Types of Tubing Strings Used in Oil and Gas Wells

Tubing strings play a vital role in transporting fluids to and from the wellbore in oil and gas operations. Several types of tubing strings exist, each designed for specific functions, materials, and applications. The primary tubing strings used in oil and gas wells include Production Tubing, Injection Tubing, and Coiled Tubing.

1. Production Tubing

Objective and Application: Production tubing is the primary conduit that transports hydrocarbons from the reservoir to the surface. It is a critical component in a well's production system, providing a passage for the produced fluids.

Design Criteria: Production tubing is specifically engineered to endure extreme pressure and temperature scenarios, making it ideal for the challenging conditions found within the reservoir. The tubing's internal diameter is crucial for ensuring a high flow rate. In contrast, the external diameter determines the wellbore size and the installation of other components, such as packers or downhole tools.

Materials and Grade: Production tubing is generally made of carbon steel or alloy steel, with corrosion-resistant alloys used in more aggressive environments, such as sour gas reservoirs. The grade of steel used depends on factors such as pressure, temperature, and the type of fluids being produced. Common material grades include API 5CT tubing grades, such as J55, K55, L80, N80, and P110.

Caution Points: When using production tubing, careful attention must be paid to corrosion, erosion, and mechanical wear. Corrosive fluids or high sand production can cause significant damage. The tubing should also be carefully monitored for signs of fatigue. Internal coatings or corrosion inhibitors enhance its durability.

2. Injection Tubing

Objective and Application: Injection tubing is used to inject fluids, such as water, gas, acid, or stimulation chemicals, into the reservoir. This is essential for enhanced oil recovery (EOR) processes, pressure maintenance, and reservoir management.

Design Criteria: Injection tubing is designed to handle the injection of large volumes of fluids at varying pressures. It must also resist the effects of corrosive injection fluids and withstand the different pressures within the reservoir.

Materials and Grade: Similar to production tubing, injection tubing is often made from carbon steel or alloy steel, with a greater emphasis on corrosion resistance. For example, CRA (Corrosion-Resistant Alloys) or SS (Stainless Steel) tubing may be used in situations involving harsh chemicals or CO2 injection. Common grades include N80, C95, and L80, with specialized materials for sour environments.

Caution Points: Special consideration must be given to the potential for scale buildup and chemical reactions that may occur with injected fluids. Pressure cycling can also lead to fatigue. Additionally, the integrity of the tubing should be monitored regularly for leaks or fractures due to aggressive injection practices.

3. Coiled Tubing

Objective and Application: Coiled tubing is a continuous, flexible tube spooled on a reel that can be deployed into the wellbore without needing to be cut into joints. It is widely used for well intervention activities such as drilling, logging, perforating, and even fracturing.

Design Criteria: Coiled tubing is typically manufactured from high-strength steel or specialized alloys. Its design allows it to be deployed rapidly and efficiently, without requiring a rig-up process, which reduces downtime. Due to its flexibility, coiled tubing can be deployed in wells with complex geometries.

Materials and Grade: Coiled tubing is made from high-strength carbon steel or specialized alloys, depending on the specific performance requirements. Materials are chosen based on pressure, temperature, and potential exposure to corrosive fluids. The typical grades used are N80 or P110.

Caution Points: While coiled tubing provides operational flexibility, it is susceptible to mechanical damage, such as fatigue or buckling, especially when exposed to high pressures or harsh conditions.

4. Tubing for Gas Lift Applications:

The injected gas reduces the equivalent density of the fluid column in the wellbore, lifting it and enhancing production. It typically has smaller diameters and is used in conjunction with specialized gas lift valves to optimize the flow of hydrocarbons.

Materials and Grade: This type of tubing must resist corrosion from sour gas and other chemicals. Tubing grades such as N80 and L80 are often used.

Caution Points: One significant risk in gas lift operations is the potential for gas migration issues, which can lead to operational inefficiencies. Additionally, tubing should be monitored for leaks or pressure loss in the gas lift system.

Key Considerations for Tubing Selection in the Field:

  1. Pressure and Temperature: Tubing must be designed to handle the reservoir’s pressure and temperature conditions. High pressures may require thicker walls, while high temperatures may necessitate the use of specific materials, such as alloy steels.

  2. Corrosion and Erosion Resistance: Depending on the composition of produced fluids (e.g., presence of CO2, H2S, or chlorides), corrosion-resistant materials or coatings may be required. This is especially critical in sour gas environments or injection fluids that can lead to scaling.

  3. Fatigue and Mechanical Wear: Tubing strings are exposed to significant mechanical stresses, particularly in production and injection wells. Fatigue resistance and the ability to withstand mechanical wear from abrasive particles (such as sand) are essential for long-term operational integrity.

  4. Wellbore Conditions: The shape of the wellbore, including its diameter and curvature, can influence the choice of tubing. Coiled tubing, for example, is ideal for horizontal or highly deviated wells.