The API 5CT Sizing System
Standard casing sizes are governed by the API 5CT specification. Casing is categorized by its outside diameter, and the industry relies on a set of standard diameters used for different sections of the well: conductor, surface, intermediate, and production. For each diameter, the wall thickness is selected by the nominal weight in pounds per foot, which reflects the pressure and collapse requirements of the section.
Production and Small-Diameter Casing
Production casing commonly uses 4-1/2 inch at 114.3 mm with weights from 9.50 to 13.50 pounds per foot and walls from 5.21 to 7.37 mm; 5-1/2 inch at 139.7 mm with weights from 14.00 to 23.00 pounds per foot and walls from 6.20 to 10.54 mm; and 7 inch at 177.8 mm with weights from 20.00 to 32.00 pounds per foot and walls from 6.91 to 11.51 mm.
Intermediate and Technical Casing
Intermediate casing uses 7-5/8 inch at 193.68 mm with weights from 26.40 to 33.70 pounds per foot; 9-5/8 inch at 244.48 mm with weights from 36.00 to 53.50 pounds per foot and walls from 8.94 to 13.84 mm; and 10-3/4 inch at 273.05 mm with weights from 40.50 to 51.00 pounds per foot.
Surface Casing and Conductor
Surface casing and conductor use 13-3/8 inch at 339.72 mm with weights from 48.00 to 68.00 pounds per foot; 16 inch at 406.4 mm with weights from 65.00 to 75.00 pounds per foot; and 20 inch at 508 mm with weights from 94.00 to 133.00 pounds per foot and walls up to 16.13 mm.
Length Ranges and Connections
API standardizes joint lengths: Range 1 is 4.88 to 7.62 m, Range 2 is 7.62 to 10.36 m, and Range 3 is 10.36 to 14.63 m, the most common for casing strings. End connections include short thread coupled, long thread coupled, buttress thread coupled, and premium connections with gas-tight seals for high-pressure or deep wells.
Selection Guidance
Casing wall thickness is not a single fixed number; it is determined by well depth, internal pressure, and environmental loads. Most common casings have walls between 5 and 16 mm, with thicker walls used in deeper sections to prevent collapse from external ground pressure and burst from internal fluid pressure.
Frequently Asked Questions
Q: What are the most common casing sizes?
A: Production casing of 4-1/2, 5-1/2, and 7 inch; intermediate of 7-5/8, 8-5/8, and 9-5/8; surface of 10-3/4 and 13-3/8; conductor of 16 and 20 inch.
Q: What governs casing wall thickness?
A> Well depth, internal pressure, and environmental loads, selected by nominal weight per foot.
Q: What is the typical wall range?
A: 5 to 16 mm, about 0.200 to 0.625 inch.
Q: What is the most common length range?
A: Range 3, from 10.36 to 14.63 m.
Q: What connections are available?
A: STC, LTC, BTC, and premium gas-tight connections.
Q: Which standard governs casing?
A: API 5CT.
For API 5CT casing quotations with size, weight, and connection requirements, contact our sales team.
