Why do oil tube casing need to be anticorrosive?
Corrosiveness of oil and drilling fluids
Petroleum composition: Petroleum contains a variety of corrosive substances, such as acidic substances, salts, sulfides, etc., which are prone to chemical reactions when in contact with the casing, resulting in corrosion of the casing metal.
Drilling fluid composition: Drilling fluid as an indispensable medium in the drilling process, its composition is complex, including a variety of chemicals such as sodium ions, calcium ions, potassium ions, sulfate, iron sulfate, polymer and so on. The content and properties of these substances have a significant impact on casing corrosion. For example, sodium ions increase the salinity of drilling fluid, reduce the pH value, and accelerate the corrosion of casing inner wall. Calcium ion, sulfate and ferric sulfate, as strong oxidants, will accelerate the corrosion of casing when the content is too high. The polymer inhibits oxidants in the drilling fluid and reduces casing corrosion.
Geological and environmental conditions
Formation conditions: Drilling in different formations will encounter different geological conditions, such as acidic formation, high salinity formation, high temperature and high pressure formation. These conditions will accelerate the corrosion process of the casing.
Environmental conditions: When drilling in an offshore environment, chloride ions in seawater can accelerate casing corrosion. In addition, environmental factors such as temperature, humidity and pressure during drilling will also affect casing corrosion.
Mechanical damage and stress corrosion
Mechanical damage: During drilling, transportation, installation and use, casing may be subjected to external impact and friction, resulting in surface damage and cracks. These damages will accelerate the corrosion process and reduce the service life of the casing.
Stress corrosion: Casing is prone to stress corrosion cracking (SCC) when subjected to complex stresses such as internal pressure, external pressure and temperature stress. This is a corrosion phenomenon that occurs under specific environmental and stress conditions, which poses a serious threat to the safe operation of casing.
Extend service life and reduce costs
Prolonging service life: By taking effective anti-corrosion measures, the corrosion rate of casing can be slowed down and its service life can be extended. This not only reduces the expense and time cost associated with casing replacement, but also ensures long-term stable production of the well.
Reduced operating costs: The implementation of anti-corrosion measures can also reduce the operating costs of the well. For example, reduce maintenance and replacement costs caused by corrosion, reduce environmental pollution caused by leakage and treatment costs.
