Steel casing pipe is the structural backbone of every oil and gas well. Run in successive strings from surface to target depth, it lines the drilled wellbore, isolates geological formations, controls downhole pressures, and provides the sealed conduit required for safe production. Without it, wellbore integrity — and the entire completion program — is impossible.
This technical reference covers the full API 5CT casing pipe specification: grade classifications and mechanical properties, OD sizes and wall thickness ranges, weight per foot, standard and premium connection options, and the selection criteria that determine which grade is appropriate for a given well environment. ZC Steel Pipe manufactures API 5CT casing in grades N80, L80, P110, C110, and Q125, with independent patents in premium connections and export experience across the Middle East, Africa, and South America.
CONTENTS
What Is Steel Casing Pipe?
Casing String Architecture
API 5CT Grade Specifications
Standard OD Sizes and Wall Thicknesses
Casing Weight Per Foot
Connection Types
Chrome Alloy Casing for Corrosive Environments
Grade Selection Guide
FAQ
1. What Is Steel Casing Pipe?
Steel casing pipe is large-diameter, heavy-wall seamless or ERW steel pipe that is cemented in place inside a drilled wellbore. After each section of a well is drilled, a casing string is lowered to the bottom of that section and cement is pumped up the annulus between the casing outside diameter and the formation wall. The cement sets to create a hydraulic seal, permanently fixing the casing in place and isolating the formations behind it.
API 5CT — SPECIFICATION FOR CASING AND TUBINGAPI Specification 5CT is the governing standard for casing and tubing used in oil and gas well construction. It covers material grades, chemical composition, mechanical properties (yield strength, tensile strength, hardness limits), dimensional tolerances, thread form specifications, and testing requirements. API 5CT is the internationally recognized baseline for casing procurement and is required by most national regulatory bodies and operating company standards.
Casing serves four primary engineering functions: wellbore support (preventing collapse of unstable formations), zonal isolation (sealing off fresh water aquifers, lost-circulation zones, and non-productive formations from the productive interval), pressure containment (providing the pressure vessel through which production fluids are brought to surface), and completion support (serving as the anchor for perforating guns, packers, tubing hangers, and wellhead equipment). See ZC's range of casing couplings.
2. Casing String Architecture
A typical well uses multiple casing strings, each cemented inside the previous one, creating a telescoping structure from surface to total depth. Each string is designed for the specific pressure, formation, and load conditions encountered at that depth interval.
Conductor Casing
Typical OD: 18-5/8 – 20 inch
Depth: 50–300 ft
Purpose: Surface hole support, returns the drilling fluid
Typical grade: K55, J55
Surface Casing
Typical OD: 13-3/8 – 16 inch
Depth: 500–4,000 ft
Purpose: Freshwater protection, BOP support
Typical grade: K55, J55, N80
Intermediate Casing
Typical OD: 9-5/8 – 10-3/4 inch
Depth: 3,000–12,000 ft
Purpose: Overpressure isolation, lost-circulation control
Typical grade: N80, L80, P110
Production Casing
Typical OD: 4-1/2 – 7 inch
Depth: TD (total depth)
Purpose: Completion conduit, reservoir isolation
Typical grade: L80, P110, Q125, C110
Field Note — Casing Program DesignThe casing program is set during well planning based on the pore pressure and fracture gradient profile of the well. The number of casing strings required is determined by the number of pressure barriers needed — typically one string per major formation transition. Deep high-pressure wells in the Gulf of Mexico or Middle East may require five or six strings; a simple shallow well in Africa may need only two or three. This directly affects the casing tonnage required and should be established clearly in any procurement discussion before sizing pipe OD and grade.
3. API 5CT Grade Specifications
API 5CT organizes casing grades into four product specification levels (PSL-1 and PSL-2) and defines the mechanical and chemical requirements for each grade. The grades most relevant to intermediate and production casing — the applications where grade selection is technically critical — are detailed below. See also our dedicated guides on API 5CT N80 casing and T95 casing pipe.
| Grade | Min. Yield (psi) | Max. Yield (psi) | Min. Tensile (psi) | Max. Hardness (HRC) | Sour Service | PSL |
| N80 Type 1 | 80,000 | 110,000 | 100,000 | — | No | 1 & 2 |
| N80Q | 80,000 | 110,000 | 100,000 | — | No | 1 & 2 |
| L80 | 80,000 | 95,000 | 95,000 | 23 | Yes (SSC) | 1 & 2 |
| L80-9Cr | 80,000 | 95,000 | 95,000 | 23 | Yes + CO₂ | 1 & 2 |
| L80-13Cr | 80,000 | 95,000 | 95,000 | 23 | Limited H₂S | 1 & 2 |
| C90 | 90,000 | 105,000 | 100,000 | 25.4 | Yes (SSC) | 1 & 2 |
| C95 | 95,000 | 110,000 | 105,000 | — | No | 1 only |
| T95 | 95,000 | 110,000 | 105,000 | 25.4 | Yes (SSC) | 1 & 2 |
| P110 | 110,000 | 140,000 | 125,000 | — | No | 1 & 2 |
| C110 | 110,000 | 130,000 | 120,000 | 30 | Yes (SSC) | 1 & 2 |
| Q125 | 125,000 | 150,000 | 135,000 | — | No | 1 only |
Critical Engineering Point — P110 Is Not Sour-QualifiedP110 is the most widely used production casing grade globally, but it carries no hardness restriction and is not qualified for sour service (H₂S exposure) under NACE MR0175 / ISO 15156. In wells where produced fluids contain H₂S, P110 is a sulfide stress cracking (SSC) risk. The correct sour-service alternatives at the 110 ksi yield level are C110 (hardness-controlled) or Q125 is not recommended for sour service either. Confirm H₂S partial pressure with your reservoir engineer before selecting P110 for any well containing sour zones.
4. Standard OD Sizes and Wall Thicknesses
API 5CT casing covers a wide OD range from 4-1/2 inch through 20 inch. Each OD is available in multiple wall thicknesses, producing different nominal weight classes. The selection of wall thickness is driven by collapse resistance (external pressure during cementing), burst resistance (internal pressure from wellbore fluids), and tensile load (the weight of the string below the hanger).
| Nominal OD (in) | OD Tolerance | Wall Thickness Range (in) | Typical String Use | Drift Diameter Reference |
| 4-1/2 | ±0.75% | 0.205 – 0.337 | Production | 3.965 – 3.813 in (varies by wt) |
| 5 | ±0.75% | 0.220 – 0.362 | Production | 4.560 – 4.276 in |
| 5-1/2 | ±0.75% | 0.244 – 0.415 | Production / Intermediate | 5.012 – 4.670 in |
| 7 | ±0.75% | 0.272 – 0.540 | Production / Intermediate | 6.456 – 5.920 in |
| 7-5/8 | ±0.75% | 0.300 – 0.595 | Intermediate | 7.025 – 6.435 in |
| 8-5/8 | ±0.75% | 0.304 – 0.595 | Intermediate | 8.017 – 7.435 in |
| 9-5/8 | ±0.75% | 0.312 – 0.608 | Intermediate / Surface | 9.001 – 8.409 in |
| 10-3/4 | ±0.75% | 0.350 – 0.797 | Intermediate / Surface | 10.050 – 9.156 in |
| 13-3/8 | ±0.75% | 0.380 – 0.514 | Surface | 12.615 – 12.347 in |
| 16 | ±0.75% | 0.438 – 0.656 | Surface / Conductor | 15.124 – 14.688 in |
| 18-5/8 | ±0.75% | 0.500 | Conductor | 17.625 in |
| 20 | ±0.75% | 0.500 – 0.635 | Conductor | 19.000 in |
5. Casing Weight Per Foot
Casing weight is expressed in pounds per foot (lb/ft) and is a primary specification parameter — it appears in the casing designation alongside OD and grade (e.g., "5-1/2 inch, 20 lb/ft, P110"). Weight per foot determines collapse and burst ratings, available drift diameter, and total string weight (critical for wellhead and hanger load calculations).
| Nominal OD (in) | Nominal Weight (lb/ft) | Wall Thickness (in) | ID (in) | Drift Diameter (in) |
| 4-1/2 | 9.50 | 0.205 | 4.090 | 3.965 |
| 11.60 | 0.250 | 4.000 | 3.875 |
| 15.10 | 0.337 | 3.826 | 3.701 |
| 5-1/2 | 15.50 | 0.275 | 4.950 | 4.825 |
| 20.00 | 0.361 | 4.778 | 4.653 |
| 23.00 | 0.415 | 4.670 | 4.545 |
| 7 | 20.00 | 0.272 | 6.456 | 6.331 |
| 26.00 | 0.362 | 6.276 | 6.151 |
| 32.00 | 0.453 | 6.094 | 5.969 |
| 38.00 | 0.540 | 5.920 | 5.795 |
| 9-5/8 | 36.00 | 0.352 | 8.921 | 8.765 |
| 47.00 | 0.472 | 8.681 | 8.525 |
| 53.50 | 0.545 | 8.535 | 8.379 |
| 13-3/8 | 54.50 | 0.380 | 12.615 | 12.459 |
| 68.00 | 0.480 | 12.415 | 12.259 |
Engineering Insight — Weight Selection and Collapse RatingHeavier wall casing has higher collapse resistance but reduces drift diameter (the minimum ID a tool must pass through). In production casing, selecting too-heavy a weight class can restrict later completion tools — particularly in wells using large-bore completion equipment or where future recompletion with a bridge plug is anticipated. The drift diameter must be checked against the largest tool expected to pass through that string before wall thickness is finalized.
6. Connection Types
The threaded connection is the weakest link in any casing string under tension. Connection selection affects gas-tightness, torque capacity, fatigue life, and running speed. Three tiers of casing connections exist in commercial use.
Standard API Connections
| Connection | Abbreviation | Thread Form | Sealing Mechanism | Typical Use | Gas-Tight? |
| Short Round Thread Casing | STC | Round (tapered) | Thread compound + dope | Surface / conductor casing | No |
| Long Round Thread Casing | LTC | Round (tapered) | Thread compound + dope | Surface / intermediate casing | No |
| Buttress Thread Casing | BTC | Buttress (square shoulder) | Thread compound | Intermediate / production casing | Liquid-tight only |
Premium Connections
Premium connections are non-API proprietary designs offering superior performance over standard API threads in one or more of: gas-tightness, torque capacity, fatigue resistance, and make-up reliability. They use metal-to-metal seals (eliminating reliance on thread compound) and typically a positive make-up torque shoulder. Common applications include gas wells, HPHT completions, deviated and extended-reach wells, and any completion where gas migration through the connection is unacceptable.
Field Note — ZC Premium ConnectionsZC Steel Pipe holds independent patents in
premium casing and tubing connections. Our proprietary premium connection design provides metal-to-metal sealing with a torque shoulder, meeting gas-tightness requirements for high-pressure gas completions without reliance on thread compound. For buyers sourcing premium connections with Chinese manufacture, ZC's patented designs offer an alternative to licensed third-party premium connections such as VAM TOP or TenarisHydril Blue. Contact us with your connection performance specification for a technical comparison.
7. Chrome Alloy Casing for Corrosive Environments
Where wellbore fluids contain significant concentrations of carbon dioxide (CO₂) or hydrogen sulfide (H₂S), standard carbon steel casing grades corrode rapidly. Chrome alloy casing grades within the API 5CT framework provide enhanced corrosion resistance through chromium additions to the base steel chemistry.
| Grade | Cr Content | CO₂ Resistance | H₂S Resistance | Typical Application |
| L80 | <1% Cr | Limited | SSC-qualified | Standard sour service; sweet CO₂ wells not recommended |
| L80-9Cr | ~9% Cr | Good | SSC-qualified | CO₂-aggressive environments with moderate H₂S |
| L80-13Cr | ~13% Cr | Excellent | Limited (<0.1 psi H₂S pp) | High-CO₂ sweet wells; geothermal production casing |
| Super 13Cr (non-API) | ~13% Cr + Mo/Ni | Excellent | Moderate H₂S tolerance | High-CO₂ wells with low H₂S; condensate production |
Critical Engineering Point — 13Cr and H₂S Partial Pressure LimitsL80-13Cr has excellent CO₂ resistance but is susceptible to stress corrosion cracking above relatively low H₂S partial pressures — typically above 0.05–0.1 psi H₂S partial pressure depending on chloride content and temperature. In wells with co-produced H₂S even at low concentrations, full corrosion engineering analysis (per ISO 15156 Part 3) is required before specifying 13Cr. Specifying 13Cr casing in a sour environment without this analysis is a documented failure mode.
8. Grade Selection Guide
The following matrix summarizes grade selection across the primary engineering criteria for production and intermediate casing applications. Conductor and surface casing selection follows a simpler structural analysis and is not included here.
| Selection Factor | N80 | L80 | C95 / T95 | P110 | C110 | Q125 |
| Sweet service (<8,000 ft) | Preferred | Acceptable | Overspec | Overspec | Overspec | Overspec |
| Sweet service (deep) | Marginal | Marginal | Acceptable | Preferred | Preferred | Ultra-deep |
| Sour service (H₂S present) | Not qualified | Preferred | Acceptable | Not qualified | Preferred | Not qualified |
| CO₂-aggressive environment | Not recommended | With inhibitor | With inhibitor | With inhibitor | With inhibitor | Not recommended |
| HPHT wells | Insufficient | Insufficient | Marginal | Acceptable | Acceptable | Preferred |
| Gas tightness required | Premium conn. | Premium conn. | Premium conn. | Premium conn. | Premium conn. | Premium conn. |
| Cost sensitivity | Lowest | Low | Moderate | Moderate | Higher | Highest |
9. Frequently Asked Questions
What is steel casing pipe used for?
Steel casing pipe lines the drilled wellbore in oil and gas wells. After each section is drilled, a casing string is lowered to the bottom of that section and cemented in place. It prevents wellbore collapse, isolates fresh water zones from hydrocarbons, controls formation pressures, and provides a structural conduit for the production tubing and completion equipment that follow. Each well uses multiple casing strings of decreasing diameter from surface to total depth.
What are the main API 5CT casing grades?
The primary API 5CT grades for intermediate and production casing are N80 (Types 1 and Q), L80 (standard and chrome variants), C90, T95, P110, C110, and Q125. For surface casing and conductor, K55 and J55 are commonly used. Grade selection depends on well depth, collapse and burst pressure requirements, tensile load, and whether sour service (H₂S) conditions are present.
What is the difference between L80 and N80 casing?
Both L80 and N80 have a minimum yield strength of 80,000 psi, but they differ importantly in hardness control and sour service qualification. L80 carries a maximum hardness limit of 23 HRC and is qualified for sour service under NACE MR0175 / ISO 15156. N80 has no hardness restriction and is not recommended where H₂S is present. L80 is also available in chrome alloy variants (L80-9Cr, L80-13Cr) for CO₂-corrosive environments. For sweet service, N80 is typically the more economical choice; for sour service, L80 is the default selection at the 80 ksi yield level.
What OD sizes are available for steel casing pipe?
API 5CT casing is available from 4-1/2 inch through 20 inch OD. Production casing is typically 4-1/2, 5, 5-1/2, or 7 inch. Intermediate casing runs from 7-5/8 to 10-3/4 inch. Surface casing is commonly 13-3/8 or 16 inch, and conductor casing is 18-5/8 or 20 inch. Wall thickness options within each OD produce different weight classes, each with distinct collapse, burst, and tensile ratings.
What connection types are used on steel casing pipe?
Standard API connections include Short Round Thread (STC), Long Round Thread (LTC), and Buttress Thread (BTC). These are suitable for liquid service but are not gas-tight. For gas wells and HPHT applications, premium connections with metal-to-metal seals are required. Premium connections — including ZC's patented proprietary designs — provide full gas-tightness through a metal seal rather than relying on thread compound, and offer higher torque capacity for directional applications.
Request Steel Casing Pipe Specifications & Pricing
ZC Steel Pipe manufactures API 5CT steel casing pipe in grades N80, L80, L80-13Cr, P110, C110, and Q125 across the full OD range. We hold independent patents in premium connections and supply casing to oil and gas projects across the Middle East, Africa, and South America.
Provide your casing program requirements — grade, OD, weight class, connection type, quantity, and delivery destination — and our technical team will respond with a full specification sheet and competitive quotation.
mandy.w@zcsteelpipe.com | WhatsApp: +86-139-1579-1813
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