Legal Definitions - casing

Casing refers to the steel pipes that are inserted into an oil or gas wellbore (the hole drilled into the earth) and then cemented into place. Its primary purpose is to maintain the structural integrity of the well, prevent the collapse of the wellbore, and most importantly, isolate different geological layers to prevent fluids (like oil, gas, water, or drilling mud) from mixing or escaping into the environment, particularly groundwater.

There are several types of casing, each serving a specific function:

• Surface Casing: This is the first string of casing installed in a well, extending from the surface down to a certain depth, typically below any freshwater aquifers. Its main role is to protect shallow groundwater from contamination during drilling and production operations.
• Intermediate Casing: Installed deeper than the surface casing, this type of casing is used to seal off unstable rock formations, high-pressure zones, or zones that might contain undesirable fluids. It allows drilling to continue to deeper targets safely and without contamination. A well might have multiple strings of intermediate casing.
• Production Casing: This is the innermost and final string of casing set in the well, extending to the deepest point where oil or gas will be extracted. It provides a conduit for the hydrocarbons to flow from the reservoir to the surface, while also isolating the producing zone from other formations.

Here are some examples illustrating the application of casing:

• Imagine an energy company drilling a new natural gas well in an area with a significant underground freshwater supply used by local communities. To comply with environmental regulations and protect the drinking water, the company first installs surface casing. This pipe is cemented into place through the freshwater aquifer, creating a protective barrier that prevents drilling fluids or any potential gas from contaminating the water supply as drilling progresses deeper.

  This example illustrates casing by showing its initial and crucial role in environmental protection, specifically how surface casing safeguards shallow groundwater resources from potential pollution during the early stages of well development.

• During the drilling of an offshore oil well, engineers encounter a zone of highly unstable shale rock that frequently collapses into the wellbore, making further drilling difficult and risky. To stabilize this section and continue safely to the deeper oil reservoir, they decide to install intermediate casing. This pipe is run through the unstable shale and cemented, effectively sealing off the problematic formation and providing a stable conduit for the drill bit to pass through without further collapse.

  This example demonstrates intermediate casing's function in maintaining wellbore integrity and enabling safe drilling through challenging geological formations that would otherwise impede progress.

• After successfully drilling to the target depth and confirming the presence of a commercially viable oil reservoir, a company prepares the well for production. They install production casing from the surface down into the oil-bearing rock formation. This final string of casing creates a sealed pathway, allowing the crude oil to flow from the reservoir up to the surface facilities without mixing with water from other geological layers or escaping into the surrounding rock.

  This example highlights production casing's role as the final conduit for bringing hydrocarbons to the surface, ensuring efficient and isolated extraction from the target reservoir.