According to API SPEC 5CT Version 11 (released in December 2023), the permissible deviation for casing wall thickness is 0 to -12.5%, meaning the actual wall thickness must not be less than 87.5% of the nominal wall thickness.
Common wall thickness values include: 5.21 mm, 6.45 mm, 7.01 mm, 8.53 mm, 9.52 mm, 11.18 mm, 13.46 mm, and 16.13 mm, etc., which are standardized specifications matched with the outer diameter.
How Thick Should Casing Be? A Step-by-Step Selection Guide Based on Reservoir Pressure.
API 5CT Casing Wall Thickness Specification Table
| Outer Diameter (OD - inch) | Nominal Weight (lb/ft) | Wall Thickness (inch) | Wall Thickness (mm) | Inside Diameter Reference (ID - mm) |
| 4 1/2" | 9.50 | 0.205 | 5.21 | 103.88 |
| (114.30 mm) | 11.60 | 0.250 | 6.35 | 101.60 |
| 13.50 | 0.290 | 7.37 | 99.56 | |
| 5" | 11.50 | 0.220 | 5.59 | 115.82 |
| (127.00 mm) | 13.00 | 0.250 | 6.35 | 114.30 |
| 15.00 | 0.296 | 7.52 | 111.96 | |
| 5 1/2" | 14.00 | 0.244 | 6.20 | 127.30 |
| (139.70 mm) | 15.50 | 0.275 | 6.98 | 125.74 |
| 17.00 | 0.304 | 7.72 | 124.26 | |
| 20.00 | 0.361 | 9.17 | 121.36 | |
| 7" | 20.00 | 0.272 | 6.91 | 163.98 |
| (177.80 mm) | 23.00 | 0.317 | 8.05 | 161.70 |
| 26.00 | 0.362 | 9.19 | 159.42 | |
| 29.00 | 0.408 | 10.36 | 157.08 | |
| 32.00 | 0.453 | 11.51 | 154.78 | |
| 9 5/8" | 36.00 | 0.352 | 8.94 | 226.58 |
| (244.48 mm) | 40.00 | 0.395 | 10.03 | 224.41 |
| 43.50 | 0.435 | 11.05 | 222.38 | |
| 47.00 | 0.472 | 11.99 | 220.50 | |
| 13 3/8" | 48.00 | 0.330 | 8.38 | 322.96 |
| (339.72 mm) | 54.50 | 0.380 | 9.65 | 320.42 |
| 61.00 | 0.430 | 10.92 | 317.88 | |
| 68.00 | 0.480 | 12.19 | 315.34 | |
| 20" | 94.00 | 0.438 | 11.13 | 485.75 |
| (508.00 mm) | 106.50 | 0.500 | 12.70 | 482.60 |
| 133.00 | 0.635 | 16.13 | 475.74 |
API 5CT Casing Wall Thickness Tolerance
| Parameter | Applicable Product | Tolerance (Limit) |
| Minimum Wall Thickness | All Casing & Tubing Sizes | -12.5% of the nominal wall thickness |
| Maximum Wall Thickness | All Casing & Tubing Sizes | Controlled by Weight Tolerance |
API 5CT Casing Wall Thickness Application & Selection Guide
| Well Section / Application | Primary Engineering Concern | Wall Thickness (WT) Strategy | Typical Examples (OD/Weight/Grade) |
| Surface Casing | Supporting surface soil layers and protecting shallow freshwater zones. | Light Wall (Standard): Focus on large OD to accommodate subsequent drilling tools; high collapse resistance is usually not required. | 9 5/8", 36.00 lb/ft (8.94mm) J55/K55 |
| Intermediate / Technical Casing | Isolating complex formations, high-pressure water zones, or sloughing shales. | Medium Wall: A balance between collapse resistance and internal clearance (Drift) for the next casing string. | 7", 26.00 lb/ft (9.19mm) N80/L80 |
| Deep Production Casing | Extreme external reservoir pressures that could lead to casing collapse. | Heavy Wall: Significantly increases Collapse Resistance to handle high geostatic loads at great depths. | 5 1/2", 23.00 lb/ft (10.59mm) P110 |
| High-Pressure (HP) Gas Wells | Extreme internal fluid pressures (Burst loads) and the need for gas-tight integrity. | Heavy Wall: High Burst Pressure rating. Thicker walls are also ideal for machining Premium Gas-Tight Connections. | 7", 32.00 lb/ft (11.51mm) P110/Q125 |
| Sour Service (
H2S/CO2H2S/CO2 ) |
Gradual wall thinning caused by chemical corrosion over the life of the well. | Corrosion Allowance: Adding extra thickness beyond structural needs or combining with specialized alloys (e.g., 13Cr). | Design-specific, usually 5%-10% thicker than standard. |
| Thermal Recovery (Steam Injection) | Extreme axial stress caused by thermal expansion and contraction cycles. | Thick Wall: Enhances tensile strength and creep resistance to prevent pipe deformation under high temperature. | 7", 29.00 lb/ft (10.36mm) L80/C90 |
| Directional / Horizontal Wells | High torque, drag, and bending stresses (dogleg severity). | Balanced Wall: Must be thick enough to prevent buckling, but thin enough to remain flexible and reduce rig hoist loads. | 5 1/2", 17.00 - 20.00 lb/ft (7.52 - 8.94mm) |
GNEE API Casing factory
From 5.21mm to 16.13mm: All API 5CT Casing Wall Thicknesses in Stock for Immediate Shipment.
FAQ
What are the common casing sizes?
Common sizes of casing include: 4½", 5", 5½", 6⅝", 7", 7⅝", 8⅝", 9⅝", 10¾", 11¾", 13⅜", 16", 18⅝", 20", etc. Common wall thickness series: such as 5.21 mm, 6.35 mm, 7.72 mm, 9.17 mm, 10.36 mm, etc.
What is an oil well casing?
Casing is large diameter pipe that is assembled and inserted into a recently drilled section of a borehole and typically held into place with cement. Casing that is cemented in place aids the drilling process in several ways: Prevent contamination of fresh water well zones.
