Steel Grades of OCTG Casing Pipe

API 5CT is the primary standard for oil well casing and tubing. It defines steel grades (such as J55, N80, P110) with different strength levels and corrosion resistance. This article compares common casing steel grades and provides selection guidance for different well conditions.

▶ Note: Most oil well casing uses seamless steel pipe. ERW casing is limited to shallow, low-pressure wells.

Grades of OCTG Casing Pipe:

OCTG casing pipe is manufactured in various grades, each with its own unique characteristics, in order to meet the needs of specific applications. The API 5CT standard divides oil casing into different steel grades. Common OCTG casing steel grades include API J55, API K55, API L80, API N80, API P110 and API Q125. These steel grades have different tensile strength and yield strength to meet different usage requirements. 

Comparison of Common Casing Steel Grades in API 5CT Standard:

Note: YS = Yield Strength, TS = Tensile Strength. 1 MPa ≈ 145 psi.

Let’s look at these grades in more detail:

API J55 

API J55 casing is a general-purpose grade that is the most commonly used grade for oil and gas wells. It is known for its affordability and versatile nature. It is usually selected when cost is the main consideration. J55 casing has relatively low yield and tensile strength properties, ranging from 55,000 psi to 80,000 psi respectively. It is often used in shallow wells that require only modest pressure containment or when there is little risk of hydrogen sulfide corrosion.

API K55 

K55 casing is slightly stronger than J55 and is classified as a medium-strength grade. It is best suited for use in shallow wells with less stringent containment and corrosion requirements. It is also more durable and can withstand greater pressure than J55. It has an average yield strength of 55,000 psi and an average tensile strength of 80,000 psi. It is often used in onshore wells requiring medium tensile and yield strength.

API L80 

L80 casing has higher strength than J55 and K55. It combines strength and toughness, making it particularly suitable for acidic environments containing hydrogen sulfide. Its sulfur resistance primarily targets sulfide stress corrosion cracking, rather than providing comprehensive corrosion resistance. L80 is a common choice for casing and tubing in sulfur-containing oil and gas fields.

L80 is divided into L80-1, L80-9Cr, and L80-13Cr. They have the same mechanical properties and delivery condition. The differences lie in their applications, production difficulty, and price. L80-1 is the standard type, while L80-9Cr and L80-13Cr are both high-corrosion-resistant tubing, which are difficult to produce and expensive, typically used in heavily corroded oil wells.

API N80 

N80 casing is a higher grade than L80, offering superior strength and performance in medium-deep wells and conventional pressure environments. It is the most widely used general-purpose steel grade in onshore and offshore drilling. Its average yield strength is 85,000 psi, and its average tensile strength is 100,000 psi. N80 does not have hydrogen sulfide resistance; sulfur-resistant steel grades such as L80 must be selected in sulfur-containing environments.

N80 is available in two types: N80-1 and N80Q.
N80-1: A standard type with normalizing heat treatment, ensuring basic toughness through Charpy impact testing, suitable for conventional operating conditions.
N80Q: A high-performance type treated with quenching and high-temperature tempering (quenching and tempering), featuring a more uniform microstructure and better overall mechanical properties, suitable for deep well applications requiring higher strength and toughness.

API P110 

P110 casing is a high-strength steel grade commonly used in deep wells and high-pressure environments. It combines high strength with good ductility, with an average yield strength of 105,000 psi and an average tensile strength of 125,000 psi. Suitable for deep well and high-pressure applications. Note: P110 does not resist hydrogen sulfide corrosion; caution should be exercised or other protective measures should be taken in sulfur-containing environments.

API Q125 

Q125 casing is a premium-grade product with exceptional strength and ductility. It is often selected for demanding applications, such as extreme depths and high pressures. It has an average yield strength of 125,000 psi and an average tensile strength of 150,000 psi. It is most commonly used in offshore wells, but can also be found in some onshore wells.

Oil Casing Pipe Steel Grades: How to Choose Correctly?

● Shallow wells, low pressure, no corrosion: Choose J55 or K55 for the lowest cost.
● Medium-deep wells, conventional pressure: Choose N80 for the best cost-performance ratio.
● H₂S-containing environments: Regardless of well depth, sulfur-resistant steel grades such as L80 must be selected.
● Deep wells, high pressure: Choose P110 or Q125.

The specific steel grade and wall thickness need to be determined by an engineer based on strength calculations.

Frequently Asked Questions (FAQ):

Q1: Are OCTG and casing the same concept?

No. OCTG is an abbreviation for "Oil Country Tubular Goods," a general term encompassing three main product categories: casing, tubing, and drill pipe. Casing is only a part of OCTG, used to construct the wellbore structure; tubing is used to transport oil and gas during the production phase; and drill pipe is used to transmit torque and deliver drilling fluid during drilling.

The three correspond to different API standards:
Casing and tubing: Follow API 5CT
Drill pipe: Follow API 5DP

Q2: Do drill pipe and casing pipe use the same steel grade?

Not entirely. Although both belong to OCTG, they are subject to different standard systems:

● Commonly used casing steel grades: J55, K55, N80, L80, P110, Q125 (API 5CT)
● Commonly used drill pipe steel grades: E75, X95, G105, S135, V150 (API 5DP)

The numbers in the drill pipe steel grade also represent the minimum yield strength (e.g., S135 is 135,000 psi), but drill pipe needs to withstand complex loads such as torque, tension, and bending during drilling. Its design requirements and performance testing differ from those of casing. Casing is mainly used to withstand the radial pressure and internal fluid pressure of the formation, while drill pipe focuses more on torsional, bending, and fatigue resistance. The two cannot be used interchangeably.

Q3: What is the difference between J55 and K55 casing?

J55 and K55 casing have the same yield strength, 55,000 psi. The main difference lies in tensile strength: K55 requires a higher tensile strength (≥95,000 psi, while J55 requires ≥75,000 psi), meaning K55 can withstand greater tensile loads. In practical selection, if the casing string of a shallow well is subjected to significant tensile forces, K55 should be given priority.

Q4: N80 and L80 have the same strength, so why is L80 more expensive?

Both N80 and L80 have a yield strength of 80,000 psi. The difference lies in the heat treatment process and chemical composition control:

N80: Normalized or normalized + tempered treatment, with relatively relaxed control over chemical composition and hardness, resulting in lower cost.
L80: Quenched + tempered (quenched and tempered) treatment, with strict limitations on hardness (≤23 HRC), and a specially designed chemical composition to ensure resistance to sulfide stress cracking in H₂S environments.

These additional process and control requirements make L80 more expensive to manufacture than N80. Therefore, L80 is only necessary in sulfur-containing environments; N80 is more economical for conventional environments.

Q5: How do I determine the steel grade required for my well?

The selection of casing steel grade needs to be determined by the drilling engineer based on strength design calculations considering the following factors:

a. Well depth: determines the tensile force of the casing string's own weight.
b. Formation pressure: determines the external extrusion pressure and internal pressure the casing needs to withstand.
c. Fluid properties: whether it contains corrosive media such as H₂S and CO₂.
d. Design safety factor: API recommended extrusion resistance ≥1.0, internal pressure resistance ≥1.1, tensile strength ≥1.8.

The final selection should meet the following requirements: within the safety factor range, select the lowest steel grade that meets the strength requirements to achieve a balance between safety and cost. If you have any questions, please consult the casing supplier (Email: info@hu-steel.com) or the drilling design agency.

Read more: 

● Casing Pipe Material Types and Selection 

● OCTG Casing Pipe Size Chart

● API Coupling and Threads for Tubing & Casing Pipe Connection