What Are The Types of Casing Pipe Threads?

In oil and gas drilling, casing pipe is a crucial component for ensuring wellbore stability, preventing formation collapse, and isolating water layers. A key aspect of casing pipe design is the threaded connection, which plays a vital role in securing the casing pipe and ensuring a leak-free connection. The three most common types of casing pipe threads are Short Thread Coupling (STC), Long Thread Coupling (LTC), and Buttress Thread Coupling (BTC). Different thread structures vary significantly in strength, sealing performance, pressure resistance, and cost.

Casing Pipe Threads

Casing pipe threads are threaded at both ends, allowing one casing pipe to be securely connected to another.

Casing threads are designed to:

Provide high sealing performance;

Withstand high pressure and high loads;

Facilitate installation (assembly) and disassembly (removal);

Prevent pipe failure due to tension, pressure, or torque.

The following are the three most common types of casing pipe threads: STC, LTC, and BTC.

Short Thread Coupling (STC)

STC is the most common and lowest-cost API 5CT casing pipe thread configuration, suitable for shallow and low-pressure wells.

1. Properties

Thread Length: STC threads are relatively short, making them easier to manufacture and assemble.

Thread Profile: The threads are typically tapered, allowing for tightening of the connection by applying torque, thus improving sealing.

Joint Design: Uses a short coupling, resulting in a lighter overall weight.

2. Applications

Shallow Wells: Lower pressure and less demanding requirements on the casing pipe string.

Low-Cost Drilling Operations: STC joints are more cost-effective than long thread coupling due to simpler manufacturing processes and easier assembly.

Onshore Oil Fields: Relatively mild environmental conditions.

3. Advantages

Lowest cost; 

Easy to process and assemble; 

Lighter casing pipe string weight.

4. Disadvantages

Limited load-bearing capacity, unsuitable for deep wells; 

Weaker resistance to high pressure and extreme well conditions.

Long Thread Coupling (LTC)

LTC is stronger than STC and has a longer thread, making it widely used in medium-deep wells and medium-high pressure environments.

1. Properties

Thread Length: LTC threads are much longer than STC threads, providing a more robust connection.

Thread Profile: LTC threads feature a tapered design, enhancing sealing performance under torque application.

Joint Design: The use of a long coupling significantly improves mechanical strength.

2. Applications

Medium-depth wells: Medium to high pressure and forces, but not extreme conditions.

Onshore and offshore drilling: Particularly suitable for environments where the casing pipe may be subjected to additional mechanical stress.

Medium-cost operations: LTC achieves a good balance between strength and cost-effectiveness.

3. Advantages

Strength higher than STC;

Better tensile, compressive, and bending resistance;

More stable seal, suitable for medium-high pressure environments.

4. Disadvantages

More complex and time-consuming assembly due to the longer thread length;

Slightly more expensive than STC threads.

Buttress Thread Coupling (BTC)

BTC is the strongest and most suitable thread structure for extreme operating conditions among the three API standard threads.

1. Properties

Thread Profile: Unlike STC and LTC threads, BTC threads have a square or near-square profile.

Thread Length: BTC threads are similar to LTC threads, but their square design significantly improves strength and load-bearing capacity.

Coupling Design: BTC uses a large coupling capable of withstanding extremely high forces and preventing downhole thread failure.

2. Applications

Deep and Ultra-Deep Wells: Downhole environments with extremely high pressure and mechanical stress.

Offshore Oil Fields: Particularly suitable for high-pressure, high-temperature (HPHT) wells.

Critical Drilling Operations: These operations allow no room for error; the casing pipe must withstand extreme conditions for extended periods.

3. Advantages

Excellent load-bearing capacity.

Extremely high resistance to pressure and torque.

Suitable for the most challenging drilling environments, including high-temperature, high-pressure wells.

4. Disadvantages

Higher manufacturing and installation costs due to the complex thread design.

Assembly and disassembly require more time and expertise.

Casing Pipe Threads Comparison: STC, LTC, and BTC

API 5CT Thread Clearance

API standard threaded fittings primarily seal pipes through thread grease, metal plating, and thread engagement.

Note:

STC relies on a smaller clearance to form a seal, but its load-bearing capacity is limited.

BTC achieves stronger mechanical locking even with a larger clearance due to its square thread structure.

How to Choose the Right Casing Pipe Thread?

1. Well Depth

Shallow Wells: STC

Medium-Deep Wells: LTC

Ultra-Deep Wells/HPHT: BTC

2. Formation Pressure/Temperature

High-pressure, high-temperature wells typically require the strength of BTC threads.

3. Project Budget

STC has the lowest cost;

LTC offers the best cost-performance ratio;

BTC is the most expensive but the safest.

4. Downhole Environments

Offshore or harsh environments may require LTC or BTC threads for their higher strength and sealing performance.

FAQ

1. Which casing pipe thread is best for deep wells?

BTC (Buttress Thread) is the best choice for deep wells, ultra-deep wells, and HPHT environments.

Its square thread has the largest bearing area, better able to withstand extreme pressure, torque, and axial tension.

2. What is the API 5CT thread standard for STC, LTC, and BTC?

API 5CT specifies the thread type, taper, tooth profile, thread accuracy, tolerances, and matching coupling dimensions for STC, LTC, and BTC threads.

Specifically:

STC and LTC are API Round Threads with a 1:16 taper;

BTC is an API Buttress Thread with a near-square tooth profile and a larger bearing area.

These standards ensure uniform strength, sealing, and interchangeability of casing pipe connections.

Summary

In conclusion, the choice of casing pipe thread—STC, LTC, or BTC—depends on the specific requirements of the drilling environment. STC threads offer a cost-effective solution for shallow wells, LTC threads provide higher strength for medium-depth applications, while BTC threads offer maximum strength and durability for deep wells and extreme conditions.