Casing Strength and Load Analysis: Burst Criterion

 

Introduction

"Can you calculate the burst pressure for a casing section?" Burst failures can be catastrophic if internal pressures exceed the casing's strength. In this section, we'll explore the effects of internal pressures, how burst failure occurs, and the importance of accurate burst pressure calculations in ensuring well integrity.

Understanding Internal Pressures and Their Effects

Internal pressures in a well are typically caused by the presence of fluids or gases that exert force on the casing from the inside. These pressures can vary depending on the well depth, formation characteristics, and production or injection activities.

Internal Pressures:

• Fluid Pressures: Often result from the injection of drilling mud or fluids used during completion and production phases.
• Gas Pressures: In high-gas wells, such as gas reservoirs or during the production of natural gas, internal pressures can spike dramatically, stressing the casing to the point of failure.

The casing must withstand these internal pressures without failing. If the internal pressure exceeds the casing's burst resistance, the casing will rupture, which can lead to severe safety hazards and loss of well control.

Types of Burst Failure

Burst failure occurs when the internal pressure exceeds the casing's ability to resist the expansion forces. There are two main scenarios for burst failure:

1. Elastic Burst

Elastic burst occurs when the casing material deforms elastically under internal pressure but can return to its original shape once the pressure is relieved. This type of failure generally happens at shallow depths or low-pressure conditions.

• Impact: While elastic burst doesn't cause permanent casing damage, it can cause temporary wellbore instability and loss of well control.

2. Plastic Burst

Plastic burst happens when the casing material reaches its yield strength and deforms permanently. This often occurs in high-pressure environments, such as deepwater or high-gas wells. The casing will not return to its original shape once plastic deformation has occurred, leading to permanent damage.

• Impact: Plastic burst failure compromises well integrity and may require remedial operations to repair or replace the casing.

Calculating Burst Pressure for a Casing Section

To prevent burst failure, engineers calculate the burst pressure that the casing can withstand before failure occurs. The burst pressure is influenced by the casing's material properties, diameter, wall thickness, and the internal fluid or gas pressures.

Burst Pressure Calculation:

The burst pressure can be calculated using the following formula:

$$P_b = \frac{2S}{D_o - D_i} \times T$$

Where:

• $P_b$
• $S$
• $D_o$
• $D_i$
• $T$

This formula calculates the maximum internal pressure the casing can withstand before failure. The burst pressure is influenced by the material's strength (yield strength) and the casing's wall thickness.

API Burst Pressure Standards:

The American Petroleum Institute (API) provides standards for calculating burst pressure, specifically API 5C3, which defines burst pressure limits based on casing size and material specifications.

Case Study: Burst Failure in a High-Gas Well

In a high-gas well, burst failure can occur if the casing is not designed to withstand the extreme internal pressures generated by gas production. For example, in a deep gas well, the internal pressure might exceed the design strength of the casing during production, especially when the gas reservoir is under high pressure.

Case Study: A High-Gas Reservoir

In a case study of a high-gas well, engineers failed to account for sudden gas pressure surges during production. The internal pressure reached a level that exceeded the casing's burst pressure, causing the casing to rupture and leading to a blowout. The failure resulted in loss of well control, environmental hazards, and significant financial costs.

This case highlights the importance of proper burst pressure calculations, especially in gas wells where internal pressures can vary significantly. Proper casing design, including thicker walls and stronger materials, can prevent such failures.

Practical Application of Burst Pressure Calculations

Burst pressure calculations play a crucial role in selecting the right casing for high-pressure wells. Engineers use the burst pressure calculation to ensure the casing will withstand the internal pressures expected during drilling, completion, and production.

Example: Designing for a High-Pressure Gas Well

For a high-pressure gas well, engineers might select a casing material with a higher yield strength and increase the casing wall thickness to ensure the casing can resist burst failure. By performing burst pressure calculations, they can confirm that the casing will withstand internal gas pressures, protecting well integrity and ensuring safe operations.

Conclusion

Burst failure is a critical consideration in casing design. By understanding the effects of internal pressures, calculating burst pressure using API standards, and applying these calculations in real-world scenarios, engineers can ensure that casing will perform safely under high-pressure conditions. Proper burst pressure calculations help prevent catastrophic failures, preserve well integrity, and reduce operational risks in challenging well environments.