What are the functions of tubing and casing in oil wells? What are the types of casing according to their functions?

In oilfield drilling and production operations, "casing" refers to the pipe used as a borehole lining to prevent the wellbore from flowing or collapsing. It is a permanent part downhole, and the bottom of the casing is fixed with cement. Cement sometimes returns to the surface. Most casings have an outer diameter of 114.3mm or larger. The innermost pipe in the well is called tubing. Downhole fluid is sent to the surface through the tubing, which can be separated from the casing by a production separator. The tubing is often pulled out of the well and sometimes needs to be replaced. Most tubing has an outer diameter of 114.3mm or smaller.

The drilling environment often requires several layers of casing to reach the expected total well depth. Each layer of casing can be divided into the following according to its function:

(1) Conduit. The outermost layer of the well, its main function is to reinforce the wellbore wall and prevent the surface gravel layer and unbonded rock from falling into the well.

(2) Structural pipe. Located between the guide tube and the surface pipe, the purpose of this layer of casing includes solving the problem of recurring leakage or well collapse and avoiding the problem of shallow gas well kick as much as possible.

(3) Surface casing. The purpose of the lower surface casing is multiple, including isolating the freshwater layer, preventing collapse and leakage, isolating the weak layer that cannot withstand the back pressure applied by controlling the well kick, providing conditions for installing blowout preventers, and supporting the weight of all layers of casing smaller than the surface casing.

(4) Intermediate casing. The main purpose is that high-pressure layers will be encountered during drilling, and high-density drilling fluid is required to control abnormally high pressure. Shallow and weak formations must be protected to prevent leakage or stuck drill. In some special cases, intermediate casing is also used to isolate salt layers or expansive and easily collapsed shale layers.

(5) Liner. The liner has the same function as the intermediate casing. The liner extends from the bottom of the well upward into the intermediate casing but does not extend to the ground. To save money, the liner does not need to extend the pipe string to the ground but can still achieve the purpose of controlling pressure and fracture gradient.

(6) Production casing. Also called oil layer casing. The function of this string is to separate the production layer from other formations, form a working wellbore with a given diameter that can reach the production layer, and protect the production oil pipe and equipment.

(7) Back-up string. The liner is also often used as part of the production casing, instead of running another string from the ground to the production layer. The liner can be returned to the ground from the top of the liner with an appropriate amount of pipe.