Types of Drilling Casing Pipe

In oilfield drilling and production operations, "casing" refers to the tube used as a lining for a borehole to prevent the well wall from flowing or collapsing. It is a permanent part underground, and the bottom of the casing is fixed with cement. Cement sometimes returns to the surface. Most casing outer diameters are 114.3mm or larger. The innermost pipe in the well is called tubing. Downhole fluids are brought to the surface through tubing, which can be separated from the casing pipe by a production separator. The tubing is often pulled out of the well and sometimes needs to be replaced. Most oil pipes have an outer diameter of 114.3mm or less.

Types of Drilling Casing Pipe:

Drilling environments often require several layers of casing pipe to achieve the desired total well depth. Each layer of casing can be divided into:

(1) Conductor Casing.

Located at the outermost layer of the well, the conductor provides critical structural support for the wellhead and subsequent drilling operations. Its main function is to support the wellbore during drilling and prevent surface gravel and unconsolidated rock from falling into the well. It is equivalent to the foundation of the entire wellbore structure. The diameter of the conductor casing usually ranges from 20 inches to 36 inches. In some contexts, when its structural support function is emphasized, it is referred to as "structural casing," but it is not a separate type.

(2) Surface Casing

The surface casing is set below the conductor. Its purposes include: isolating freshwater aquifers to protect them from contamination, preventing collapse and leakage, isolating weak sections that cannot withstand wellbore backpressure, providing a stable base for installing the blowout preventer (BOP) , and supporting the weight of all smaller casings run below it.

In some contexts, the surface casing is also referred to as "structural casing" when its role in supporting wellhead equipment is emphasized. Common sizes are 13-3/8 inches, 16 inches, and 9-5/8 inches.

(3) Intermediate Casing (Technical Casing)

Usually, the intermediate casing is the longest section of casing in the well. It is used to isolate problematic formations encountered during drilling, such as high-pressure zones, lost circulation zones, or salt sections. It helps to maintain hydrostatic pressure at an appropriate level to prevent blowouts. In some special cases, intermediate casing is also used to isolate salt layers or expansive, collapsing shale layers.

(4) Production Casing

Also known as oil layer casing, production casing is the last and deepest casing string in the well. It provides a permanent pipeline from the producing formation to the surface. Its function is to separate the production layer from other formations, form a working wellbore of a given diameter, and protect the production tubing and equipment.

(5) Liner

The function of the casing liner is the same as that of the intermediate casing. The liner extends from the bottom of the well upwards into the intermediate casing, but does not extend to the surface. Liners are often used to save cost by not running a full string to the surface while still achieving the purpose of controlling pressure and isolating zones.

Essentially, a casing liner is a string of casing that is suspended from the previous casing shoe and is usually cemented throughout its entire length. The installation of the liner helps achieve more efficient well control and management.

Drilling Depth Comparison:

Casing Type	Relative Position	Typical Depth Range
Conductor Casing	Outermost layer	20 – 100 m
Surface Casing	Inside conductor, second layer	100 – 500 m
Intermediate Casing	Inside surface casing, middle layer	Hundreds to thousands of meters
Production Casing	Innermost layer, final layer	Deepest (to reservoir)

Drilling Depth and Position Logic:

From shallow to deep, from large to small: The drilling process is like a relay race. First, drill a large, shallow wellbore from the surface and run the conductor casing. Then, use a smaller drill bit to drill a deeper wellbore inside the conductor casing and run the surface casing. Repeat this process, layer by layer, until the final production casing is run.

Functional Progression:
a. Conductor & Surface Casing → Focus on a "Safe Start": They address issues near the wellhead and shallow depths, focusing on environmental protection and establishing a safe working platform. Without them, a well simply cannot be drilled.
b. Intermediate casing → Focuses on "Process Assurance": It addresses obstacles encountered during drilling. It is dynamic and tailored to ensure the drilling team can safely reach its target.
c. Production casing → Focuses on "Ultimate Goal": It addresses long-term production issues throughout the lifecycle of the oil and gas well. Its quality and integrity are directly related to the oilfield's recovery rate and economic performance.

The unique characteristics of the "casing liner":

The liner is not a separate type of casing, but rather a running technique. It does not extend to the wellhead, but is suspended at the bottom of the upper casing. It can be either a technical liner or a production liner. Its main advantages are cost savings and reduced string weight.

The Meaning of Steel Grade for Drilling Casing Pipe:

In the API SPEC 5CT standard, the steel grade of oil casing and tubing designation indicates the pipe's minimum yield strength and general performance characteristics.

The number in the grade (e.g., the "80" in N80) represents the minimum yield strength in 1,000 psi (ksi). For example, N80 casing has a minimum yield strength of 80,000 psi (approximately 552 MPa).

The letter (e.g., "N" in N80) generally indicates performance type, such as standard (N) or sour-service (L). It does not follow alphabetical order to indicate strength levels.

The most common API 5CT casing steel grades include J55, K55, N80, L80, P110, and Q125.

FAQs:

Q1: How many casing layers are typically needed for a well?
A: It depends on the well depth and geological complexity. Generally, at least 3 layers (operating casing, surface casing, production casing), and deep wells can have 4-5 layers (including technical casing and tailpipe).

Q2: What is the difference between conductor casing and surface casing?
A: Conductor casing is the outermost and shallowest layer, mainly serving as structural support and isolating loose surface layers; surface casing goes deeper and is used to isolate freshwater layers and install blowout preventers.

Q3: Does the casing steel grade (e.g., J55, N80) have a significant impact on selection?
A: Very much. The steel grade determines the casing's strength, corrosion resistance, and applicable depth. Deep wells and high-pressure wells require higher steel grades (e.g., P110 casing), and sulfur-containing environments require special sulfur-resistant steel grades (e.g., L80).

Q4: What are the functional differences between technical casing and production casing? 

A: Technical casing is "process-oriented" and used to isolate complex formations encountered during drilling (high pressure, leakage, etc.); production casing is "permanent" and used for the final oil and gas production pathway.

Q5: When is a liner used?
A: When the well is deep but steel costs are desired, a liner is used instead of a full set of casing run from the wellhead. It only covers the lower section of the well and hangs at the bottom of the upper casing.

Related Resources:

●  Casing pipe vs. Drill pipe: What’s the Difference and Why It Matters

●  Steel Grades of OCTG Casing Pipe - A Complete Guide

●  API Coupling and Threads for Tubing & Casing Pipe Connection

●  Drilling Casing Specifications and Dimensions for Oil Fields