Oil pipeline engineering is a crucial component of the oil industry, and corrosion prevention is a key aspect of pipeline projects. Casing pipes are used to transport various media, including oil, natural gas, and chemical products, so ensuring their corrosion protection is vital for safe operation.
Casing pipe, as a specialized mode of transportation, have been in use for over a century. With the development of the national economy, the advantages of pipeline oil transportation have become increasingly evident. Most oil pipelines use
carbon steel seamless pipes, straight seam resistance welded pipes, or spiral welded pipes. Pipelines are generally installed either above ground or buried underground. Regardless of the installation method, when metal pipes come into contact with surrounding media, chemical or electrochemical reactions can cause corrosion. Corrosion is a key factor affecting the reliability and lifespan of pipeline systems. It can alter the appearance, color, and mechanical properties of metal pipes, impact the quality of transported oil, shorten the lifespan of pipelines, and, in severe cases, lead to leaks, environmental pollution, or even make the pipeline unusable. As oil extraction progresses to the mid and late stages, corrosion issues become more prominent, often resulting in significant economic losses and severe social consequences due to accidents caused by corrosion. Therefore, understanding the causes of corrosion, the mechanisms, influencing factors, and control methods of corrosion in oil and gas pipelines, and implementing effective protective measures are of utmost importance. Given the complexity and variability of corrosion, no single corrosion prevention technology is universally effective. Therefore, a comprehensive understanding of the corrosion environment and the adoption of appropriate material selection and design strategies for prevention are necessary.
Corrosion Prevention Techniques for Casing Pipes
Corrosion of casing pipes is influenced by various factors, including the partial pressure of corrosive gases, environmental temperature, water content, salinity, flow rate, and pH value. The following are several common corrosion prevention techniques for casing pipes:
Cement Sealing Corrosion Prevention
Early Analysis: It was observed early on that casing damage was primarily due to incomplete cement sealing, leading to the intrusion of corrosive gases from the formation. To enhance the corrosion resistance of casing pipes, the method of increasing the amount of cement returned was proposed in the 1980s. Although this approach reduced the degree of corrosion to some extent, it did not completely prevent it. Thus, its effectiveness in improving corrosion resistance remains limited.
Using Corrosion-Resistant Alloy Pipes
Material Solutions: Since 2000, the corrosion prevention strategy for marine oil casing pipes has shifted towards material solutions to enhance corrosion resistance. Through years of laboratory research, alloy materials suitable for marine oil and gas fields, such as chromium-containing pipes (e.g., 1Cr, 3Cr, 9Cr, 13Cr), have been developed and evaluated under specific temperature, pressure, and corrosive gas conditions.
Injecting Corrosion Inhibitors
Chemical Protection: During the completion process, if the formation contains corrosive oil gases, chemical corrosion inhibitors are added to the well fluids to prevent scaling, wax deposition, and corrosion. The use of chemical inhibitors is a common protection method abroad. Depending on the specific conditions of the oil and gas field, intermittent or continuous injection of inhibitors can be used, with the amount adjusted according to the corrosion environment. Although the use of inhibitors increases costs, it has a positive impact on production.
Applying Corrosion-Resistant Coatings
Coating Methods: For casing pipes, applying corrosion-resistant coatings is an effective prevention method. Some oil fields use nitriding processes to provide internal and external corrosion resistance for pipes, which has good wear resistance but may accelerate internal corrosion of the casing to some extent. Additionally, epoxy powder coatings, titanium nano coatings, and epoxy phenolic coatings are used to provide extra protection.
Additional Corrosion Prevention Measures of Casing Pipe
1.Ensure Water Quality Meets Standards
Implement Clean and Dirty Water Separation: During water injection, distinguish between clean and dirty water to avoid introducing dirty water into the system. This helps reduce the introduction of corrosion sources, thus lowering the risk of corrosion to the casing pipes.
Enhance Main Water Pipeline Cleaning: Regularly clean the main water pipeline to ensure water quality meets standards and reduce the accumulation of corrosion sources. Ensure that the water quality at the station and at the bottom of the well meets standards to control corrosion from the source.
2.Protect Newly Commissioned Injection Wells
Use of Biocides: For newly commissioned injection wells, primarily use biocide-containing annular protection fluids to inhibit bacterial growth and reduce corrosion.
Regular Addition: Establish a system for regular addition of biocides to maintain the effectiveness of the annular protection fluid and ensure long-term corrosion protection.
3.Minimize Mechanical Wear
Add Rubber Rings: Install rubber rings on the oil pipe connectors to reduce friction and wear during operation, preventing the oil pipe from scratching the casing.
Use Centralizers: Install centralizers in areas with severe well deviation to keep the oil pipe stable during operation and reduce direct contact with the casing, thereby minimizing mechanical wear.
4.Address Sulfate-Reducing Bacteria (SRB) Corrosion
Inject High-Temperature Water or Steam: For severe SRB corrosion under scale, periodically inject 100°C high-temperature water or steam into the annular space to effectively kill SRB and suppress its growth.
Avoid Premature Adoption of Cathodic Protection and Coated Casing Pipes
Delay Adoption: Before effectively controlling casing pipe corrosion, avoid promoting the use of cathodic protection and coated casing pipes. This is because the effectiveness of cathodic protection and coatings may not meet expectations if corrosion control measures are not yet fully implemented, potentially leading to additional maintenance costs and complexity.
By implementing the above measures, it is possible to effectively control and reduce the corrosion of casing pipes, thereby improving their lifespan and safety. In practical applications, corrosion prevention strategies should be adjusted based on specific conditions to ensure the stability and safety of oil extraction. In summary, corrosion prevention for casing pipes involves multiple techniques, each targeting different corrosion issues. Effective application of these methods can significantly enhance the safety and durability of casing pipes, ensuring their reliable operation during oil and gas extraction.