Manufacturing Process of Casing and Tubing for Oil and Gas Wells

Casing and tubing are essential for oil and gas wells. They provide structural integrity and facilitate the transport of hydrocarbons. Casing lines the wellbore to prevent collapse and protect groundwater, while tubing transports oil or gas to the surface. Their manufacturing process involves precise steps to meet rigorous standards. API 5CT standards govern oil & gas tubular manufacturing to ensure reliability and safety.

1. Raw Materials Used in Manufacturing

The manufacturing of casing and tubing begins with high-quality steel, selected for its strength, corrosion resistance, and weldability to withstand the harsh conditions of oil and gas wells. The primary raw materials include:

Carbon Steel: This steel is used for its cost-effectiveness and adequate strength in less corrosive environments. It typically contains low to medium carbon content (around 0.3% or less) and is the base material for many API grades.

Alloy Steel: Alloy steel incorporates chromium, molybdenum, and nickel to enhance strength and corrosion resistance. It is suitable for deeper or more corrosive well conditions.

Stainless Steel: Employed in highly corrosive conditions of high hydrogen sulfide (H2S) or carbon dioxide (CO2) levels, due to its superior corrosion resistance.

2. Steel Grades and Alloy Compositions

API 5CT specifies a range of steel grades for casing and tubing, each designed for specific well conditions based on yield strength, tensile strength, and corrosion resistance. Common grades include H40, J55, K55, N80, L80, C90, T95, P110, and Q125, with numbers indicating the minimum yield strength in thousand psi (ksi). The chemical compositions of these grades are tightly controlled to achieve the desired properties. Below is a table summarizing the chemical compositions for select API grades, based on API 5CT standards:

Chemical Composition of API Steel Grade for Casing and Tubing

3. Manufacturing Processes

Casing and tubing are manufactured using either seamless or welded processes, depending on the pressure, cost, and performance requirements of each application. Seamless pipes dominate oil and gas wells due to their strength and reliability in high-pressure environments. Welded pipes are cost-effective and used in low-to-medium-pressure applications, such as surface casing or less critical well sections. 

Seamless Pipe Manufacturing

Seamless pipes are produced through a series of steps starting with a solid steel billet:

  1. Heating and Piercing: The billet is heated to a high temperature (around 1200–1300°C) and pierced to create a hollow tube.

  2. Hot Rolling or Cold Drawing: The hollow tube is elongated and shaped through hot rolling to achieve the desired diameter and wall thickness. Cold drawing may follow for precise dimensions and smoother surfaces.

  3. Sizing and Straightening: The pipe is sized to meet dimensional tolerances and straightened to ensure uniformity.

Welded Pipe Manufacturing

Welded pipes are made by joining steel strips or plates, typically using:

  1. Electric Resistance Welding (ERW): Steel coils are cold-formed into a cylindrical shape, and the edges are welded together using an electric current, forming a longitudinal seam without the need for filler metal.

  2. Submerged Arc Welding (SAW): Steel plates are bent and welded using a submerged arc process, which is suitable for larger-diameter pipes.

  3. High-Frequency Induction (HFI): Similar to ERW but uses high-frequency current for welding.

4. Heat Treatment and Finishing Processes

Heat treatment is a pivotal step in manufacturing casing and tubing. It determines the mechanical properties (yield strength, tensile strength, hardness) that differentiate API grades. Finishing processes ensure the pipes meet dimensional, functional, and durability requirements.

Heat Treatment Processes

Heat treatment methods vary by steel grade to achieve the desired properties:

  • Normalizing: The steel is heated to 850–950°C and air-cooled. This refines the grain structure, improving strength, toughness, and machinability. Normalizing is used for grades like J55 and K55.

  • Quenching: Applied to grades like N80, L80, and P110. The steel is heated to 850–950°C and rapidly cooled (e.g., in water or oil) to increase hardness and strength.

  • Tempering: Quenching also makes steel brittle. Hence, after quenching, the steel is reheated to a temperature of 400–650°C to reduce brittleness while maintaining its strength. This process is called Tempering and is critical for high-grade steels like L80 and P110.

  • Combined Quenching and Tempering: This process is used for high-performance grades, such as L80 and P110, to optimize mechanical properties for deep or corrosive wells.

  • Stress Relieving is applied to welded pipes to reduce residual stresses in the weld seam. It is typically performed at a minimum temperature of 540°C to eliminate untempered martensite (Central Steels).

Specific requirements include:

  • N80 Type 1: Normalized or normalized and tempered at the manufacturer’s discretion.

  • N80Q: Quenched and tempered for higher strength and collapse resistance.

  • L80 (9Cr, 13Cr): Quenched and tempered to avoid brittleness.

  • P110: Quenched and tempered for high-pressure environments.

Finishing Processes

After heat treatment, casing and tubing undergo several finishing steps:

  • Straightening: Ensures the pipe is free of bends or distortions.

  • Dimensional Control: Pipes are sized to meet API-specified diameters and wall thicknesses.

  • Threading: Threaded connections (e.g., STC, LTC, BTC, or premium threads like VAM) are cut to allow secure joining of pipe sections.

  • Coating: Protective coatings, such as anti-corrosion paints or varnishes, are applied to prevent rust during storage and transport.

  • Quality Inspection: Non-destructive testing (e.g., ultrasonic, magnetic particle) ensures compliance with API standards for strength, consistency, and defect-free surfaces.

  • Packaging: Pipes are bundled and prepared for shipping. The color-coded bands indicate steel grades.

These processes ensure that casing and tubing can withstand the compression, tension, collapse, and burst pressures encountered in oil and gas wells, as well as the corrosive conditions from reservoir fluids.