Understanding OCTG: Casing vs Tubing
OCTG (Oil Country Tubular Goods) encompasses three principal product categories used in oil and gas well construction: casing, tubing, and drill pipe. This guide focuses on casing and tubing, which are governed by API 5CT.
Casing Types and Functions
- Conductor Casing (20"-30" OD): The first and largest-diameter casing string. Provides hole stability during top-hole drilling and supports the wellhead. Typically Grade B or H40.
- Surface Casing (13-3/8"-20" OD): Isolates freshwater zones, provides blowout protection, and supports subsequent casing strings. Common grades: H40, J55, K55.
- Intermediate Casing (9-5/8"-13-3/8" OD): Isolates troublesome formations (lost circulation, high pressure, unstable shales). Grades: K55, N80, L80.
- Production Casing (4-1/2"-9-5/8" OD): The final casing string, run through the producing zone. Must withstand full reservoir pressure. Grades: N80, L80, C95, T95, P110.
- Liner (4-1/2"-7" OD): A shortened casing string hung from the previous casing shoe rather than extending to surface. Used for cost optimization in deep wells.
API 5CT Grade Comparison Table
| Grade | Yield Strength (ksi) | Tensile Strength min (ksi) | Hardness max (HRC) | Sour Service | Typical Application |
|---|
| H40 | 40–80 | 60 | — | No | Conductor, shallow surface casing |
| J55 | 55–80 | 75 | — | No | Surface casing, shallow wells |
| K55 | 55–80 | 95 | — | No | Surface/intermediate, higher collapse |
| N80-1/Q | 80–110 | 100 | — | No | Intermediate/production, medium depth |
| L80-1 | 80–95 | 95 | 23 | Yes | Production casing, CO₂/sour service |
| C90 | 90–105 | 100 | 25.4 | Yes | Sour gas wells, restricted yield |
| T95 | 95–110 | 105 | 25.4 | Yes | Sour gas wells, higher strength |
| C95 | 95–110 | 105 | — | No | High-pressure, non-sour wells |
| P110 | 110–140 | 125 | — | No | Deep/high-pressure wells, production |
| Q125 | 125–150 | 135 | — | No | Ultra-deep, HPHT wells |
Grade Selection by Well Depth
| Well Depth | Typical Casing Grade | Typical Tubing Grade | Key Considerations |
|---|
| Shallow (<3,000 ft) | H40, J55 | J55 | Minimal pressure, low cost priority |
| Medium (3,000–10,000 ft) | K55, N80 | N80, L80 | Burst/collapse design, standard service |
| Deep (10,000–18,000 ft) | L80, C95, P110 | L80, C95, P110 | High pressure/temperature, collapse resistance |
| Ultra-Deep (18,000–30,000+ ft) | P110, Q125 | P110, Q125 | Extreme pressure, axial load, premium connections |
| Sour Service (any depth) | L80, C90, T95 | L80, C90, T95 | NACE MR0175 compliance, restricted hardness |
OCTG Connection Types
The connection is one of the most critical elements of OCTG performance. Connections fall into three categories:
API Threaded Connections
- STC (Short Thread Coupling) — API 8-Round: 8 threads per inch, shorter thread length. Adequate for shallow to medium-depth wells. Limited tension capacity and sealing performance.
- LTC (Long Thread Coupling) — API 8-Round: Longer thread engagement than STC, providing higher tension capacity. Common for intermediate and production casing in standard wells.
- BTC (Buttress Thread Coupling): Trapezoidal thread profile with higher tension capacity and better jump-out resistance than 8-round threads. Widely used for deeper wells. Not gas-tight without supplementary seal.
Premium Connections
Premium connections are proprietary designs from manufacturers such as Tenaris (Blue, Dopeless), Vallourec (VAM series), and Hunting (SEAL-LOCK). They provide:
- Gas-tight metal-to-metal seal: Essential for high-pressure gas wells
- 100% tension efficiency: Connection equals or exceeds pipe body strength
- Enhanced compression and bending capacity: Critical for horizontal and ERD wells
- Sour service qualification: Tested per NACE TM0177 for H₂S environments
- Thermal cycling resistance: For steam injection and thermal recovery wells
Sour Service OCTG Selection
Critical Rule: For sour service (H₂S-containing environments), API 5CT requires restricted-yield grades (L80, C90, T95) with maximum hardness HRC 23-25.4 per NACE MR0175 / ISO 15156. Standard grades J55 through P110 are generally NOT acceptable for sour service unless specifically qualified with restricted chemistry and hardness.
- L80 Type 1: The workhorse sour service grade. 13Cr-L80 for CO₂ environments. Hardness max HRC 23.
- C90: Higher strength than L80 with sour service qualification. Max HRC 25.4.
- T95: Highest API sour service grade. Max HRC 25.4. Requires careful manufacturing control.
- C110 (non-API): Proprietary grade for very high-pressure sour wells, requires extensive qualification testing.
Testing and Inspection Requirements
- Hydrostatic Test: Each length tested to 3,000 psi (standard) or as specified; some mills test to higher pressures for premium products
- NDT: Full-body UT/EMI inspection for seamless pipe; 100% UT of ERW weld seam
- Drift Test: Mandatory full-length drift to verify minimum ID for passage of tools
- Thread Inspection: Gauge inspection of thread form, lead, height, taper per API 5B
- UCS (Upset Characterization Studies): For upset tubing, verification of upset geometry and mechanical properties
- Sour Service Testing: NACE TM0177 Method A (tensile) and Method D (DCB) for sour grades
