Overview of API Casing
API Connections & Common Casing Specifications
Connections of API casing
API casing can be furnished with all standard API 5CT casing couplings:
BTC – Buttress Thread Coupling
LTC – Long Round Thread Coupling
We also provide premium casing connections as a lower‑cost alternative to proprietary connections:
Tenaris Connections: TenarisHydril CS, TSH W511, Blue Series, Wedge Series 500, Legacy Series
Hunting Connections: Seal-Lock-Flush (SLX), Seal-Lock Semi-Flush (SLSF), and other premium connections
Standard Specifications
Standard: API 5CT
Steel Grades: J55/K55, N80-1, N80Q, L80, C90, T95, P110, Q125, 13Cr
Length Range: R1, R2, R3
Casing Types & Grade
The chemical composition of most casing steel grades is essentially similar. What differentiates each grade is the heat treatment process applied, which determines its unique mechanical properties. The primary distinctions among casing grades lie in yield strength, tensile strength, and hardness.
API 5CT requires additional testing for certain casing groups to ensure each grade is suitable for its intended service conditions.
| Casing Type | Grades | Application Conditions |
|---|---|---|
| General Casing | H40, J55, K55, M65, N80, R95 | Shallow wells, surface casing, standard production service with moderate pressure and load, non‑corrosive or low‑corrosion environments. Cost‑effective choice for wells without extreme strength or anti‑corrosion requirements. |
| Corrosion‑Resistant | L80 (1 / 9Cr / 13Cr), C90, T95, C110 | Sour service (H₂S) with controlled hardness and mandatory SSC testing; CO₂ environments for 9Cr and 13Cr grades. Suitable for corrosive wells requiring higher strength and resistance to sulfide stress cracking. |
| Deep‑Well / High‑Strength | P110, Q125 | Deep wells, high‑pressure formations, ultra‑deep wells and HPHT conditions requiring extreme yield strength and load capacity. Not suitable for H₂S environments without additional corrosion protection. |
Casing Types
| Casing Type | Function |
|---|---|
| Conductor Casing | First casing set below structural casing. Isolates unconsolidated formations and water sands, protects against shallow gas, and provides foundation for casing head and BOP stack. |
| Surface Casing | Provides blowout protection, isolates freshwater zones from contamination, prevents lost circulation, and supplies shoe strength for deeper drilling. Most strictly regulated due to environmental concerns. |
| Intermediate Casing | Isolates unstable hole sections, lost‑circulation zones, low‑pressure zones, and production zones. Often set in the transition from normal to abnormal pressure; multiple strings may be required. |
| Production Casing | Final full casing string that isolates production zones and contains formation pressure if tubing leaks. Also withstands injection, gas lift, and production loads. Requires excellent primary cementing. |
Mechanical Properties
All products shall meet the tensile performance requirements specified in API 5CT. For upset casing and tubing, the tensile properties (except elongation) shall be consistent with the requirements of the pipe body. In case of any dispute regarding the upset area, properties shall be determined by cutting specimens directly from the upset.
| Grade | Type | Total Elongation Under Load % | Yield Strength (MPa) | Tensile Strength (MPa) | Hardness (max) | ||
|---|---|---|---|---|---|---|---|
| min | max | min | HRC | HBW | |||
| H40 | - | 0.5 | 276 | 552 | 414 | - | - |
| J55 | - | 0.5 | 379 | 552 | 517 | - | - |
| K55 | - | 0.5 | 379 | 552 | 655 | - | - |
| N80 | 1 | 0.5 | 552 | 758 | 689 | - | - |
| N80 | Q | 0.5 | 552 | 758 | 689 | - | - |
| R95 | - | 0.5 | 655 | 758 | 724 | - | - |
| M65 | - | 0.5 | 448 | 586 | 586 | 22.0 | 235 |
| L80 | 1 | 0.5 | 552 | 655 | 655 | 23.0 | 241 |
| L80 | 9Cr | 0.5 | 552 | 655 | 655 | 23.0 | 241 |
| L80 | 13Cr | 0.5 | 552 | 655 | 655 | 23.0 | 241 |
| C90 | 1 | 0.5 | 621 | 724 | 689 | 25.4 | 255 |
| T95 | 1 | 0.5 | 655 | 758 | 724 | 25.4 | 255 |
| C110 | - | 0.7 | 758 | 828 | 793 | 29.0 | 279 |
| P110 | - | 0.6 | 758 | 965 | 862 | - | - |
| Q125 | 1 | 0.65 | 862 | 1034 | 931 | - | - |
In case of dispute, laboratory Rockwell C hardness testing shall be used as the referee method. Where hardness limits are not specified, maximum variation is controlled by the manufacturer per API 5CT requirements.
Testing Services
To ensure every API casing joint meets your specified grade, size, and performance requirements, we provide comprehensive testing services covering the full inspection chain – from steelmaking to final delivery. Below are the key tests we perform and offer to our customers:
Mechanical Performance Testing
The following destructive tests are performed to verify that the casing meets API 5CT mechanical property requirements:
Hardness Test – Measures HRC / HBW values to confirm controlled hardness, especially critical for sour‑service grades (L80, C90, T95, C110) to prevent SSC (sulfide stress cracking).
Tensile Test – Determines yield strength, tensile strength, and elongation to ensure the pipe body and upset areas meet grade‑specific requirements.
Flattening Test – Evaluates ductility and soundness of the steel by flattening a ring specimen under controlled pressure.
Impact Test – Measures material toughness and resistance to brittle fracture under low‑temperature or high‑pressure conditions.
Non‑Destructive Testing (NDT)
NDT methods are applied to detect surface and internal flaws without damaging the product. These include ultrasonic testing, electromagnetic inspection, and magnetic particle inspection to identify surface cracks, laminations, and internal defects that could compromise pipe integrity under downhole conditions.
Applications of API casing
API casing is extensively used across the oil and gas industry, serving as the primary structural component in well construction for both onshore and offshore drilling operations. Its core applications span oil wells, natural gas wells, and oil and gas fields, where it provides wellbore stability, formation isolation, and pressure containment throughout the life of the well. In sour wells containing H₂S, specialized anti‑sulfur grades such as L80, C90, T95, and C110 are required to prevent sulfide stress cracking, while high‑strength grades like P110 and Q125 are selected for high‑pressure / high‑temperature and ultra‑deep wells to withstand extreme downhole conditions. Beyond well construction, API casing is also used in oil and gas transportation, serving as pipelines to convey hydrocarbons from wellheads to refineries or processing facilities, where it must reliably withstand transport pressures.
It is important to note that casing is a single‑use, permanent installation – once it is run into the wellbore and cemented in place, it cannot be retrieved or reused. This makes the initial selection of casing grade, type, and specification critical to the long‑term safety and productivity of the well. Selecting the appropriate casing based on specific well conditions - including depth, pressure, temperature, and corrosion potential - is essential to ensure well integrity and production efficiency throughout the life of the well.
FAQ
1. What are the types of casing?
The main casing types are conductor casing, surface casing, intermediate casing, and production casing – each installed at different depths to serve specific functions from stabilizing shallow formations to isolating production zones.
2. What is the purpose of casing?
Casing stabilizes the wellbore, prevents collapse, isolates pressure zones and fluids, protects freshwater aquifers, and provides long‑term structural integrity for the well.
3. What are the common sizes of casing?
API casing is available in outer diameters from 4‑1/2″ to 20″, with standard length ranges R1 (4.88–7.62m), R2 (7.62–10.36m), and R3 (10.36–14.63m).
4.What thread types are available for API casing?
API 5CT casing offers three standard connections: STC (Short Thread Coupling), LTC (Long Thread Coupling), and BTC (Buttress Thread Coupling).
5. What is the difference between casing and tubing?
Casing is large‑diameter pipe cemented in the wellbore permanently to support the well structure; tubing is smaller‑diameter pipe installed inside casing to transport oil and gas to the surface, and can be retrieved and replaced.
6. Which casing grade should I choose for my well?
J55/K55 for shallow low‑pressure wells, N80 for medium‑depth wells, P110 for deep high‑pressure wells, Q125 for ultra‑deep HPHT conditions, and L80/C90/T95 for sour service (H₂S environments).
7.How long will a well casing last?
Casing is a permanent installation designed to maintain integrity for 20‑to‑30‑year production lifecycles, with some applications exceeding 30 years under proper conditions.
