Type of Casings in Oil and Gas Wells

Casing is a critical steel pipe used in oil and gas wells to ensure structural integrity, environmental protection, and operational efficiency. Each type serves a specific function, from stabilizing the wellbore to isolating production zones. Below are detailed descriptions of the various types of casings used in well construction, their objectives, selection criteria, and significance in well design.

Conductor casing is typically the first casing string installed in a well. It can be driven or drilled into place and is set between 30 and 300 feet, depending on the soil characteristics and well design. It is usually cemented back to the surface to ensure stability and isolation.

The primary objective of conductor casing is to stabilize unconsolidated surface soil and the loose formation at the surface. It shields shallow freshwater sands from contamination by drilling fluids or hydrocarbons. Conductor casing provides a stable foundation for the wellhead and subsequent casings. In some cases, a blowout preventer (BOP) system is also installed on it.

Conductor casing is an essential part of well design. It enables smooth initial drilling by providing a conduit for drilling fluid, preventing surface washouts, and establishing a structural foundation to bear the weight of subsequent casings and wellheads.

2. Surface Casing

The surface casing is typically 9 ½ to 30 inches, depending on the well design. It is set deep enough to cover all freshwater zones and shallow formation anomalies. Conductor casing is cemented from the bottom to the surface to achieve effective isolation. The material of surface casing is selected for compatibility with drilling fluids and formation conditions.

The surface casing stabilizes near-surface sediments and protects freshwater aquifers from contamination by drilling and formation fluid. It also provides a base for blowout preventers and supports the load of subsequent casing strings in the well.

Surface casing is a significant part of the well construction. It meets environmental regulations for aquifer protection, maintains well stability, and provides critical operational safety by ensuring the well can handle kicks or blowouts through secure BOP attachment.

3. Intermediate Casing

Intermediate casing is the casing string run between the surface and production casing or production liner. Intermediate casing is not always cemented to the surface but is often cemented above the previous casing shoe. The primary criterion for the top of cement in intermediate casing is to isolate any hydrocarbon zones. In certain scenarios, cementing the intermediate casing to the surface may be necessary or desirable to avoid annular pressure.

Intermediate casing strings help isolate troublesome zones and allow drilling deeper with higher mud weights without exposing the shallower, lower-strength zones to higher pressures. They facilitate Deeper Drilling and enable the safe progression of the well through challenging formations. Depending on the complexity of the well and the geological formations, multiple intermediate casing strings might be used.

4. Production Casing

Production casing is a vital component in oil and gas wells. It is installed through the production zone and perforated to allow reservoir fluids to flow into the tubing string. Its primary roles include isolating distinct production zones to prevent fluid mixing, containing formation pressures if the tubing leaks, and withstanding pressure from surface operations such as fluid injections or gas lift. 

Typically, the production casing is cemented to isolate the reservoir effectively. A high-quality primary cementing job is essential to ensure selective production free from contamination by fluids from other zones. In certain situations, operators may use a slotted production casing or a liner instead. The casing’s design, size, grade, and material selection are crucial, as it must withstand the mechanical stresses of production and resist corrosion throughout the well’s operational life.

5. Liner

Unlike a casing string, the liner does not extend back to the wellhead but is hung from another casing string. It is typically cemented over its entire length. Special tools, such as liner hangers and liner top packers, are required to complete these installations. Using a liner instead of a full casing string can reduce costs. It allows more flexibility in well design, especially in complex well configurations or for specific purposes, such as drilling or production. Liners improve hydraulic performance during deep drilling, allowing large tubing above the liner top. However, running a liner also exposes the casing string above it to higher pressures, which it should be designed to withstand.

Liners can be either intermediate or production strings. An intermediate or drilling liner extends the intermediate casing through problematic zones. It can be either a planned string or used as an additional string if the intermediate casing is short landed due to unfavourable downhole conditions.

Additional Casing Types

Structural/Drive Pipe: Occasionally used before conductor casing, especially in offshore wells, to provide initial hole support at shallow depths (30–300 feet). It prevents erosion and stabilizes the foundation.

Tieback Casings: A tieback string is the casing that connects the liner to the surface. It provides additional pressure integrity from the liner top to the wellhead. An intermediate tieback isolates worn-out or weak casing that cannot withstand possible pressure loads if drilling is continued. Similarly, a production tieback isolates an intermediate string from production loads. Depending on operational loads, tiebacks can be uncemented or partially cemented.