The wellbore structure is mainly composed of conduit, surface casing, technical casing, oil casing and cement sheath outside each casing.
1. Conduit: The first layer of casing installed in the well structure is called the conduit. Its role is to maintain the surface layer near the wellhead.
2. Surface casing: The second layer of casing in the well structure is called the surface casing, generally tens to hundreds of meters. After running in, cement the well back to the surface with cement slurry. Its function is to isolate the upper unstable soft stratum and water layer.

3. Technical casing: The casing between the surface casing and the oil casing is called technical casing. It is the casing that is drilled to the destination layer when encountering complex formations such as high-pressure oil and gas water layers, leakage layers and collapse layers in the middle of drilling, and its level is determined by the number of complex layers. The function is to isolate the complex formation that is difficult to control and keep the drilling work going smoothly.
4. Oil layer casing: The innermost layer of casing in the well structure is called oil layer casing. The running depth of the reservoir casing depends on the drilling depth and the completion method of the well. Generally, the cementing cement is required to return to the top of the uppermost oil and gas layer 100-150 meters. Its function is to seal off oil and gas water layers and establish a channel for long-term oil and gas exploitation.
5. Cement return height: refers to the distance between the cement slurry along the casing and the well wall and between the upper return surface of the annular space and the plane of the turntable during cementing.