Types of drilling casing pipe

In oilfield drilling and production operations, "casing" refers to the tube used as a lining for a borehole to prevent the well wall from flowing or collapsing. It is a permanent part underground, and the bottom of the casing is fixed with cement. Cement sometimes returns to the surface. Most casing outer diameters are 114.3mm or larger. The innermost pipe in the well is called tubing. Downhole fluids are brought to the surface through tubing, which can be separated from the casing pipe by a production separator. The tubing is often pulled out of the well and sometimes needs to be replaced. Most oil pipes have an outer diameter of 114.3mm or less.

Drilling environments often require several layers of casing pipe to achieve the desired total well depth. Each layer of casing can be divided into:

(1) Conductor. In the outermost layer of the well, its main function is to reinforce the well wall and prevent the surface gravel layer and unbonded rocks from falling into the well.

(2) Structural pipe. Located between the conductor and the surface pipe, the purpose of this layer of casing is to solve the problem of recurring losses or well collapse and to avoid the problem of shallow gas kick.

(3) Surface casing. The purpose of the lower surface casing is multiple, including isolating the freshwater layer, preventing collapse and leakage, isolating weak sections that cannot withstand the back pressure exerted by controlling well kicks, providing conditions for installing blowout preventers, and supporting all surface casings. The weight of each layer of casing is small.
(4) Intermediate casing. The main purpose is because high-pressure layers will be encountered during drilling, and high-density drilling fluid is required to control abnormal high pressure. Shallow, weak formations must be protected to prevent leakage or stuck drilling. In some special cases, intermediate casings are also used to isolate salt layers or expansive and collapsible shale layers.

(5) Liner. The liner has the same function as the intermediate casing. The liner extends upward from the bottom of the well into the intermediate casing, but does not extend to the surface. Liners are often used to save money without extending the pipe string to the ground but still achieve the purpose of controlling pressure and fracture gradient.
(6) Production casing. Also called reservoir casing. The function of this pipe string is to separate the production layer from other formations, form a working wellbore with a given diameter that can be lowered to the production layer, and protect the production tubing and equipment.

(7) Take back the pipe column. Liners are also often used as part of the production casing, rather than running a separate string of tubing from the surface to the production layer. The liner can be run from the top of the liner back to the surface with just the right amount of pipe.

The meaning of steel grade for drilling casing pipe:

In the API SPEC 5CT standard, the steel grade of casing and tubing indicates its yield strength and some special characteristics. Steel grade markings are usually represented by 1 letter and 2 or 3 numbers, such as N80. In most cases, according to the order of letters in the alphabet, the later the letter, the greater the yield strength of the pipe. For example, the yield strength of N80 grade steel is greater than that of J55.

Numerical notation is determined by the minimum yield strength of the pipe expressed in thousands of pounds per square inch. For example: the minimum yield strength of N80 steel grade is 550Mpa.

The casing steel grades listed in the API SPEC 5CT standard are: H40, J55, K55, N80, M65, L80, C90, C95, T59, P110, Q125; the casing steel grades are: H40, J55, N80, L80, C90 , T59, P110.

Read more: Casing pipe VS Drill pipe or API 5CT Steel Casing and Tubing Chart