In the oil and gas industry, the grade of casing and tubing refers to the chemical composition and mechanical properties of the steel. Specifically, it tells you the minimum yield strength of the pipe.
Steel grades are standardized by API Spec 5CT. The name of a grade consists of a letter followed by a number (e.g., J55, P110). The number represents the minimum yield strength in thousands of pounds per square inch (psi).
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API Oilfield Casing Pipes Steel Grades
API 5CT classifies grades into four groups based on their performance characteristics:
Group 1: General Purpose Grades
These are the most common and cost-effective grades used in shallow to medium-depth wells where high pressures or corrosive gases are not a major concern.
H40: Low strength; used in shallow wells.
J55: Very common for both casing and tubing. Reliable and economical.
K55: Similar to J55 but has a higher tensile strength (used primarily for casing).
N80 (N80-1 / N80-Q): A mid-range grade used in deeper wells. "Q" stands for Quenched and Tempered.
Group 2: Restricted Yield (Sour Service / H2S) Grades
These grades are specially treated to be "softer" or more ductile to resist Sulfide Stress Cracking (SSC) in environments containing Hydrogen Sulfide (H2S).
L80 (L80-1, 9Cr, 13Cr): The industry standard for Sour Service.
13Cr is a "Chrome" grade used to resist CO2 corrosion.
C90 & T95: High-strength grades designed specifically for sour environments.
C110: The highest strength grade for H2S service (restricted hardness).
Group 3: High-Strength Grades
Used for deep, high-pressure wells where the weight of the pipe string itself requires extreme strength.
P110: High-strength grade. Very common for production casing in deep horizontal wells (shale gas/oil).
Group 4: Special High-Strength
Q125: Extremely high strength. Used for very deep wells or specialized high-pressure designs.
API Standard Casing Connections
API standard connections rely on thread interference and thread compound (dope) to create a seal.
| Connection Name | Abbreviation | Thread Profile | Features & Applications |
| Short Thread Coupled | STC | Round | Good sealability but lower tensile strength. Used for shallow wells or low-load strings. |
| Long Thread Coupled | LTC | Round | Longer threads provide higher tensile strength. The most common API casing connection. |
| Buttress Thread Coupled | BTC | Trapezoidal | Buttress threads provide extremely high tensile strength to support heavy strings. Ideal for deep wells. |
API Standard Tubing Connections
Tubing connections focus on maintaining the strength of the pipe end and ensuring a reliable seal for production fluids.
| Connection Name | Abbreviation | Structural Features | Features & Applications |
| Non-Upset End | NUE | Threaded directly on the pipe OD | Lower strength because the wall is thinned by threading. Rarely used for production today. |
| External Upset End | EUE | Pipe ends are forged thicker before threading | Industry standard for tubing. The upset design compensates for strength loss, making it very durable. |
| Integral Joint | IJ | Male and female threads on the pipe itself | No coupling required; has a slim profile. Used in restricted spaces or small-diameter completions. |
API 5CT Deep Well Casing specification
| API Grade | Min Yield Strength (psi) | Max Yield Strength (psi) | Environment |
| H40 | 40,000 | 80,000 | Shallow / Low Pressure |
| J55 / K55 | 55,000 | 80,000 | Standard Onshore Wells |
| N80 | 80,000 | 110,000 | Mid-depth Wells |
| L80 | 80,000 | 95,000 | Sour Service (H2S) |
| C90 | 90,000 | 105,000 | Sour Service |
| T95 | 95,000 | 110,000 | Sour Service |
| P110 | 110,000 | 140,000 | Deep / High Pressure |
| Q125 | 125,000 | 150,000 | Severe Conditions |
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FAQ
What size is API casing?
API casing dimensions comply with the API 5CT standard and are primarily used for supporting the walls of oil and gas wells. The outer diameter (OD) ranges from 114.3 mm to 508 mm (approximately 4½" to 20"), with wall thicknesses between 5.21 mm and 16.13 mm. Lengths are standardized into three categories: R-1 (4.88–7.62 m), R-2 (7.62–10.36 m), and R-3 (>10.36 m), with specific dimensions selected based on well depth, pressure, and geological conditions.
What are the sizes of tubing?
The outer diameter (OD) of tubing typically ranges from 60.3 mm to 114.3 mm, with common sizes including 2⅜ inches (≈60.3 mm), 2⅞ inches (≈73.0 mm), 3½ inches (≈88.9 mm), and 4½ inches (≈114.3 mm).
According to API 5CT specifications, tubing is classified into three structural types-non-upset (NU), external-upset (EU), and integral joint (IJ)-each suitable for fluid transport under varying well conditions.
