Oil casing includes wellhead casing and downhole casing.

According to the purpose of cementing and the function of the casing, the casing run into the well can be divided into surface casing, technical casing, and reservoir casing.

(1) Surface casing: The outermost casing in the casing program of an oil and gas well. After drilling, drill to the bedrock below the topsoil layer, or drill to a certain depth, and then run the surface casing.

The function of surface casing is: ① to isolate the upper aquifer and prevent surface water and surface groundwater from seeping into the wellbore; ② Protect the wellhead and reinforce the wellbore of the topsoil well section; ③ For those who will encounter high-pressure oil and gas layers if they continue to drill, install a blowout preventer on the surface casing to prevent blowout. All gaps between the surface casing and the wellbore must be sealed with cement, that is, when cementing and injecting cement, the cement slurry must return to the wellhead in order to isolate the formation and protect the wellbore.

The depth of the surface casing, at least 100 meters.

(2) Technical casing: also known as intermediate casing. It is a casing in the middle layer or two layers of the casing program cover. In deep wells, it plays a role in isolating formations and protecting the wellbore, such as those prone to collapse, leakage, high pressure, and salt content in the middle section of the wellbore.

The insertion of technical casing can ensure smooth drilling of the lower wellbore; It can also ensure the safety of drilling into oil and gas reservoirs; The technical casing is equipped with a casing head and a four-way blowout preventer, which can prevent blowout.

The intermediate casing is inserted due to the technical requirements of drilling, so it is also known as technical casing. The height of cement sealing between the technical casing and the wellbore should be at least 200 meters above the isolated formation.

(3) Reservoir casing: also known as production casing. It is the last layer of casing in the casing program of an oil and gas well, running down from the wellhead to below the oil and gas layer it passes through. The depth of the oil layer casing is basically the depth of drilling.

The function of oil layer casing is to provide a passage for oil and gas to reach the ground, isolating oil and gas from all formations and ensuring that oil and gas pressure does not leak. After the oil and gas well is put into production, the quality of the oil layer casing should ensure that it can maintain a certain production life.

The cementing quality of oil layer casing is crucial for oil and gas testing, as it relates to exploration wells; On the other hand, as a production well, it directly affects the lifespan of the well. The cement sealing height between the oil layer casing and the wellbore wall should be at least 500 meters above the oil and gas layer, or up to 200 meters inside the previous layer of casing.