Once a well has been drilled, if it is to become a production well, the well must undergo completion. While drilling a well cuts through the rock formations and allows drilling engineers to reach the reservoir below, the raw sides of the well cannot support themselves. Similar to the bones of your spine protecting the spinal cord, casing is tubing that is set inside the drilled well to protect and support the wellstream.
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In addition to providing stabilization and keeping the sides of the well from caving in on themselves, casing protects the wellstream from outside contaminants, as well as any fresh water reservoirs from the oil or gas that is being produced.
Also known as setting pipe, casing a well involves running steel pipe down the inside of a recently drilled well. The small space between the casing and the untreated sides of the well is filled with cement to permanently set the casing in place.
Casing a Well
The casing is fabricated in sections, or joints, that are usually about 40 feet long and screwed together to form longer lengths of casing, called casing strings. Each end of the casing joint has male threads that are protected by cap called a thread protector until the casings are ready to be jointed. Then, a collar or coupling, composed of a short cylindrical steel pipe that is slightly larger in diameter than the joints and also has female threads, is used to connect the two male joint ends. A thread compound is used on the two ends to ensure a tight seal.
Casing is run from the rig floor, connected one joint at a time by casing elevators on the traveling block and stabbed into the previous casing string that has been inserted into the well. Hanging above the drill floor, casing tongs screw each casing joint to the casing string.
Casing is run into the well and officially landed when the weight of the casing string is transferred to the casing hangers, which are located at the top of the well and use slips or threads to suspend the casing in the well.
A rounded section of pipe with an open hole on the end, a guide shoe is connected to the first casing string to guide the casing crew in running the casing into the well. Additionally, the outside of the casing has spring-like centralizers attached to them to help position in casing string in the center of the well.
After running the casing and before the cementing the well, a used drill bit is inserted into the well via a drillstring, and drilling fluid is then circulated for a certain amount of time to remove any remaining cuttings from the well. Also wall scratchers are dispatched into the well to remove any filter cake that may have formed on the sides of the well.
A cement slurry is then pumped into the well and allowed to harden to permanently fix the casing in place. After the cement has hardened, the bottom of the well is drilled out, and the completion process continues.
Casing Programs
Sometimes the well is drilled in stages called a casing program. Here, a well is drilled to a certain depth, cased and cemented, and then the well is drilled to a deeper depth, cased and cemented again, and so on. Each time the well is cased, a smaller diameter casing is used.
The widest type of casing is called conductor pipe, and it usually is about 30 to 42 inches in diameter for offshore wells and 16 inches in diameter for onshore wells. The next size in casing string is the surface casing, which can run several thousand feet in length.
In some wells, protection or intermediate casing is run to separate challenging areas or problem zones, including areas of high pressure or lost circulation.
The last type of casing string that is run into the well, and therefore the smallest in diameter, is the production or oil string. The oil string is run directly into the producing reservoir.The last type of casing string that is run into the well, and therefore the smallest in diameter, is the production or oil string. The oil string is run directly into the producing reservoir.
Casing Alternatives
In an effort to save money, sometimes a liner string is run into the well instead of a casing string. While a liner string is very similar to casing string in that it is made up of separate joints of tubing, the liner string is not run the complete length of the well. A liner string is hung in the well by a liner hanger, and then cemented into place.
Casing is a large diameter pipe that is assembled and inserted into a recently drilled section of a borehole. Similar to the bones of a spine protecting the spinal cord, casing is set inside the drilled borehole to protect and support the wellstream. The lower portion (and sometimes the entirety) is typically held in place with cement.[1] Deeper strings usually are not cemented all the way to the surface, so the weight of the pipe must be partially supported by a casing hanger in the wellhead.
Casing that is cemented in place aids the drilling process in several ways:[2]
Optimum design of the casing program decreases the well construction costs, enhances the efficiency of operations and also diminishes the environmental impacts.[3]
Casing arranged on a rack at a drilling rig in preparation for installationA slightly different metal string, called production tubing, is often used without cement inside the final casing string of a well to contain production fluids and convey them to the surface from an underground reservoir.
Design
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In the planning stages of a well, a drilling engineer, usually with input from geologists and others, will pick strategic depths at which the hole will need to be cased in order for drilling to reach the desired total depth. This decision is often based on subsurface data such as formation pressures and strengths, well integrity,[4] and is balanced against the cost objectives and desired drilling strategy.[2]
With the casing set depths determined, hole sizes and casing sizes must follow. The hole drilled for each casing string must be large enough to accommodate the casing to be placed inside it, allowing room for cement between the outside of that casing and the hole. Also, subsequent bits that will continue drilling obviously must pass through existing casing strings. Thus, each casing string will have a subsequently smaller diameter. The inside diameter of the final casing string (or penultimate one in some instances of a liner completion) must accommodate the production tubing and associated hardware such as packers, gas lift mandrels and subsurface safety valves.
Casing design for each size of designed pipes is done by calculating the worst conditions that may be faced during drilling and over the producing life of the well. Mechanical properties such as longitudinal tensile strength, and burst and collapse resistance (calculated considering biaxial effects of axial and hoop stresses), must be sufficient at various depths. Pipe of differing strengths often comprises a long casing string, which typically will have the greatest axial tension and perhaps highest internal burst pressure differentials in the upper parts, and the greatest collapsing loads deeper in the well from external pressure vs lowered internal pressure.
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Casing strings are supported by casing hangers that are set in the wellhead, which later will be topped with the Christmas tree. The lower members of the wellhead usually are installed on top of the first casing string after it has been cemented in place.
Intervals
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Typically, a well contains multiple intervals of casing successively placed within the previous casing run.[2] The following casing intervals are typically used in an oil or gas well:
The conductor casing serves as a support during drilling operations, to flowback returns during drilling and cementing of the surface casing, and to prevent collapse of the loose soil near the surface. It can normally vary from sizes such as 18" to 30".[5]
The purpose of surface casing is to isolate freshwater zones so that they are not contaminated during drilling and completion. Surface casing is the most strictly regulated due to these environmental concerns, which can include regulation of casing depth and cement quality. A typical size of surface casing is 13⅜ inches.[5]
Intermediate casing may be necessary on longer drilling intervals where necessary drilling mud weight to prevent blowouts may cause a hydrostatic pressure that can fracture shallower or deeper formations. Casing placement is selected so that the hydrostatic pressure of the drilling fluid remains at a pressure level that is between formation pore pressures and fracture pressures.[6][5]
In order to reduce cost, a liner may be used which extends just above the shoe (bottom) of the previous casing interval and hung off downhole rather than at the surface. It may typically be 7", although many liners match the diameter of the production tubing.[5]
Few wells actually produce through casing, since producing fluids can corrode steel or form deposits such as asphaltenes or paraffin waxes and the larger diameter can make flow unstable. Production tubing is therefore installed inside the last casing string and the tubing annulus is usually sealed at the bottom of the tubing by a packer. Tubing is easier to remove for maintenance, replacement, or for various types of workover operations. It is significantly lighter than casing and does not require a drilling rig to run in and out of hole; smaller "service rigs" are used for this purpose.
Cementing
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Cementing is performed by circulating a cement slurry through the inside of the casing and out into the annulus through the casing shoe at the bottom of the casing string. In order to precisely place the cement slurry at a required interval on the outside of the casing, a plug is pumped with a displacement fluid behind the cement slurry column, which "bumps" in the casing shoe and prevents further flow of fluid through the shoe. This bump can be seen at surface as a pressure spike at the cement pump. To prevent the cement from flowing back into the inside of the casing, a float collar above the casing shoe acts as a check valve and prevents fluid from flowing up through the shoe from the annulus.
Casing Wear
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A prolonged, recurrent axial and rotational movement within casing would cause wear to the casing interior, with the probability of blowouts, production loss, and other hazardous and costly complications.
The following conditions contribute to casing wear:
The following are recommendations for preventative measures to minimize casing wear:
References
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