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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 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 of oil and gas wells.
**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 drilling operations. This type of casing is typically designed with specifications such as ∮762mm (30in) × 25.4mm and ∮762mm (30in) × 19.06mm. It helps maintain stability during the early stages of drilling, especially in challenging environments.
**Surface Casing:**
Surface casing is installed during the initial phase of drilling to pass through soft 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 depends on the depth of the soft formation, usually ranging from 80 to 1500 meters. It is typically made from K55 or N80 steel grades due to its relatively low pressure requirements.
**Technical Casing:**
Used in complex drilling conditions, technical casing is essential when encountering difficult formations such as unstable rock, oil, gas, water, salt, or gypsum layers. Proper sealing with technical casing is crucial to avoid drilling complications. In deep or complex wells, multiple layers of technical casing may be required, demanding high mechanical strength and excellent sealing properties. Common steel grades include N80 and P110, while deeper wells may use non-API grades like Q125 or V150. Typical dimensions 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 well reaches the target oil or gas layer, oil layer casing is installed to isolate the hydrocarbon-bearing zone and protect the overlying strata. This is the deepest and most demanding casing in the wellbore, requiring superior mechanical strength and sealing capabilities. Common steel grades include K55, N80, P110, Q125, and V150. Standard sizes for oil layer casing include 177.8mm (7in), 168.28mm (6-5/8in), 139.7mm (5-1/2in), 127mm (5in), and 114.3mm (4-1/2in), among others.
Understanding the different types of casing and their specific roles is essential for successful well construction and long-term production efficiency. Each casing serves a unique purpose, contributing to the integrity and safety of the entire drilling operation.