Drilling Casing Specifications and Dimensions

During the process of oil field exploitation, oil drilling casing plays a vital role. It can not only support the well wall and prevent formation collapse, but also ensure the smooth extraction of oil and gas resources. In order to meet different geological conditions and exploitation needs, various sizes of casing are used in oil fields. So, what are the commonly used specifications and dimensions of drilling casing in the process of oil drilling?

Casing pipes are ordinarily produced with outside diameter sizes of 114.3 mm to 508.0 mm. The specifications and weight of oil casing pipe are as follows:

Common Casing Sizes and Their Applications:

The table below summarizes key specifications for several commonly used casing pipe sizes in oil fields, from 4-1/2″ to 9-5/8″. The data includes outer diameter, typical wall thickness range, and general application depth. 

Size	Outer Diameter (mm)	Common Wall Thickness (mm)	Application Depth
4-1/2″	114.3	6.35 – 11.05	Shallow wells
5″	127.0	7.52 – 11.07	Shallow to medium wells
5-1/2″	139.7	9.17 – 12.70	Medium-depth wells
7″	177.8	11.05 – 15.88	Shallow to deep wells
9-5/8″	244.5	11.05 – 20.24	Deep wells, high pressure

Note: The application suggestions above are for general reference only. The final selection of casing size and grade must be determined by a drilling engineer based on specific well conditions, formation pressures, and safety factor calculations.

Detailed Specifications by Casing Size:

1. 4 1/2-inch casing

The outer diameter of this casing is 114.3 mm (4.5 inches). Its wall thickness varies by grade and weight, with common thicknesses ranging from 5.21 mm to 11.05 mm (e.g., 9.5–15.1 lb/ft). Due to its relatively small diameter, it is often used in shallow to medium-depth wells where lower formation pressures allow for cost-effective completion.

2. 5-inch casing
The outer diameter of 5-inch oil casing is 127.0 mm (5.0 inches). Typical wall thicknesses range from 5.59 mm to 11.07 mm (e.g., 11.5–24.1 lb/ft). This size is widely used in shallow to medium-depth wells and offers a good balance of flow area and structural strength.

3. 5 1/2-inch casing
This casing has an outer diameter of 139.7 mm (5.5 inches), with common wall thicknesses from 6.20 mm to 12.70 mm (e.g., 14.0–23.0 lb/ft). It is frequently selected for medium-depth wells where higher formation pressures require stronger support and a more robust casing design.

4. 7-inch casing
The outer diameter of 7-inch casing is 177.8 mm (7.0 inches), with wall thicknesses typically ranging from 5.97 mm to 15.88 mm (e.g., 17.0–38.0 lb/ft). This versatile size is suitable for a broad range of well depths, from shallow to deep. Its larger inner diameter and enhanced bearing capacity make it a common choice for complex geological conditions and higher production requirements.

5. 9 5/8-inch casing

This casing has an outer diameter of 244.5 mm (9.625 inches), with wall thicknesses commonly ranging from 8.94 mm to 20.24 mm (e.g., 36.0–75.6 lb/ft). As one of the larger sizes used in oil fields, it is primarily employed in deep wells to withstand high formation pressures and temperatures. Its wide inner diameter also helps reduce flow resistance, improving production efficiency.

FAQs about Drilling Casing Sizes:

1. What are the common API standard casing sizes and steel grades?

API SPEC 5CT defines 14 standardized outer diameters for casing, ranging from 4-1/2″ (114.3 mm) to 20″ (508 mm). The most widely used sizes are 9-5/8″ and 13-3/8″. For each diameter, wall thickness and strength vary by steel grade. Common grades include J55/K55 casing (basic, for shallow wells), N80 casing (versatile, for medium depths), and P110 casing (high-strength, for deep/high-pressure wells). Higher steel grades generally allow a wider range of wall thicknesses but require careful engineering selection.

2. Casing with the same outer diameter has multiple wall thicknesses; how to choose?

Different wall thicknesses for the same outer diameter are identified by "nominal weight" (lb/ft). For example, 9-5/8″ casing offers weights from 36 lb/ft to 75.6 lb/ft. A higher weight means a thicker wall, which increases collapse and internal pressure resistance but reduces the inner diameter. The right choice depends on well depth, expected formation pressures, and required safety factors. Selection depends on well depth and pressure. For a complete list of weight and wall thickness options by size, see our casing size reference table.

3. What are the available casing length specifications? How to choose?

API standards classify casing length into three ranges:
R1: 4.88 - 7.62m
R2: 7.62 - 10.36m
R3: 10.36m and longer

Selection advice:
a. Most land drilling operations use R-2, with a length of approximately 9.5 meters being the de facto standard;
b. R-3 can be used for deep wells or when high efficiency is desired, reducing the number of downhole connections;
c. Note that the number of casing lengths greater than 6m but shorter than the standard range should not exceed 20% of the total.

4. What is the allowable range of casing dimensional tolerances?

Outer Diameter (Pipe Body):
D ≤ 101.6mm: ±0.79mm
D ≥ 114.3mm: -0.5%D ~ +1.0%D

Outer Diameter (API Coupling): ±1%D
Wall Thickness: -12.5% (Negative deviation only allowed)
Single Wire Weight: +6.5% ~ -3.5%

Conclusion:

Selecting the right casing size is a critical first step in well design, impacting both safety and production efficiency. This guide covers the most common API 5CT sizes from 4-1/2″ to 9-5/8″. For a complete reference including all weights and dimensions, see our OCTG casing size chart.

Learn about API casing steel grade and thread types:

● How to Choose Casing Steel Grade (J55, N80, P110)?

● API Casing Thread Types: BTC, LTC, and Premium Connections

● Casing pipe vs. Drill pipe 

● Seamless Casing vs. ERW Casing