OCTG pipe meaning

OCTG(Oil Country Tubular Goods) and oil and gas transmission pipe (Line Pipe) are collectively referred to as oil pipes. Special oil pipes are actually deep-processed products of steel pipes, including special oil well pipes and oil and gas transportation pipes. Drill pipe, casing, tubing, drill collar, kelly, etc. are collectively referred to as special pipes for oil wells.

1.Production and inspection standards of OCTG pipe
The service conditions of special oil pipes are quite harsh, and their quality requirements are much stricter than seamless pipes. They must be produced and inspected in accordance with special standards or technical conditions. At present, the production and inspection standards widely used for petroleum special pipes at home and abroad are mainly American Petroleum Institute (API) standards. The casing and tubing adopt API SPEC5 CT standard, the drill pipe adopts API SPEC5 standard, and the drill pipe joint adopts API SPEC7 standard.

2.Types of non-API tubing and casing
In addition to API standard casing, non-API casing that meets the special geological conditions of oil fields has also been researched and developed at home and abroad, including: ultra-high strength tubing and casing for deep wells; high collapse resistance casing; Hydrogen sulfide stress corrosion-resistant tubing and casing used in oil and gas wells containing hydrogen sulfide; high-strength tubing and casing used in low-temperature oil and gas wells; tubing used in corrosive environments with only carbon dioxide and chloride ions and almost no hydrogen sulfide , casing; used in oil pipes and casing used in highly corrosive environments where hydrogen sulfide, carbon dioxide and chloride ions coexist.

3. Drill pipe  
Drill pipe is a steel pipe with a thread at the end, used to connect the surface equipment of the drilling rig to the drilling and grinding equipment or bottom hole device at the bottom of the drilling well. The purpose of the drill pipe is to transport drilling mud to the drill bit and to raise, lower or rotate the bottom hole device together with the drill bit. Drill pipe must be able to withstand tremendous internal and external pressure, twisting, bending and vibration. In the process of oil and gas extraction and refining, drill pipe can be used multiple times. Drill pipe is divided into three categories: kelly, drill pipe and weighted drill pipe.

4. The role of tubing and casing in oil wells
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 downhole, 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 via tubing, which can be separated from the casing by a production separator. The tubing is frequently pulled out of the well and sometimes needs to be replaced. Most oil pipes have an outer diameter of 114.3mm or less.

5.Types of OCTG casing
Drilling environments often require several layers of casing to achieve the desired total well depth. Each layer of casing can be divided into:
(1) Catheter. 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 recurrence of lost circulation or well collapse and to try to avoid the problem of well kick in shallow gas.

(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 and weak formations must be protected to prevent leakage or stuck drilling. In some special cases, intermediate casings are also used to isolate salt formations or expansive and collapsible shale formations.

(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 of 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 go down 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 zone. The liner can be run from the top of the liner back to the surface with just the right amount of pipe.

6.Steel grade of tubing and casing
In the API SPEC5CT 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 SPEC5 CT 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.

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