion resistance
- AISI 316 (UNS S31600) — Molybdenum-enhanced austenitic grade; superior resistance to pitting and crevice corrosion in chloride-containing environments
- AISI 316L (UNS S31603) — Low-carbon variant of 316; enhanced weldability and resistance to intergranular corrosion
- AISI 321 (UNS S32100) — Titanium-stabilized austenitic grade; elevated-temperature creep resistance
- Alloy 625 (UNS N06625) — Nickel-chromium-molybdenum superalloy; exceptional performance in aggressive sour gas (H₂S) environments
Typical Mechanical Properties
| Property | Unit | Grade 304 | Grade 316L | Alloy 625 |
|---|---|---|---|---|
| Tensile Strength (ASTM A270) | MPa (ksi) | ≥ 515 (75) | ≥ 485 (70) | ≥ 550 (80) |
| Yield Strength (0.2% Offset) | MPa (ksi) | ≥ 205 (30) | ≥ 170 (25) | ≥ 275 (40) |
| Elongation (min. in 50 mm gauge length) | % | ≥ 40 | ≥ 35 | ≥ 35 |
| Hardness (Rockwell B, annealed) | HBR | ≤ 92 | ≤ 92 | ≤ 100 |
| Density | g/cm³ | 8.0 | 8.0 | 8.4 |
| Operating Temperature Range | °C (°F) | −196 to 870 (−320 to 1600) | −196 to 870 (−320 to 1600) | −269 to 1090 (−452 to 1994) |
Corrosion Resistance Characteristics
Stainless steel coil tubing exhibits a passive chromium oxide film (Cr₂O₃) on its surface, providing inherent resistance to oxidation and chemical attack. The addition of molybdenum (Mo ≥ 2%) in grades such as 316L significantly improves resistance to localized corrosion, including pitting and crevice corrosion, in environments containing chlorides, sulfides, and carbonic acid (H₂CO₃) generated by dissolved CO₂. Alloy 625 demonstrates outstanding resistance to stress corrosion cracking (SCC) and uniform corrosion in high-temperature, high-chloride, and sour gas service conditions.
Dimensional Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 1/2" to 3" (12.7 mm to 76.2 mm) | ASTM A270 / ASTM A269 |
| Wall Thickness (WT) | 0.035" to 0.188" (0.89 mm to 4.78 mm) | ASTM A270 |
| OD Tolerance | ±0.015" (±0.38 mm) | ASTM A270 |
| Wall Thickness Tolerance | ±10% (±12.5% for WT < 0.060") | ASTM A270 |
| Roundness (Ovality) | ≤ 0.5% of OD | ASTM A270 |
| Coil Length | 1,000 ft to 20,000 ft (305 m to 6,096 m) per spool | Manufacturer specification |
| Coil Inner Diameter | 12" to 60" (305 mm to 1,524 mm) | Manufacturer specification |
| Surface Finish | Pickled and Passivated (2B finish, Ra ≤ 0.4 μm) | ASTM A478 |
| Bend Radius (Minimum) | 5 × OD | ASTM A270 |
| End Condition | Beveled / Plain / Threaded (optional) | Customer requirement |
Manufacturing Process
1. Hot Rolling & Annealing
Stainless steel coils are produced via hot-rolling of continuous cast billets, followed by controlled annealing at temperatures between 1010°C and 1150°C (1850°F to 2100°F) to achieve a fully austenitic grain structure. The annealing cycle ensures complete recrystallization and dissolution of carbide precipitates, restoring ductility and corrosion resistance.
2. Pickling & Passivation
The hot-rolled coil undergoes acid pickling (typically with a mixture of hydrofluoric acid and nitric acid) to remove mill scale and oxide films. This is followed by passivation treatment to re-establish the protective chromium oxide layer, ensuring a clean, corrosion-resistant surface suitable for downhole service.
3. Cold Reduction (Cold Rolling)
The annealed coil is cold-rolled through multiple passes to achieve the target outer diameter and wall thickness. Cold working introduces dislocation density, thereby increasing yield strength and tensile strength while reducing elongation. Intermediate annealing may be performed to manage accumulated work hardening and prevent cracking.
4. Seamless Tube Forming
The coil is converted to seamless tubing through one of the following processes:
- Pilger Milling — For small OD tubing (≤ 76.2 mm), achieving tight dimensional tolerances
- Plattner Process — For medium to large OD tubing, producing uniform wall thickness and high straightness
- Autoclave Expansion (ADEX) — For high-pressure applications requiring wall thinning with controlled dimensional accuracy
5. Final Annealing & Passivation
Following forming, the tubing undergoes a final solution anneal to relieve residual stresses and restore full corrosion resistance. The passivation step ensures the surface is free of free iron contamination that could compromise the passive film.
6. Coil Winding & Spooling
The finished tubing is wound onto a spool in a continuous helical pattern. The winding tension and layer-to-layer contact pressure are precisely controlled to prevent ovalization, kinking, or surface damage. Each coil is inspected for continuity and dimensional conformity before packaging.
7. Non-Destructive Testing (NDT)
Every length of coil tubing is subjected to comprehensive NDT, including:
- Eddy Current Testing (ECT) — For surface and near-surface defect detection
- Ultrasonic Testing (UT) — For volumetric and wall thickness verification
- Hydrostatic Pressure Testing — Per ASTM A270, at 1.5× the maximum operating pressure (typically 20–50 MPa), held for a minimum of 2 seconds with no weeping or leakage
Applications in Horizontal Drilling
Primary Applications
- Horizontal Well Drilling & Completion — Coil tubing serves as the primary conduit for circulating drilling fluids, carrying cuttings to the surface, and enabling real-time wellbore monitoring during extended-reach horizontal drilling operations. The continuous length eliminates connections, reducing the risk of fluid leaks and improving flow efficiency in long lateral sections.
- Well Intervention & Workover — Deployment of downhole tools (sweepers, wireline tools, logging instruments, and perforating guns) through the coil tubing string into horizontal laterals without removing the production tubing.
- Stimulation & Acidizing — Controlled injection of acid, fracturing fluids, or chemical treatments into the formation through the coil tubing string, with precise flow-rate control at the injection point.
- Gas Lift & Artificial Lift — Continuous injection of lifting gas into the annulus or directly into the production zone to enhance reservoir fluid delivery in horizontal wells.
- Scale & Paraffin Removal — Mechanical and chemical cleaning of the production tubing and wellbore, maintaining flow capacity and reducing maintenance intervals.
- Well Logging & Monitoring — Continuous logging while drilling (LWD) and real-time production monitoring through the flexible, continuous-length tubing.
Advantages in Horizontal Drilling Context
The use of stainless steel coil tubing in horizontal drilling provides several critical advantages over conventional jointed pipe: (1) Continuous length eliminates the need for connectors, reducing potential leak paths and improving pressure integrity over extended lateral lengths; (2) High flexibility (minimum bend radius of 5× OD) allows navigation through deviated and horizontal wellbore geometries; (3) Corrosion resistance of austenitic stainless steels ensures long-term integrity in environments containing H₂S, CO₂, chlorides, and produced water; (4) Rapid deployment significantly reduces rig time, with coil tubing capable of reaching depths of 3,000–5,000 meters in a single run; (5) Real-time monitoring is facilitated by the continuous string, enabling pressure and temperature measurement at any point along the wellbore.
Quality Certifications & Compliance
| Certification / Standard | Description |
|---|---|
| ASTM A270 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing |
| ASTM A269 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Pipe |
| ASTM A478 | Standard Specification for Chemical Requirements for General Delivery Austenitic Stainless Steel Wrought Products |
| API 5CT | Specification for Casing and Tubing (for well service applications) |
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System certification |
| NACE MR0175 / ISO 15156 | Materials for use in H₂S-containing environments in oil and gas production |
| ASME B31.3 | Process Piping standards (where applicable) |
| EN 10216-5 | European standard for seamless stainless steel tubes for general applications |
| Mill Test Reports (MTR) | Full chemical and mechanical property documentation for each heat lot |
Unique Selling Points
- Seamless Construction — Eliminates weld-induced metallurgical discontinuities, ensuring uniform mechanical and corrosion performance along the entire coil length
- Continuous Length Up to 6,096 m — Enables single-run deployment into horizontal laterals without connectors, maximizing operational efficiency and pressure integrity
- Multi-Grade Availability — From cost-effective 304 for benign environments to high-performance Alloy 625 for sour gas service, providing material optimization for specific downhole conditions
- Full Traceability — Each coil is traceable to its heat lot, with complete Mill Test Reports documenting chemical composition, mechanical properties, and NDT results
- Comprehensive NDT Coverage — 100% eddy current, ultrasonic, and hydrostatic testing ensures defect-free product delivery
- High Ductility — Elongation values exceeding 35–40% provide excellent formability and resistance to plastic deformation under cyclic loading in horizontal wellbore configurations
- Weldability — Low-carbon grades (304L, 316L) offer excellent weldability for field fabrication of tool assemblies and connections
Handling & Storage Recommendations
To preserve the mechanical integrity and corrosion resistance of the stainless steel coil tubing during storage and handling, the following practices are recommended:
- Store on a flat, level surface with adequate support to prevent coil deformation
- Protect from contact with carbon steel (to prevent cross-contamination and galvanic corrosion)
- Maintain in a dry environment; avoid exposure to atmospheric chlorides and acidic contaminants
- Do not drop or impact the coil; use appropriate lifting equipment rated for the spool weight
- Inspect the coil for surface damage, ovalization, or kinking prior to deployment
Conclusion
The Stainless Steel Coil Tubing manufactured by Wenzhou Jixing Metal Materials Co., Ltd. represents a state-of-the-art solution for horizontal drilling operations, combining the superior corrosion resistance and mechanical properties of austenitic stainless steels with the operational advantages of continuous-length coil configuration. Compliance with international standards (ASTM, API, ISO, NACE) and comprehensive quality