Stress of API 5CT Oil Casing in Oil Well

The stress on the API 5CT oil casing in the oil well: to ensure that the casing running into the well is continuous, not cracked or deformed, the casing is required to have a certain strength, enough to resist the external force it receives. Therefore, it is necessary to analyze the stress on the inner well casing.

Types of casing stress:

1. Tensile force: When the casing is lowered into the well, the weight of the entire casing string applies a tensile load, especially in the upper part. This tensile force pulls on the casing, so a high tensile strength is required in the upper part of the casing.
2. Compression force: In the lower part of the well, the casing is subjected to external compression force due to the downward pressure exerted by the weight of the overlying formation. The casing must withstand this compression force without buckling.

3. Internal pressure: During production, the internal pressure from the oil, gas or fluid in the well exerts a large force on the casing. If the casing strength is insufficient, the internal pressure increases the risk of rupture.
4. Bending force: Although the bending force has a smaller effect than other forces, in deviated or horizontal wells, when the casing string is bent, bending force may be generated, causing stress in the casing along the length direction.

Casing string stress distribution:

Upper part: The upper part of the casing string is mainly subjected to tensile stress because it supports the weight of the entire casing string.
Lower Section: The lower section of the casing string is subject to compressive forces from the formation and the casing's own weight.
Middle Section: The middle section is subject to less forces than the upper and lower sections, but still requires careful design.

Safety factors:
The design of the casing string takes these stresses into account by selecting the appropriate steel grade and wall thickness. Safety factors are used to ensure that the casing can withstand unexpected stress changes:

Tensile Strength Safety Factor: Typically 1.6 to 2.0 to account for tensile forces.
Impact Safety Factor: Typically between 1.00 and 1.50 (typically 1.125) to protect against impact loads.
Internal Pressure Safety Factor: Varies between 1.0 and 1.33 to ensure that the casing can withstand internal fluid pressure.
Compression Safety Factor: At the cement injection point, the recommended safety factor is about 0.85 to account for external pressure.

Casing design considerations:

The selection of safety factors and casing specifications depends on a variety of factors, including regional conditions, geological formations, and future production processes. Empirical data is often used to fine-tune these safety margins.

Because different parts of the casing string are subject to different stress levels, the design may use thicker walls or higher grades of steel in the upper and lower parts of the casing, while the middle part is relatively thin. Casing is usually numbered before it is lowered into the well to ensure correct placement based on these stress changes.

Corrosion resistance and sealing:

In addition to strength, casing must also be able to operate in potentially corrosive environments. This requires not only high joint strength, but also excellent corrosion resistance and effective sealing to prevent leaks during the service life of the well.

Conclusion:

The steel casing in the well mainly bears the first three forces. The stress conditions of the various parts are different, the upper part receives the pulling force, the lower part has the outer pressing force, and the middle part receives less external force. When designing the casing string, the steel grade and wall thickness of the casing are selected based on the above consideration of the safety factor. 

For the API standard casing, the general safety factor for tensile is 1.6-2.0, the safety factor for impact resistance is 1.00-1.50, generally 1.125, the safety factor for internal pressure is 1.0-1.33, and the safety factor for the compression resistance at the cement injection site is The desirable value is 0.85. 

It should be emphasized that the safety factor in the design of casing string strength is carefully selected according to the region, stratum and the later oil extraction and gas production process. He is an empirical figure. Due to the different external forces applied to the upper, middle and lower parts of the casing string, the designed casing string is often thicker or more steel grades in the upper and lower walls, and opposite in the middle, so it is necessary to number the casing. Into this well. In most cases, the casing is working in corrosive media. Therefore, in addition to requiring a certain degree of joint strength, the casing is required to have good sealing performance and to be resistant to corrosion.

Read more: API 5CT Steel Casing and Tubing Chart or Types of Drilling Casing Pipe