20-gauge seamless steel pipe

I. What are oil-use pipes?

Oil tubing (also known as oil-specific tubing) is a general term for all steel pipes used in oil and natural gas drilling and production operations. It forms the “skeleton” and “blood vessels” of an oil well, and its quality directly affects the safety, efficiency, and cost of drilling operations.

Due to their extremely harsh working environment—characterized by high pressure, high temperature, high corrosion, and complex stress—oilfield tubing places stringent demands on materials, manufacturing processes, and quality control.

II. Main Classification of Oil-Use Pipes (by Purpose)

This is a crucial part of understanding oil tubing, typically divided into two major systems:

System 1: Oil Well Tubing (OCTG - Oil Country Tubular Goods)

This is the core component of oilfield tubing—special steel pipes required during well construction (well drilling). It mainly includes the following three types, which are nested inside one another like “Russian nesting dolls”:

1. Drill pipe

The connectors at both ends, which are thickened and referred to as tool connectors, are joined to the pipe body via friction welding.

The pipe body is subjected to tremendous tensile, compressive, torsional, and bending stresses.

It is hollow in the middle and used to convey high-pressure drilling fluid.

Function: Connects the drill collar to the drilling rig, transmits torque and drilling fluid, and serves as the “driving shaft” of the drilling operation.

Features:

Marking: Drill rods are typically painted silver.

2. Casing

It is the “main structure” of the oil well and is used in large quantities.

Depending on the depth of installation and their functions, they are classified into surface casing, intermediate casing, and production casing.

Withstands external compressive stress, internal pressure, and axial tensile force.

Connected via a special thread (casing thread).

Function: From the wellhead down into the wellbore, it reinforces the well wall to prevent formation collapse; seals off different geological formations (such as oil layers, water layers, and high-pressure layers); and forms the wellbore, providing a passage for subsequent production operations.

Features:

Marking: The casing is typically painted green or another color (but not silver).

3. Oil pipe

Smaller in size than the casing.

Withstands internal pressure, axial tensile force, and corrosion (in direct contact with produced fluids).

Also connected via precision threads (oil pipe threads).

Function: It is run into the inside of the production casing and serves as a conduit for extracting oil or natural gas from the reservoir to the surface.

Features:

Marking: Oil pipes are typically painted blue.

Simple analogy:

Drill rod = the drill bit’s shaft used during construction (to be removed after completion).

Casing = the “retaining piles” and “sewer pipes” installed during house construction—they are permanent structures.

The oil pipe is a water supply pipe housed inside the “sewer pipe,” responsible for final delivery.

System 2: Oil and Gas Transmission Pipe (Line Pipe)

This is the pipeline pipe we discussed in detail in the previous section. It is responsible for transporting crude oil and natural gas—once extracted—from the source to refineries or end users over long distances via onshore or offshore pipelines.

III. Key Technical Requirements for Oilfield Tubing (Especially OCTG)

Due to its extreme operating conditions, the performance requirements for OCTG have reached the pinnacle of steel pipe manufacturing:

Ultra-high strength and high toughness: It must be able to withstand enormous tensile forces and complex stresses at depths of thousands of meters underground, while also preventing brittle fracture.

Resistance to high external and internal pressures—especially for casing, which must withstand both formation compaction and internal fluid pressure.

Strict thread machining accuracy: The threaded connection is the weakest link in the entire pipe string, and it is essential to ensure both airtightness and connection strength.

Excellent corrosion resistance:

Hydrogen Sulfide (H₂S)-Resistant Stress Corrosion Cracking: Tubing used in “acidic environments” must have extremely low hardness control and pure metallurgical quality.

Resistance to carbon dioxide corrosion: Chromium-containing alloy steel is required.

Resistance to chloride-ion corrosion: Particularly important in deep-sea environments or formations with high mineralization.

High temperature and high-pressure resistance: In deep wells and ultra-deep wells, temperatures can exceed 200℃, and pressures can surpass 100 MPa.

IV. Main Materials and Steel Grades

Standards: The American Petroleum Institute’s API standards are widely adopted worldwide, but many high-performance non-API standards also exist (such as proprietary standards from companies like V&M and Tenaris).

Common API steel grades:

H-40, J-55, K-55: Lower strength, used for shallow wells.

N-80: Commonly used steel grade.

L-80, C-90, T-95: High strength and resistant to hydrogen sulfide corrosion.

P-110, Q-125: Ultra-high strength, used for deep wells and ultra-deep wells.

Drill pipe: E-75, X-95, G-105, S-135 (the numbers represent the minimum yield strength, in thousands of pounds per square inch).

Casing and tubing:

High-performance materials:

13Cr martensitic stainless steel: resistant to CO₂ corrosion.

Duplex stainless steel/superduplex stainless steel: Simultaneously resistant to corrosion by CO₂, Cl⁻, and H₂S.

Nickel-based alloys (such as G-3, C-276, 825): Used in harsh corrosive environments (high temperature, high Cl⁻ concentration, high H₂S content).

V. Production Process

Seamless pipe: This is the mainstream production process for OCTG (drill pipes, casing, and tubing) because it is entirely seamless and thus offers higher safety and reliability. It is primarily produced via hot rolling.

Welded pipes: Some conveyance pipes and casing for shallow wells will use welded pipes (such as ERW and LSAW).