Tension Criterion in Casing Design

 

Introduction

"How do you prevent casing failure in extended-reach drilling?" In extended-reach drilling (ERD), managing the tensile load on casing is critical to prevent casing failure. Understanding the tension criterion is essential to ensure the casing can withstand the stresses imposed by the well trajectory, depth, and operational conditions.

What Is the Tension Criterion?

The tension criterion is a critical aspect of casing design that ensures the casing can withstand the axial tensile forces encountered during the drilling process. These forces arise due to the weight of the casing string, operational loads, and the well’s geometry (especially in directional or extended-reach wells). If the tensile load exceeds the casing’s tensile strength, failure can occur, potentially compromising well integrity and leading to costly delays and safety hazards.

Calculating Tensile Loads and Safety Factors

1. Tensile Loads:

Tensile load refers to the force exerted along the length of the casing string, which pulls the casing in opposite directions (from the surface to the bottom). The total tensile load on the casing depends on:

• Casing weight: The weight of the casing material itself contributes to the tensile load.
• Mud weight: The weight of the drilling fluid inside the casing exerts additional force.
• Operational loads: During operations, especially in directional drilling or during tripping, the casing experiences additional tensile forces.

2. Safety Factors:

To prevent casing failure, engineers use safety factors when designing the casing string. The safety factor accounts for uncertainties and variations in material properties, operational conditions, and unexpected loads. Typically, a safety factor between 1.5 and 3 is applied to ensure the casing can safely handle the predicted tensile loads.

Formula for Tensile Load:

$$Tensile\ Load = Casing\ Weight + Mud\ Weight + Operational\ Loads$$

Where:

• Casing Weight = (Casing Weight per Unit Length) × (Length of Casing in the Hole)
• Mud Weight = (Mud Weight per Unit Length) × (Length of Casing in the Hole)
• Operational Loads include any additional forces due to tripping, pulling, or wellbore configuration.

Practical Example: Tension Failure in Directional Wells

Directional and extended-reach drilling wells experience a more pronounced tensile load due to the wellbore trajectory. In a typical vertical well, the casing weight is primarily vertical, but in a horizontal or directional well, the casing string is subjected to more axial tension, especially as the angle increases.

Example:

In a deep directional well, the casing string may be exposed to tens of thousands of pounds of tensile load. If the casing’s tensile strength is insufficient to withstand these forces, failure could occur. This is especially critical in extended-reach drilling (ERD), where the horizontal section of the well can impose significant tension on the casing string, increasing the risk of failure.

Solution:

To prevent failure in ERD, operators must design the casing string with the following considerations:

• Use stronger casing materials or larger-diameter casing to withstand higher tensile loads.
• Apply appropriate safety factors based on the expected loads in ERD operations.
• Consider the well trajectory and casing handling techniques to minimize tension buildup.

Key Considerations for Tension Criterion in Casing Design

1. Wellbore Trajectory:

In directional and ERD wells, the angle of the wellbore affects the amount of tension on the casing. Horizontal or highly deviated sections of the well require special attention, as the casing will be subject to higher axial forces in these sections.

2. Casing Material and Strength:

The selection of casing material plays a crucial role in ensuring that the casing can handle the required tensile load. Materials like high-tensile steel or premium grades are often used in wells where tensile stresses are more pronounced.

3. Depth of the Well:

The deeper the well, the more tensile force the casing experiences due to the weight of the casing itself and the weight of the drilling fluid. Deep wells often require higher safety factors and stronger casing grades.

4. Casing Design and Operational Factors:

Using the correct casing design, including the right casing diameter, length, and material grade, is essential for managing tensile loads. In addition, operational factors such as tripping speed, the frequency of pulling and running the casing, and the handling of casing during these operations can influence the tensile load on the casing.

Conclusion

The tension criterion is crucial for ensuring that the casing string in directional or extended-reach drilling wells can handle the axial forces it will encounter during the drilling process. By calculating tensile loads accurately, applying appropriate safety factors, and selecting the right materials and design, engineers can prevent casing failure and maintain the integrity of the well throughout its life. Understanding the tension forces at play is vital for safe and efficient well operations.