Previously on the Eiffel Trading blog, we’ve discussed drill strings, drill collars, and tool joints, which all play a part in actually drilling the well to reach oil and natural gas deposits trapped beneath the earth’s surface. We’ve also discussed how rotary steerable drilling systems are a popular choice due to the reduced chance of the drill string sticking to the wall of the borehole.
Now it is time to look at what some refer to as the second phase of oil and gas drilling well construction: casing strings, sometimes called casing pipe. Casing strings are hollow steel pipes used to line the wellbore and cemented in place to protect the surrounding areas from contamination and reinforce the well. There are different types of casing pipes, depending on the depth and pressure of the well as well as local/state regulations.
Keep reading to learn about casing pipe strings, its makeups, and its roles in completing the well.
Casing strings are series of steel pipes made up of individual joints (pipe segments) that can vary in length between different types of casing strings. These joints are male-threaded on each end and connected together with short sections of double-female threaded pipe, known as collars or couplings. (Some manufacturers do produce pipe joints with female threads already attached to one end.) Each string of pipe has a smaller diameter than the previous, tapering the strings down into the wellbore.
Casing strings and drill strings alternate in the wellbore. Once a new section of drilling is completed, the drill string is pulled from the hole, a new section of casing string is placed and cemented, and the drill string is re-inserted with a smaller bit to accommodate the narrower pipe.
Once in the wellbore, the steel pipe casing strings are cemented in place around the outside diameter of the pipe. This is done by pumping a cement slurry and drilling mud mix down the casing string with a bottom plug and a top plug. The mixture then moves through the casing and fills the space between the outside of the pipe and the well wall (the annular space/annulus). After the casing pipe is cemented, drilling can continue.
The outermost layer of casing pipe is the conductor casing string. Conductor casing pipe joints are usually 40 feet in length, but are only used at the upper-most levels of the well (typically 40-300 feet deep). This section of pipe prevents contamination of the surrounding soil and groundwater/freshwater zones, as well as minimizes the risk of the well walls collapsing.
Conductor pipe is sometimes called drive pipe because it can be set with a pile driver in many offshore environments, or simply run into a pre-drilled hole and cemented.
The second-largest diameter steel casing pipe is the surface casing string, which can run several thousand feet long. Surface casing pipe is cemented to the surface and covers freshwater zones, which helps to prevent contamination during drilling and well completion. Additionally, surface casing helps reinforce the walls of the wellbore and supports the weight of all other casing strings (with the exception of the production liner).
Surface casing pipe also serves as the foundation for the blowout preventer and provides kick resistance for deeper drilling.
Intermediate casing strings are set between the surface casing and production casing strings in order to isolate sections of the openhole (uncased areas of the well) to enable deepening the well. Intermediate casing strings work to absolve surrounding high formation pressure areas or protect against caving in abnormally weak zones. Depending on wellbore conditions, this type of casing pipe may have a higher pressure rating than prior casing strings. Intermediate casing is also intended to isolate hole sections intended to deviate the wellbore.
Liners are used as a cost-saving casing string. Liners are hung from the bottom of the previous casing string, rather than at the surface. They may be run instead of full production casing to provide isolation across pay zones.
The production casing string, sometimes called the oil string, is inserted into the wellbore once the drill pipe has reached its target formation. It can rest above, below, or within the pay zone. Few wells are capable of producing directly through the casing pipe. The producing fluids, oil or natural gas, corrode or form deposits within the pipe, so an additional layer of casing is necessary to extract the oil or natural gas from the pay zone.
If the well is to be drilled deeper at a later date, the production casing pipes must be large enough that a drill bit can pass through.
In the event that the well cannot produce through the production casing strings, steel production tubing is installed in the last casing string. Production tubing is lighter than steel casing pipes and can be removed and re-run using a small service rig, making maintenance or replacement work much easier.
Production tubing is the conduit through which oil, natural gas and water are brought from the pay zones to the surface for processing. It must be carefully selected; tubing that is too narrow inhibits optimum production rates, but tubing that is too large will increase costs on the overall casing design (remember that all casing strings “nest” within one another) of the well.
The two primary purposes of casing strings are to protect the surrounding earth and fresh water zones from contamination, and to reinforce the well itself to prevent collapse. Casing pipe strings also work together to help protect the well in many other ways, including:
Casing strings are a large structural component of a well. Once drilling is completed, casing strings provide surfaces for the blowout preventer, wellhead equipment, and production packers and tubing. Casing also accounts for a large part of the overall well set-up cost.
Eiffel Trading helps customers buy and sell used steel casing pipe, as well as other types of used and surplus oilfield materials like drill pipe, steel pipe casing, and more. We also help customers buy and sell used oilfield equipment, like drill rigs and pumps.
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