Casing vs. Tubing: Key Differences, Functions, and API Standards

Casing and tubing are steel pipes that are vital to use in oil, gas and water wells. Despite their similarities in appearance, casing and tubing have different functions in the wells. The function of casing is to support the wellbore and separate formations underground.

Casing vs tubing information provides an insight for customers to choose the right pipe that fits their requirements. It all comes down to several factors including the depth of the well, pressure, temperature, size, grade, and type of connections.

Before buying casing and tubing (casing pipe and tubing pipe), buyers are advised to review the specifications and API standards.

What is OCTG?

OCTG means Oil Country Tubular Goods. These are the steel pipes utilized in oil and gas wells. Mainly these pipes are used for drilling and transferring the oil and gas to the surface.

OCTG includes three kinds of steel pipes:

• Casing – pipe used in the well in order to reinforce it and provide stability.
• Tubing – pipe which extracts oil or gas from the well and brings it to the surface.
• Drill Pipe – pipe used in drilling operations to rotate the drill bit and provide drilling fluid.

Both casing and tubing are made in accordance with API Spec 5CT standard, while drill pipes should conform to API Spec 5DP. Line Pipes (API 5L) differ from OCTG pipes. Line pipes are used for transportation of oil, gas or water above the ground level.

What Is Casing Pipe?

Casing pipe refers to a pipe made of steel that is set into a borehole well in order to provide structural stability to the hole. The process of drilling leads to instability of the surrounding rock formation.

Casing pipes are typically set in stages as the drilling progresses downwards. Casing pipes can be accompanied by cement in order to hold the pipe in place.

Main Functions of Casing Pipe

Casing performs several important jobs:

• Supports the well: Helps prevent the well walls from collapsing.
• Separates underground formations: Helps stop unwanted movement of fluids between different layers.
• Protects the well: Helps keep loose rock and other materials from entering the well.
• Supports well equipment: Provides a structure for other equipment used in the well.
• Helps maintain well integrity: Works as part of the well's protective system when correctly designed and installed.

Choosing the correct steel casing pipe is determined by well design and underground conditions. People who need oil well casings must verify certain requirements such as diameter and steel grade before ordering.

Types of Casing Strings

There are usually multiple casing strings used for any single well. Different strings serve different purposes:

• Conductor casing: the first and largest casing string, placed close to the surface to provide stability to the upper part of the wellbore.
• Surface casing: protects fresh water zones and provides support to the wellhead and blowout prevention equipment.
• Intermediate casing: isolates difficult areas, such as unstable zones or areas where there are pressure transitions, between the surface and the production zones.
• Production casing: passes through the producing zones, in which the tubing string is placed.
• Liner: a smaller casing string suspended from the bottom of the previous casing string without reaching the surface.

What Is a Tubing Pipe?

A tubing pipe is used to transport fluids such as oil, gas, and other production fluids from the well to the surface. A tubing pipe is typically placed inside the casing and forms part of the well's production string.

The basic difference between the two is that the casing will provide support for the well, while the tubing will give a channel through which the fluids will move. Based on how the well is designed, it is possible to remove the tubing without removing the casing.

Main Functions of Tubing Pipe

Some of the important functions of tubing pipe include the following:

• Fluid transport: Transport of produced oil, gas, and other fluids towards the surface.
• Production support: Becomes part of the system responsible for producing hydrocarbons from the well.
• Connection with other equipment: Can use appropriate connections to interact with other completion equipment.
• Facilitation of maintenance: Usually can be easily changed in case of need, depending on the well configuration.
• Meets operating needs: Must suit the pressure, temperature and fluids expected inside the well.

Oil well tubing and other types of oilfield tubing should be selected according to the well's production needs and operating conditions.

Casing vs. Tubing: Key Differences

The key differences between casing and tubing are their functions. Casing aids in the strengthening of the wellbore, whereas tubing facilitates the transportation of produced fluids from the wellbore to the surface.

The table below illustrates their primary distinctions.

Although both products may be made from steel, they cannot simply replace one another. Each must meet the technical requirements of its intended use.

Main Applications of Casing and Tubing

Casing and tubing have primary applications in a well that are distinct from one another.

Applications of Casing

Casing pipe is used to:

• Support the well during and after drilling.
• Help separate underground formations.
• Reduce the risk of formation material entering the well.
• Provide support for well equipment.
• Help maintain well integrity when properly designed and installed.

Applications of Tubing

Tubing pipe is used to:

• Carry oil, gas and other produced fluids to the surface.
• Provide a clear flow path inside the well.
• Connect with suitable production equipment.
• Allow maintenance or replacement when the well design permits.
• Meet the operating needs of the production system.

Requirements for oil and gas casing and tubing will vary based on the project. Although casing is used in water wells as well, it will have to be specific to the drilling requirements.

Casing and Tubing Specifications

It is essential to check the casing and tubing specifications prior to purchase. Two pipes can be of similar outside diameter with varying thicknesses, materials, or connections. This can determine the suitability of a pipe for a certain well.

Pipe Size and Dimensions

Buyers should check the following details:

• External diameter: The outside diameter of the pipe is determined by the well design.
• Thickness: Affects the structural integrity and inner diameter of the pipe.
• Weight per unit length: Helps identify the required pipe specification.
• Length: Must suit the installation plan.
• Dimensional tolerances: Must meet the agreed technical requirements.

The project or drilling team should confirm the required dimensions before an order is placed.

Connections and Threads

Connections are used to connect pipe segments. They should be appropriate for the well operating conditions and compatible with other equipment.

Ensure the following prior to ordering:

• Type of thread or connection needed.
• Compatibility with the current well equipment.
• Thread and inspection specifications.
• Connection specifications provided in the purchase order.
• Requirement for thread protection when transporting.

API specification 5CT deals with the requirements of casing and tubing products. API specification 5B provides the specified threading, gauging and inspection specifications. The buyer should verify applicable specifications for his/her products.

Material and Operating Conditions

Proper pipe choice is also based on the well's operating conditions. Such conditions include:

• Operating pressure and temperature.
• Interaction with corrosive fluids.
• Mechanical loads.
• Well depth and construction.
• Material certification and inspection reports.

Considering all these parameters will help the buyer choose the proper casing pipe and tubing, rather than choosing based on price alone.

Common Size, Connection, Length

Verify the most recent API 5CT edition to ensure accuracy of all sizes, connection information and dimensions prior to printing.

API 5CT Casing and Tubing Standards

The API 5CT standard, issued by the American Petroleum Institute (API), provides technical criteria for steel casing and tubing. This standard involves all necessary details about the products concerned, such as grades, sizes, and required end finishes.

An understanding of the API 5CT casing and tubing will help the buyer formulate clear specifications and verify if the supplied items satisfy the specifications provided by the buyer.

Prior to making the purchase, make sure that the following details are verified:

• Material grade: Verify if the correct grade is needed.
• Pipe sizes: Look at the diameter, wall thickness, weight, and length.
• End finishes: Confirm that the pipe ends conform to requirements.
• Testing and inspection: Confirm what testing is needed.
• Documentation: Look at the certificates and test reports of the material.
• Edition of the standard: Verify the edition of the standard relevant to the purchase.

API Spec 5CT gives the specifications of casing and tubing that are utilized in the oil and gas industries. Buyers are advised to make reference to the most recent versions of API specification in order to ensure compliance with the necessary specification standards.

Note, however, that simply because a product is stated to conform to an API standard, this is not enough to conclude that the manufacturing company holds an API license. Potential buyers must confirm this information before buying API 5CT steel pipe.

Common Casing and Tubing Grades

Casing & tubing grades are classifications of materials with particular technical and mechanical characteristics. Each grade is chosen depending on the design of the well and working conditions.

Some common grades are:

• J55: Selected where its particular properties are applicable to the design of the well.
• L80: Family of grades with particular mechanical and material requirements. The type of L80 needs to be confirmed.
• C90: Grade with particular requirements according to the specification.
• P110: High strength grade selected when the design of the well requires particular properties.

These grades cannot be substituted. It doesn't mean that the high strength grade is suitable for all wells. Buyers need to confirm the grade and product type according to the project specification.

API 5CT Grades at a Glance

Sour service (hydrogen sulfide) selection in accordance with NACE MR0175/ISO 15156. 1ksi is equivalent to 6.9 MPa. Values obtained from API 5CT. Check this against the latest version and mill test report.

How to Choose the Right Casing and Tubing

Selecting the correct piping begins with an assessment of the requirements of the well. The customer must provide precise technical information to enable the product to be evaluated in light of these requirements.

The following must be taken into account prior to ordering:

• Well depth: Familiarise yourself with the well configuration and loading requirements.
• Pipe dimensions: Verify diameter, wall thickness, length and weight.
• Operating conditions: Consider the pressure and temperature requirements.
• Material grade: The selection of the appropriate grade that provides the required mechanical properties.
• Corrosion resistance: An assessment of the fluid and operational conditions that the piping is exposed to.
• Connection: Ensure compatibility with the well equipment connections.
• API specification requirements: Specify the appropriate specification and its edition.
• Quality certification: Specify the relevant certificates, inspection documents and traceability documents.

A clear purchase order should specify the type, grade, dimensions, connection, quantity and documentation requirement of the piping.

Why Choose Ochnology for Casing and Tubing Requirements?

Choosing a casing and tubing supplier involves more than comparing prices. Buyers also need clear product information, suitable documentation and reliable communication.

Ochnology lists casing and tubing products for oil, gas and water-well applications. Information on its products includes choices like J55, L80, and C90. It is advisable for purchasers to establish the current availability and connections beforehand.

Product Specifications and Documentation

The company describes checks covering pipe dimensions, threads, material-grade documentation, surface finish and traceability. Its website also lists COA and MSDS documents among its shipment documents. Buyers should confirm which documents apply to their particular order.

Export Experience and Communication

Ochnology claims to have exported to over 40 countries and The company also claims that their export team answers all queries within 24 working hours. The buyers need to verify the cost, lead time, documentation, and other delivery terms while quoting.

Shipping Arrangements

The website mentions FOB, CIF, and FOR in their methods of shipping. Depending upon the location, number of orders, and business terms, the method can be determined.

In case of a comparison between casing and tubing manufacturers or casing and tubing exporters in India, the buyers need to verify the real manufacturers and documents.

Requesting a Quote

In order to be able to provide you with a quotation that suits your project, it is required that you give us information regarding pipe size, thickness, grade, type of connection, length and number of units. The port of destination, as well as any other specifications you may require, must also be considered.

Contact Ochnology to discuss product availability and your project's casing and tubing requirements.

Conclusion

Deciding between casing and tubing will be easier once you know what each type of pipe is used for. Before purchasing your pipes, think about the requirements of your well, pipe size, pipe material grade, and connections.

If you need help deciding what kind of pipes you should choose for your project, contact us at Ochnology!