Casing Strength and Load Analysis: Collapse Criterion

 

Introduction

What happens when external pressure exceeds casing strength? The collapse of casing under excessive external pressure can have disastrous consequences for the well's integrity. This section explores the different types of collapse and how to calculate collapse resistance, ensuring the casing can withstand the pressures it faces during drilling and production.

Types of Collapse

Casing collapse can occur when external pressures such as the weight of the overburden or formation pressures exceed the strength of the casing. There are three primary types of collapse:

1. Elastic Collapse

Elastic collapse occurs when the casing material deforms under external pressure but can return to its original shape once the pressure is relieved. This type of collapse is typically seen in shallow wells or in situations where the casing is exposed to moderate external pressures.

• Impact: Elastic collapse does not cause permanent damage to the casing, but it can temporarily reduce the casing's ability to resist further pressure.

2. Plastic Collapse

Plastic collapse happens when the casing material deforms permanently under pressure. Once the yield point of the material is reached, the casing does not return to its original shape. Plastic collapse is more common in deeper wells with higher pressures, where the casing material is exposed to severe external forces.

• Impact: Plastic collapse can lead to permanent deformation of the casing, compromising its ability to isolate formations and resist additional external pressures.

3. Yield Collapse

Yield collapse occurs when the casing material reaches its yield strength and deforms irreversibly. The casing will collapse under this type of pressure and will not return to its original shape, resulting in significant structural failure. Yield collapse is particularly problematic in high-pressure and deepwater environments.

• Impact: Yield collapse can severely compromise well integrity, leading to wellbore instability, loss of well control, and the need for costly remedial work.

Calculating Collapse Resistance

To ensure that the casing can withstand the expected external pressures, engineers use specific calculations to determine the casing's collapse resistance. The American Petroleum Institute (API) provides standards that help guide these calculations. The basic calculation involves evaluating the casing material's strength and the external pressures it will face, including the weight of overburden, formation pressures, and other external forces.

Collapse Resistance Calculation: API Standard

The collapse resistance is calculated using the formula:

$$C = 2.5 \times \left( \frac{(D_o - D_i)}{D_i} \right) \times \left( \frac{T}{S} \right)$$

Where:

• $C$
• $D_o$
• $D_i$
• $T$
• $S$

This formula considers the difference between the internal and external diameters of the casing, along with the material strength and wall thickness. The higher the yield strength and wall thickness, the better the casing will resist collapse under external pressure.

API Standards

API 5C3 is the standard used for calculating the collapse resistance of casing. This standard provides specific guidelines for the material properties and the conditions under which collapse resistance should be evaluated, ensuring consistency and reliability in casing design.

Practical Application of Collapse Resistance

In real-world applications, collapse resistance calculations help engineers select the appropriate casing for specific well conditions. For example, in high-pressure and deepwater wells, casing must be designed to handle significantly higher external pressures than shallow wells.

Example: Deepwater Well Design

In a deepwater drilling operation, external pressures are much higher due to the water column and formation pressures. To ensure the casing can resist collapse, engineers might use thicker, high-strength steel alloys, as well as multiple casing strings to distribute the external loads. These materials and designs are often chosen after conducting detailed collapse resistance calculations using API standards.

Cost Considerations

While thicker casing and high-strength materials increase upfront costs, the long-term benefits of avoiding collapse failure outweigh the risks. By calculating collapse resistance accurately, engineers can select the most cost-effective material that meets safety and performance standards without overspending on unnecessary strength.

Conclusion

Casing collapse is a critical factor in well integrity and safety, especially when external pressures exceed the casing's strength. Understanding the types of collapse elastic, plastic, and yield and how to calculate collapse resistance using API standards helps engineers design casing systems that can withstand extreme wellbore conditions. By carefully considering these calculations, engineers can ensure the casing performs reliably under pressure, protecting both the well and the environment.