Conductor Casing

What is Conductor Casing?

OCTG Casing can be divided into four types depending on its intended use: Conductor Casing, Surface Casing, Intermediate Casing, and Production Casing. Conductor casing is the first layer of casing lowered into the wellbore and is typically located near the wellhead.

The primary function of conductor casing is to provide stable support for the wellhead, guide drilling tools smoothly into the wellbore, and prevent formation collapse around the wellhead. It also effectively isolates surface water from shallow groundwater, providing a stable wellhead environment for drilling operations.

In some areas, conductor casing may be the first casing lowered and cemented. In other areas, particularly on offshore drilling platforms, conductor casing is the first casing to provide formation pore pressure protection.

Conductor Casing Specifications:

Conductor casing sizes vary widely, depending on a variety of factors, including well design, wellbore diameter, downhole geology, and installation method.
Outer Diameter Range: Typically ranges from 16 inches to 42 inches. Common sizes: 20-inch, 24-inch, 30-inch, and 36-inch are the most common.

Oversized sizes: In offshore areas or extremely soft ground (such as swamps), 42-inch or larger conductor casing may be used.

Length: The length of the conductor casing depends on the specific geology of the drilling site and the wellbore design requirements. It is typically shorter than subsequent casing strings and is primarily used to protect shallow formations from contamination and provide stability near the surface.

Running depth: The running depth of conductor casing is typically determined by the depth required to prevent shallow borehole erosion when drilling to the surface casing depth and the depth required to support the weight of the surface casing. It is typically shallow, typically between 20 and 100 meters, to penetrate the soft, unstable layer and enter the first stable formation.

Drilling diverters can be installed on the conductor casing to divert flow in the event of a shallow kick. Typically, this casing string is not required when the surface sands are stable.

Conductor Casing VS. Surface Casing:

Conductor casing: The first, outermost layer, the shortest and shallowest (20 to 100 meters) of the entire casing string. It is the first barrier in the wellbore structure to resist the complex surface environment. Its main function is to isolate the shallow soft formations, establish the wellhead, and provide a stable start for subsequent drilling operations.

Surface Casing: The second layer, which is lowered to a deeper depth (up to several hundred meters). Its main purpose is to isolate the deeper freshwater layer and shallow gas layer. It is the key to well control safety.

How to install Conductor Casing?

Setting conductor casing involves several steps:

(1) Preparation
a. Well site preparation
Before lowering the casing, the well site must be prepared, including cleaning the site, compacting the ground, and providing a solid foundation for heavy equipment.

b. Determine the lowering method

This is the most critical decision, which mainly depends on the hardness of the surface layer.

Soft formations (such as soil, sand, swamp):
Use the "driving method", which is fast, low-cost, and does not require drilling fluid. It is only suitable for soft formations. The strong impact force may damage the casing thread.

Hard formations (such as hard soil and rock): 

The "drilling method" requires excavating a starting hole, which does not cause impact damage to the casing and is suitable for more complex and deeper formations. This method is slow, expensive, and requires drilling fluid and more complex operations. 

c. Equipment and materials in place 

Conductor casing: A single  conductor casing (usually about 12 meters long) is delivered to the well site and the threads and sealing grease for connection are prepared. 

Drilling rig or pile driver: Depending on the selected method, the corresponding drilling rig or large hydraulic hammer is prepared. 

Drilling fluid: If the drilling method is used, drilling fluid (mud) needs to be prepared. 

Cement and equipment: Cementing cement trucks, cement and related mixing pumping equipment are prepared. 

(2) Installation 

The casing is usually placed in the starting hole using a casing running tool or casing elevator installed on the drilling rig. The casing is gradually lowered into the hole until it reaches the required depth. 

(3) Placement

Once the  conductor casing reaches the desired depth, cement or drilling mud is pumped through the casing string to secure it firmly in place. This process, called cementing or mud displacement, helps to secure the casing in the wellbore and forms a barrier between the casing and the surrounding formation.

(4) Verification

Once cementing is complete, the depth and integrity of the guide casing are verified using downhole tools or logging technology to ensure it is properly lowered and sealed.

(5) Completion

Once the guide casing has been successfully installed and verified, the drilling operation can proceed to the next stage, which may involve drilling deeper into the wellbore and installing additional casing strings as needed.

Key Point: There must be sufficient annular space (usually 1-2 inches) between each layer of casing and the wellbore, and between each casing string to allow for smooth lowering and to ensure the thickness and quality of the cement sheath.

Why is such a complex casing system necessary?

1. Preventing wellbore collapse: Underground rock formations are highly variable, with many loose and fractured strata. Without the support of casing, the wellbore will collapse immediately.
2. Controlling downhole pressure: High-pressure oil, gas, and water may exist in the formation. If uncontrolled, they can cause a blowout. Casing is a crucial barrier to prevent high-pressure fluids from reaching the surface.
3. Protecting groundwater sources: Strictly isolate shallow freshwater formations to prevent contamination by drilling fluid or formation fluids.
4. Facilitating the installation of well control equipment: BOPs, for example, must be installed on sturdy conduit or surface casing.

5. Providing a pathway for production: Ensuring that oil and gas can be extracted safely and efficiently.

Read more: Specifications and Dimensions of Drilling Casing for Oil Fields or Casing Pipe vs. Drill Pipe