Oil Well Casing Types and Structures

When constructing an oil or gas well, casing pipe is a series of steel pipes lowered layer by layer deep into the wellbore. Its core function is to reinforce the wellbore wall, isolate the formation, control pressure, and ensure safe drilling and production.

Oil Well Casing Structure:

Oil well casing has a relatively simple yet powerful structure, primarily consisting of a steel pipe body and connecting components.

Steel Pipe Body:
The steel pipe body is the primary component of oil casing and is typically made of high-strength, corrosion-resistant steel. Its outer diameter and wall thickness are determined based on drilling requirements and geological conditions to ensure the casing can withstand the various stresses and corrosive environments found downhole.

Connecting Components:
Oil casing pipes are connected to each other and to the wellhead assembly using connecting components. Common connecting components include threaded joints, flanges, and clamps. These connecting components must possess high strength and sealing properties to ensure a secure and reliable connection between the casing pipes and prevent downhole fluid leakage.

Other Accessories:
In some cases, oil casing may also be equipped with accessories such as leak-proof devices and anti-burst devices to enhance casing safety and reliability.

Oil Well Casing can be divided into four types based on their intended use: conductor casing, surface casing, technical casing, and production casing. These are clearly differentiated by the order of insertion, depth, and function.

The casing hierarchy of a typical oil well:

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

Surface casing: The second layer, run deeper (up to several hundred meters), primarily serves to isolate deeper freshwater and shallow gas formations and is crucial for well control safety.

Intermediate casing: Whether to run it depends on geological complexity and is used to isolate complex formations such as zones with abnormal formation pressure.

Production casing: The final layer, reaching the oil and gas reservoir, is the final pathway for oil and gas production.

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 would collapse immediately.

2. Controlling downhole pressure: High-pressure oil, gas, and water may exist in the formation. Uncontrolled pressure can cause a blowout. Casing is a crucial barrier to prevent high-pressure fluids from escaping to the surface.

3. Protecting groundwater sources: Strictly isolate shallow freshwater formations to prevent contamination by drilling fluids or formation fluids.

4. Facilitate the installation of well control equipment: BOPs, for example, must be installed on a sturdy guide pipe or surface casing.

5. Provide access for production: Ensure that oil and gas can be extracted safely and efficiently.

Difference between Drill Pipe and Casing Pipe:

Drill pipe and casing pipe are two key pipe materials used in oil drilling. Their core differences lie in their purpose, usage, and material. Drill pipe is used to transmit rotary power and transport drilling fluid and is reusable. Casing is used to support the wellbore wall and prevent collapse, and is a single-use, cemented well.

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