In oil and natural gas drilling and production, oil casing is a critical pipe material that is run into the well to reinforce the wellbore. It prevents wellbore collapse, isolates fluid flow between different formations, and provides a channel for oil and gas transportation. It is the "lifeline" for ensuring drilling safety and stable oil and gas production. Its quality and performance directly determine the lifespan of the well and production efficiency. Oil casing is primarily divided into three categories based on the production stage and function. First, surface casing, which is run into the uppermost portion of the wellbore to protect shallow groundwater from contamination and support subsequent drilling equipment. High-strength, low-carbon alloy steel is typically used to adapt to complex shallow geology. Second, technical casing, used in deep wells or complex formations, isolates high-pressure oil and gas reservoirs, salt rock formations, and other easily collapsed formations. It must be resistant to high pressure and corrosion, and is typically made of chromium-molybdenum alloy steel. Third, production casing, which comes into direct contact with the oil and gas formations and serves as the channel for oil and gas to be transported from the formation to the surface, requires a smooth, impurity-free inner wall to resist corrosion from hydrogen sulfide and carbon dioxide in the oil and gas. Some casings require anti-corrosion coatings. Oil casing must meet stringent performance requirements: First, high strength and toughness must be required to withstand the forces of gravity, torque, and formation pressure during drilling, preventing deformation or fracture. Second, corrosion resistance is crucial. Acidic gases contained in oil and gas can easily cause casing corrosion, necessitating material optimization (such as the addition of sulfur-resistant elements) or coating treatments to enhance corrosion resistance. Finally, sealing reliability is crucial. Casing connections must utilize specialized thread designs to achieve high airtightness to prevent oil and gas leakage.

In practice, oil casing serves as a "safety barrier" for deep well drilling, resisting high formation pressure and preventing wellbore collapse in wells exceeding 2,000 meters. It also serves as a "corrosion protector" for high-sulfur gas wells, using sulfur-resistant materials to prevent casing damage caused by acid gas corrosion. It also serves as a key component in the rehabilitation of older wells, replacing worn or corroded casing to extend the well's production life. As oil and gas production expands into deeper and ultra-deep wells and high-sulfur zones, oil casing is being upgraded to offer higher strength and improved corrosion resistance.