1. Concept of wellbore structure

Well bore structure refers to a group of combination of casing and cement sheath, which are formed by different diameter, depth and action, and by cementing the annular space.

2. Composition and function

Well bore structure is mainly composed of conduit, surface casing, technical casing, reservoir casing and cement sheath outside each casing.

a).Conduit: the first casing in the wellbore structure is called conduit. Its function is to maintain the surface layer near the wellhead.
b). Surface casing: the second layer of casing in the wellbore structure is called surface casing, generally tens to hundreds of meters. After running in, cement slurry is used to cement and return to the ground. Its function is to seal the unstable soft stratum and water layer in the upper part.
c). Technical casing: the casing between surface casing and reservoir casing is called technical casing. It is a casing which is used to drill to the target formation when the complex formation such as high pressure oil, gas and water layer, leakage layer and collapse layer is encountered in the middle of drilling, and its level depends on the number of complex layers. The function is to seal the complex formation which is difficult to control and keep the drilling work smoothly.
d). Reservoir casing: the innermost casing in the wellbore structure is called reservoir casing. The running depth of casing depends on the completion depth and completion method. Generally, it is required to return cementing cement to the top of the uppermost oil and gas reservoir for 100-150m. Its function is to seal off oil and gas water layer and establish a channel for long-term oil and gas exploitation.

e). Cement return height: refers to the distance between the cement slurry along the casing and the well wall and between the return surface and the rotary table plane in the annular space during cementing.

3. Related terms

a). Completion depth: the distance from the upper plane of the transfer plate to the last position drilled by the bit when the drilling is completed.

b). Casing depth: the depth from the upper plane of the turntable to the casing shoe.

c). Artificial bottom hole: when drilling or testing, the cement plug left in the casing is called artificial bottom hole. Its depth is the distance from the top plane of the rotary table to the bottom of the artificial well.