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Casing Diameters of a Borehole Casing Diagram premium gas tight connections on a casing string
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.
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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.
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.
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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 in (460 to 760 mm).[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 13+3'8 inches (340 mm).[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.
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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.
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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:
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Engineers are continually challenged with the risk of hole collapse when faced with drilling through overburden and unconsolidated formations. That is why it is prudent to use drilling casings to drill through overburden while advancing the drilling casing and protecting the hole at the same time.
The aim of this blog is to tell you everything you need to know about this ingenious product so you can make an informed decision as to whether it is the right solution for your drilling program.
After reading our guide, you will have an understanding of what casing advancement systems are, when and where to use them and which casing advancement systems are suitable for your project.
A drilling casing is a type of drilling tool that enables you to case and drill simultaneously. It can be used with rotary percussion drilling where the impact of the hammer drives the casing down the wellbore.
Installing casing, while drilling, minimizes the risk of holes collapsing and enables projects to be completed faster and more efficiently. For this reason, casing advancement systems add value whether installing foundations, support elements, or casing a hole in formations where hole collapse is possible.
Casing advancement systems, or drilled casings, are used when ground conditions make it difficult to drill, such as in loose formations or drilling through highly fractured rock.
Ninety percent of the earth's ground conditions are loose, unconsolidated, and rocky. Drilling in these formations is often challenging and the risk of collapsing, hole deviation, and failure to reach the intended depth is a real concern for drilling contractors.
Installing drilling casing as the hole is drilled helps to stabilize loose, or unpredictable formations, and shore up the hole. This is particularly the case when penetrating clay, silt, sand, gravel, cobbles, boulders, and fractured rock.
Casing and drilling simultaneously improve drilling efficiency. If you need to speed up drilling time, adding a casing advancement system to your drilling toolbox will go a long way to improving productivity and the drill time on your project.
Selecting the right system will depend on the drilling application or purpose. There are several factors to consider: the ground formation and type, the drilling method, whether a permanent or retrievable casing is required, the outside diameter of the casing, the wall thickness, the ring bit ID, as well as the casing and rock socket depth. Armed with this knowledge, we can then specify the appropriate system.
Below are examples of the most common applications:
The use of geothermal heating in houses has become increasingly popular as it provides both economical and environmental advantages for the user.
Casing advancement systems are useful when drilling a geothermal well as they keep the hole open during installation of the heat exchange system and minimizes any impact on the surrounding structures. It also prevents the collapse of the well during drilling where ground conditions are unstable.
Water well drilling is commonly done in urban areas where drilling at speed is necessary to minimize disruptions to residents, as well as in the developed area where drilling is taking place.
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