What is Oil Casing?

Oil casing is a steel pipe used to support the walls of oil and gas wells to ensure the normal operation of the entire oil well after drilling and completion. Each well uses several layers of casing according to different drilling depths and geological conditions. Cement is used to cement the well after the casing is run down. Unlike tubing and drill pipe, it cannot be reused and is a disposable material. Therefore, casing consumption accounts for more than 70% of all oil well pipes.

Different types of casing used in the process of oil extraction:
Casings can be divided into pipes, surface casings, technical casings and oil casings according to their usage.

Surface oil casing: protect the well from being contaminated by shallow water and gas layers, support wellhead equipment and maintain the weight of other layers of the casing.
Technical oil casing: Separate the pressures at different levels so that the drilling fluid can circulate normally and protect the production casing. Install anti-burst devices, leak-proof devices and liners in the well.
Oil layer oil casing: to export oil and natural gas from the reservoir below the surface. It is used to protect wells and layer drilling mud. When petroleum casing is produced, the outer diameter is usually 114.3 mm to 508 mm.

Oil casing is the lifeline to maintain the operation of oil wells. Due to different geological conditions, the downhole stress state is complex, and the combined action of tensile, compressive, bending and torsion stresses on the pipe body, which puts forward higher requirements on the quality of the casing itself. Once the casing itself is damaged for some reason, the entire well may be reduced or even scrapped.

Casing can be divided into different steel grades according to the strength of the steel itself, namely J55, K55, N80, L80, C90, T95, P110, Q125, V150, etc. Different well conditions and depths have different steel grades. In a corrosive environment, the casing itself is also required to have corrosion resistance. In places with complex geological conditions, the casing is also required to have collapse resistance.

Weight calculation:
[(Outer diameter-wall thickness)*wall thickness]*0.02466=kg/m (weight per meter)

Chemical composition:
(1) According to SY/T6194-96 regulations. The casing and its coupling are of the same steel grade. Sulfur content<0.045%, phosphorus content<0.045%;
(2) Take chemical analysis samples according to GB/T222-84. Perform chemical analysis according to the relevant part of GB223;
(3) The American Petroleum Institute API SPEC 5CT 1988 first edition regulations. For chemical analysis, samples were prepared according to the latest version of ASTME59, and chemical analysis was performed according to the latest version of ASTME350.

Physical properties:
(1) According to SY/T6194-96 regulations. For flattening test (GB246-97) tensile test (GB228-2010) and hydraulic test;
(2) Hydrostatic pressure test, flattening test, sulfide stress corrosion cracking test, hardness test (as specified in the latest version of ASTME18 or E10), tensile test, transverse impact test ( ASTMA370, ASTME23 and the latest version of relevant standards stipulate to be carried out), grain size determination (ASTME112 latest version or other methods).

Technical specifications:
(1) Casing made in China is in accordance with SY/T6194-96, and the length of the casing is variable, and the range is 8-13m. But bushings not shorter than 6m can be provided, and the quantity shall not exceed 20%.
(2) The inner and outer surfaces of the casing shall not have folds, hair lines, delamination, cracks, creasing and scarring. These defects should be completely removed, and the depth of removal should not exceed 12.5% ​​of the nominal wall thickness.
(3) The outer surface of the coupling shall not have defects such as folding, hairline, delamination, cracks, creasing, scarring, etc.
(4) The surface of the casing and coupling thread should be smooth, without burrs, tears, and other defects that are sufficient to interrupt the thread and affect the strength and tight connection.

Anti-corrosion countermeasures of oil casing:
1. First, make sure that the water quality meets the standards, and strictly monitor and manage the water quality. Implement cleaning and separate injection, strengthen the cleaning of main water pipelines, ensure that the water quality in the station and the bottom of the well reaches the standard, and reduce the introduction of corrosion sources into the annular space;
2. For water injection wells that have been newly put into production, adopt annulus protection liquid mainly for sterilization, and form a system to add regularly;
3. In view of the accelerating effect of mechanical scratches on the corrosion of oil casing, it is recommended to add a rubber ring to the tubing coupling or add a centralizer to the serious part of the well deviation to avoid the tubing scratching the casing and the casing during the operation and water injection. One side touches the casing;
4. In view of the severe bacterial corrosion under the scale and the slow growth and killing of SRB at high temperature, high temperature water or steam of 100 degrees Celsius can be injected into the annulus regularly to kill the SRB under scale;
5. It is not recommended to promote the use of cathodic protection and coated casing before the corrosion in the casing of the water injection well is effectively controlled.

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