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Oil casing, commonly referred to as casing, is a critical component of oil country tubular goods (OCTG). OCTG includes various types of steel pipes such as oil casing, tubing, drill pipe, drill collars, couplings, sucker rods, and pup joints. In this article, we will explore the classification and functions of oil casing pipes, which play a vital role in the drilling and production process.
**Drilling Guide Bush:**
The drilling guide bush is primarily used in offshore and desert drilling operations. Its main purpose is to separate seawater and sand, ensuring smooth and efficient drilling. This type of casing is typically specified with dimensions like 762mm (30in) × 25.4mm and 762mm (30in) × 19.06mm. It acts as a protective layer during the early stages of drilling, helping to maintain wellbore stability.
**Surface Casing:**
Surface casing is installed during the initial phase of drilling to pass through weak or unstable ground layers and reach the bedrock. It prevents formation collapse and ensures proper sealing. Common sizes include 508mm (20in), 406.4mm (16in), 339.73mm (13-3/8in), 273.05mm (10-3/4in), and 244.48mm (9-5/9in). The depth of the surface casing usually ranges from 80 to 1500 meters, depending on the depth of the soft formation. It generally experiences lower internal and external pressures and is commonly made of K55 or N80 steel grades.
**Technical Casing:**
Technical casing is used when drilling through complex formations that may contain unstable layers, oil, gas, water, lost circulation zones, or salt-gypsum layers. Without proper sealing, these layers can cause significant drilling issues. In deep and challenging wells, multiple layers of technical casing may be required, demanding high mechanical strength and excellent sealing properties. Common materials include N80 and P110 steel grades, while deeper wells may use non-API grades like Q125 or V150. Typical specifications for technical 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), and 177.8mm (7in).
**Oil Layer Casing:**
Once the target oil or gas reservoir is reached, oil layer casing is deployed to isolate the hydrocarbon-bearing zone and the overlying strata. As the deepest and most critical casing in the well, it requires the highest level of mechanical strength and sealing performance. Steel grades commonly used include K55, N80, P110, Q125, and V150. The typical sizes for oil layer casing are 177.8mm (7in), 168.28mm (6-5/8in), 139.7mm (5-1/2in), 127mm (5in), and 114.3mm (4-1/2in). This casing is essential for long-term well integrity and production efficiency.