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, its main function is to support the wellbore during drilling operations and prevent surface gravel and unbonded rock from falling into the well. The diameter of the conductor casing usually ranges from 20 inches to 36 inches. It is equivalent to the foundation or initial ground reinforcement when building a building.

(2) Structural Casing

This layer of casing is located between the conductor and the surface pipe. When its structural support function is emphasized, in most cases, it refers to the conductor that is first lowered and plays a structural support role. It is the foundation of the entire wellbore structure and provides structural support for all subsequent drilling operations and wellhead equipment. Structural casing can solve additional well leakage, wellbore collapse problems, and minimize wellbore kick problems in shallow gas formations. 

In some contexts, especially when the main design consideration of the surface casing is to support heavy wellhead equipment, it is also called "structural casing". In this case, it refers to the surface casing, because it provides the most direct installation foundation for the blowout preventer.

(3) Surface Casing
The purpose of the lower surface casing is various, including isolating the freshwater layer, preventing collapse and leakage, isolating the weak section that cannot withstand the back pressure of the wellbore, providing conditions for the installation of the blowout preventer, and supporting the weight of all casings smaller than the surface casing.

Surface casing and intermediate casing are required in almost every well. Regulations usually require that the surface casing must penetrate and isolate all freshwater layers. If the local freshwater layer is deep, a longer surface casing is required. Although the length does not directly determine the diameter, deeper wells usually mean more complex downhole conditions, which indirectly affects the size selection. Common sizes are 13 3/8 inches, 16 inches, and 9 5/8 inches.

Due to environmental issues, some countries/regions have some strict regulations on this. Surface casing can isolate freshwater layers from the oil well, protecting these areas from contamination during drilling and completion.

(4) Intermediate Casing (Technical Casing)
Usually, the intermediate casing is the longest section of casing in the well. The intermediate casing is used to minimize the damage that the underground formation may cause to the well. It also helps to maintain the hydrostatic pressure at an appropriate level to prevent blowouts.

Its main use is because high-pressure layers are encountered during drilling, and high-density drilling fluid is needed to control abnormally high pressure. Shallow, weak formations must be protected to prevent leakage or stuck pipe. In some special cases, intermediate casing is also used to isolate salt layers or expansive, collapsing shale layers.

(5) Production Casing
Also known as oil layer casing, production casing is the last layer section of the casing in the well and the deepest part. It can provide a pipeline from the oil-producing layer to the ground. The function of this string is to separate the production layer from other formations, form a working wellbore with a given diameter, so as to enter the production layer and protect the production oil pipe and equipment.

(6) 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 money by not extending the string to the surface while still achieving the purpose of controlling pressure and fracture gradients.

Essentially, a casing liner is a string of intermediate casing that does not extend all the way to the surface. It is suspended in or above the previous casing shoe and is usually grouted throughout its entire length to ensure it is sealed within the previous casing string. The installation of the liner helps to achieve more efficient and reliable control and management of the oil well, thereby improving the production efficiency and extending the service life of the oil well.

(7) Back-up string. 

The liner is also often used as part of the production casing, rather than running a separate string from the surface to the production layer. The liner can be returned to the surface with an appropriate amount of pipe from the top of the liner.

Drilling Depth Comparison:

Conductor casing: Located in the outermost layer, shallowest (20-100 meters).
Surface casing: Located inside the conductor casing, the second layer. Shallow (100-500 meters).
Intermediate casing: Located inside the surface casing, the intermediate layer. Medium to deep (hundreds to thousands of meters).
Production casing: Innermost layer, the final layer. Deepest (reaching the 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 layer of casing, but rather a running process. 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 weight reduction.

The Meaning of Steel Grade for Drilling Casing Pipe:

In the API SPEC 5CT standard, the steel grade of oil casing and tubing indicates its yield strength and some special characteristics. Steel grade markings are usually represented by 1 letter and 2 or 3 numbers, such as N80. In most cases, according to the order of letters in the alphabet, the later the letter, the greater the yield strength of the pipe. For example, the yield strength of N80 grade steel is greater than that of J55.

Numerical notation is determined by the minimum yield strength of the pipe expressed in thousands of pounds per square inch. For example: the minimum yield strength of N80 steel grade is 550Mpa.

The casing steel grades listed in the API SPEC 5CT standard are: H40, J55, K55, N80, M65, L80, C90, C95, T59, P110, Q125; the casing steel grades are: H40, J55, N80, L80, C90 , T59, P110.

Read more: Casing pipe VS Drill pipe or API 5CT Casing and Tubing Grade & Specification