The casing is a steel pipe supporting the shaft wall of oil and gas well. According to different drilling depth and geological conditions, each well should use several layers of casing.
After casing downhole, cement cementing is adopted. Casing is different from tubing and drill pipe, and it can not be reused. It is a disposable consumable material. Therefore, the casing consumption accounts for more than 70% of the total oil well pipe.
According to usage, casing can be divided into conductor, surface casing, intermediate casing and production casing.
1. Conductor: The conductor is mainly used for drilling in the ocean and desert to separate the sea water and sand so as to ensure smooth drilling. The main specifications of the casing are: φ762mm(30in)*25.4mm, φ762mm(30in)*19.06mm.
2. Surface casing: The surface casing is mainly used for the first time drilling, drilling the surface soft stratum to bedrock. In order to seal this part of the stratum without collapse, it is necessary to use surface casing to seal it. The main specifications of the surface casing are: 508mm(20in), 406.4mm(16in), 339.73mm(13-3/8in), 273.05mm(10-3/4in), 244.48mm(9-5/9in) etc.. The depth of the casing depends on the depth of the soft stratum, usually 80-1500m. Surface casing bears little external pressure and internal pressure, and generally adopts K55 steel grade or N80 steel grade.
3. intermediate casing: Intermediate casing is used in the drilling process of complex formation. When encountering complex parts such as collapse layer, oil layer, gas layer, water layer, leakage layer, salt and gypsum layer, intermediate casing is needed to seal, otherwise drilling can not be carried out. Some wells are deep and complex, and the depth of drilling is up to thousands of meters. This kind of deep well needs several layers of intermediate casing. Its mechanical properties and sealing performance are very high, and the steel grades used are also higher. Besides K55, N80 and P110 steel grades are more used. Some deep wells also use Q125 or even higher non-API steel grades, such as V150. The main specifications of the intermediate casing are: 339.73mm(13-3/8in), 273.05mm(10-3/4in), 244.48mm(9-5/8in), 219.08mm(8-5/8in), 193.68mm(7-5/8in), 177.8mm(7in) etc..
4. production casing: When drilling into the target stratum (oil-bearing and gas-bearing formations), the production casing should be used to seal all the oil and gas layers and the upper exposed formations. The oil layer is inside the production casing. The production casing is the deepest in all kinds of casing, and its mechanical properties and sealing performance requirements are the highest. Steel grades are K55, N80, P110, Q125, V150, etc.. The main specifications of the production casing are: 177.8mm(7in), 168.28mm(6-5/8in), 139.7mm(5-1/2in), 127mm(5in), 114.3mm(4-1/2in), etc..