The pressures, temperatures, and potentially corrosive conditions that may be found in a wellbore create the need for metals and materials that can resist such conditions. >> Oil & Gas Engineering Books <<
Metallurgical requirements are dictated both by the downhole well environment and the design and performance requirements of the packer. Consideration must be given to both when selecting and specifying materials for corrosive environments. Many types of materials that are applicable for tubing and casing in corrosive environments are not always suitable (or practical) for packer manufacture. The NACE International (formerly National Association of Corrosion Engineers) Standard MR-01-75 establishes guidelines and acceptance criteria for material selection for sour service in H2S environments.
Common materials used for downhole equipment
Some commonly used materials for manufacture of downhole equipment are as follows:
- Low-alloy steels with minimum yield strengths of 110 ksi are used for standard service in noncorrosive environments. These materials are similar in property to P110 tubing and do not meet NACE MR-01-75 requirements for sour service.
- Low-alloy steels with a maximum hardness of Rockwell 22C, which meet NACE MR-01-75 requirements, are intended for use in both standard service and service in sour H2S environments. Materials that fall into this range would be similar in properties to J-55 to L-80 grades of tubing.
- Martensitic steels such as 9% chromium, 1% molybdenum, and 13% chromium alloy steels are used in some wet CO2 environments. Certain grades of these steels meet NACE MR-01-75 requirements and can be used in limited H2S applications.
- 22% chromium and 25% chromium duplex stainless steel are commonly used in some wet CO2 and mild H2S environments.
- Austenitic stainless steels, cold worked 3% Mo high-nickel alloy steels, and precipitation-hardening nickel-based alloys are suitable for some environments containing high levels of H2S, CO2, and chlorides at moderately high temperatures.
The successful application of any of these materials depends strongly on the specific downhole well environment. Many factors such as temperature, pH, chlorides, water, H2S, and CO2 concentrations can have adverse effects on the material performance and can lead to failures associated with:
- Chloride stress cracking.
- Hydrogen embrittlement.
Materials used in packers
Ideally, the packer should be built out of materials that will last the life of the well. Also, in the case of retrievable packers that may be reconditioned and used elsewhere, the advantage of being able to reuse the packer may be lost if the well environment corrodes or damages the tool beyond repair. In potentially corrosive environments, material failure can lead to a packer leak or difficulty in removing a retrievable packer from the wellbore. In these cases, corrosion-resistant alloy materials must be properly selected that are best suited to the downhole well environment.
Because of the vast number and variations of packer designs and tensile requirements of their components, the consumer cannot know which materials are appropriate for each particular design. Ultimately, the user must rely on the packer manufacturer to help make the determination as to which materials will meet the downhole requirements without sacrificing packer performance and reliability.