Well casing

A well casing is a lining that is installed in an oil or gas well after it is drilled and surrounds the wellbore. Casing typically consists hollow steel pipe that lines the inside of the wellbore. Casings are used to support the wellbore, preventing the surrounding rock and soil from collapsing into the well.

Figure 1. A diagram of a pump jack and wellbore, including a diagram of the well casing.

In addition to providing structural support, the casing also isolates the contents of the well from the surrounding rock and soil. This helps prevent the loss of oil or gas and reduces the risk of contaminating surrounding rock formations and groundwater. Cement is pumped into the space between the casing and the surrounding rock, where it hardens to form a seal that provides the primary barrier against fluid leakage and groundwater contamination. Protecting groundwater from contamination is an important consideration when drilling for natural gas or petroleum. Additionally, casing provides a conduit for fluids produced from the well and for fluids used during well operations and treatment.

Casing a Well

Well casing is installed in multiple sections. Each section of casing is referred to as a casing string. The casing is composed of steel and has a smaller diameter than the wellbore. Casing is installed in stages, beginning with the largest-diameter casing near the surface and progressing to smaller-diameter casing at greater depths. Each section of casing is connected using threaded collars or couplings. After the steel casing is installed, cement is pumped into the space between the casing and the surrounding rock to create a protective seal. The cement is pumped into the gap between the wellbore and the steel casing after the casing has been installed. Once hardened, the cement creates another barrier against fluid migration and helps secure the casing in place.

The thickness of the casing varies with depth and is determined by engineering requirements such as the pressures, loads, and geological conditions encountered throughout the well. Different sections of casing may have different wall thicknesses depending on the mechanical and environmental conditions at each depth, rather than solely on the surrounding rock type.

Capping a Well

Oil and natural gas wells can be abandoned when they no longer produce enough oil or natural gas to be economically viable. In this case, the well must be permanently sealed to ensure that it does not pose a risk to the environment after abandoned. The process of permanently sealing a well before abandonment is commonly known as well plugging and abandonment, although it is often referred to as capping the well. The first step involves cleaning the wellbore and removing any equipment so that permanent sealing materials can be installed. The well is then inspected to ensure the casing and cement barriers remain effective. Any deficiencies identified in the casing or cement must be repaired to ensure an effective seal. Cement plugs are then placed at strategic locations within the well to isolate underground formations and prevent the movement of fluids. Once the well has been permanently plugged, the casing is cut below ground level and a cap is installed. Typically, the casing is cut below ground surface, and a steel cap is installed to complete the abandonment process.

For Further Reading

• Wellbore
• Groundwater
• Petroleum
• Natural gas
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