N80 Casing & Tubing, N80 Steel Pipe

In oil drilling, N80 steel pipe is a common grade of casing and tubing steel. N80 casing is a key material for constructing the wellbore structure (cementing), while N80 tubing serves as the production channel for oil and gas flow. Other types of steel pipe used in downhole operations include drill pipe for transmitting power, drill collars for providing pressure to the drill bit, and core tubes for obtaining formation samples.

What is N80 casing?

N80 casing refers to seamless oilfield casing that conforms to the American Petroleum Institute (API) SPEC 5CT standard, with a minimum yield strength of 80,000 psi (approximately 552 MPa). Where:

"N": Indicates that this steel grade undergoes a normalizing heat treatment process. This results in a uniform and stable microstructure and mechanical properties.
"80": Indicates its minimum yield strength of 80,000 psi (approximately 552 MPa).

Applications: Used in oil well drilling, primarily for supporting the wellbore during and after drilling to ensure the smooth operation of the well and the overall well life after completion. N80 tubing is a specialized pipeline run into the casing during the production stage of oil and gas wells to transport oil and gas products.

N80 Casing & Tubing Pipe Specifications:

N80 casing covers a variety of sizes from small to large diameters. 

Common outer diameters for N80 casing are: 4-1/2", 5-1/2", 6-5/8", 7", 9-5/8", and 13-3/8".

Common outer diameters for N80 tubing are: 2-3/8", 2-7/8", 3-1/2", 4", and 4-1/2".

Differences between N80 casing and tubing:
N80 casing: Constructs the wellbore structure, isolates the formation, and prevents well collapse. It acts as the load-bearing wall of the well and has a larger diameter. It is run during the drilling and completion phases and is a permanent structure.

N80 tubing: Transports oil and gas products; it is the internal production pipeline and has a smaller diameter. It is run during the completion and production phases and serves the entire production lifecycle.

N80 Casing Length:
API classifies single casing lengths into three ranges:

R1: 16 - 25 feet (4.88 - 7.62 meters), now less commonly used, primarily for specific confined applications.
R2: 25 - 34 feet (7.62 - 10.36 meters), the most common and mainstream range. Offers a good balance between transport convenience and drilling efficiency.
R3: 34 - 48 feet (10.36 - 14.63 meters), used for applications requiring high drilling efficiency, reducing downhole connections, but with higher transport and handling requirements.

Common Threads: N80 casing primarily uses standard API threads (STC, LTC, BTC) and special threads (such as VAM, FOX).

Mechanical Properties:

According to API 5CT standard, the performance parameters of N80 oil casing are as follows:

Yield Strength: 80,000 - 110,000 psi (552 - 758 MPa)
Tensile Strength: ≥ 100,000 psi (689 MPa)
Elongation: ≥ 19% (specific value depends on gauge length and pipe diameter)
Hardness: ≤ 24.0 HRC

Meaning of Performance Parameters:

Yield Strength Range: Ensures that the casing can undergo slight deformation under high pressure without immediate failure, which is crucial for pressure-bearing design.
Tensile Strength: Ensures the casing can withstand its own significant weight and resist tensile fracture.
Elongation: Represents the casing's toughness; a higher value indicates a stronger ability of the material to plastically deform before fracture, making it less prone to brittle fracture.
Hardness: Controls the casing's hardness to prevent cracking under specific conditions.

Features of N80 Casing & Tubing:

The core feature of N80 casing & tubing lies in its balanced overall performance:

1. Moderate Strength: Its strength is higher than low-grade steels such as J55 and K55, but lower than high-grade steels such as P110 and Q125, achieving one of the best balances between strength, toughness, and cost.
2. Good Toughness and Machinability: Due to normalizing treatment, internal stress is eliminated, resulting in a uniform internal structure and good impact toughness and weldability.
3. Good Economic Efficiency: Due to its wide application and large-scale production, and mature manufacturing process, N80 casing offers high cost-effectiveness and is the first choice for many conventional well locations.
4. Strong Versatility: Suitable for various well conditions, from surface casing to production casing.

Important Limitation: N80 casing does not have resistance to hydrogen sulfide stress cracking; therefore, it cannot be used in acidic oil and gas environments containing hydrogen sulfide.

Main Applications:

N80 steel pipe is mainly used as casing and tubing, in the following applications:

1. Intermediate Casing (Main Application): This is the most classic application of N80 casing. During drilling, it is used to isolate complex formations (such as high-pressure zones and lost circulation zones), creating safe conditions for drilling deeper target layers.
2. Production Casing/Tubing: In many medium-deep wells and producing formations with normal pressure, N80 casing can serve as the final production layer, forming a channel for oil and gas production.
3. Surface Casing: In areas with favorable geological conditions, N80 can also be used as surface casing.

Comparison of N80, L80, and P110 Steel Grades:

N80 Casing: No sulfur resistance. 

Minimum Yield Strength: 80,000 psi. 

Heat Treatment: Normalizing. 

Maximum Hardness: 24.0 HRC. 

Main Applications: Technical/production casing for conventional wells, with a lower price.

L80 casing: Sulfur-resistant
Minimum yield strength: 80,000 psi
Heat treatment: Quenching + Tempering
Maximum hardness: 23.0 HRC
Main applications: Acidic environments containing hydrogen sulfide; higher price.

P110 casing: No sulfur resistance
Minimum yield strength: 110,000 psi
Heat treatment: Quenching + Tempering
Main applications: Higher strength than N80; used in deep and ultra-deep wells; high-strength sections; higher cost.

Comparison conclusion:
a. N80 vs L80: Both have the same strength, but the core difference lies in sulfur resistance. L80 achieves sulfur resistance through more stringent heat treatment and composition control, thus increasing its cost. N80 should never be used as a substitute for L80 in hydrogen sulfide-containing wells.
b. N80 vs P110: P110 has higher strength and is used in more demanding mechanical environments, but its toughness and sulfur resistance are generally inferior to N80, and its cost is also higher.

N80-1 and N80Q:

N80 steel is divided into N80-1 and N80Q. They share the same tensile properties, but differ in delivery condition and impact performance.

N80-1: "-1" refers to "Supplementary Requirement SR2" in the API SPEC 5CT standard. SR2 stands for "Charpy V-Notch Impact Test Requirement." N80-1 means that in addition to meeting all the basic requirements of N80 steel grade, it must pass the Charpy impact test to prove sufficient toughness.

N80Q: In the API SPEC 5CT standard, N80 steel grade can have two heat treatment states:
Type 1: Normalized or normalized + tempered. This is what we usually refer to as standard N80.
Type 2: Quenched + tempered. This is N80 Q. The impact energy should comply with API 5CT requirements and should be subjected to non-destructive testing.

In procurement or technical specifications, you will see annotations like these:
Casing N80, Type 2 (i.e., N80 Q)
Casing N80, with SR2 (i.e., N80-1)
Casing N80, Type 2 with SR2 (i.e., satisfying both conditions)

Casing Product Introduction:

Domestic casing to geological drilling steel by hot-rolled or cold drawn, steel with "geological" (DZ) said that the commonly used casing steel DZ40, DZ55, DZ753 species.

(1) Major importing countries for oilfield casing include: Germany, Japan, Romania, Czech Republic, Italy, UK, Austria, Switzerland, USA, and also Argentina and Singapore.
(2) API specifies three lengths: R-1 (4.88–7.62 m), R-2 (7.62–10.36 m), and R-3 (10.36 m and longer).
(3) Some imported goods are marked with "LTC," indicating long-thread casing.
(4) In addition to API standards, a small portion of casing imported from Japan adheres to Japanese manufacturer standards.
(5) In claims cases, issues have included external defects such as blackened threads, damaged threads, folded pipe body, broken threads, out-of-tolerance thread tightness, and out-of-tolerance J-value of couplings, as well as internal quality problems such as brittle casing and low yield strength.

Conclusion:

N80 casing is a versatile and cost-effective option. Its perfect balance between strength, toughness, and cost has made it one of the most widely used oilfield tubing materials globally. As long as the downhole environment is free of hydrogen sulfide and the pressure and depth are within acceptable limits, N80 casing is almost always a top choice for engineers.

Read more: Casing Pipe Specifications and Dimensions or N80 VS J55 Casing