Casing (borehole)

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Casing Diameters of a Borehole Casing Diameters of a Borehole.JPG
Casing Diameters of a Borehole
Casing Diagram Casing-Diagram.jpg
Casing Diagram
premium gas tight connections on a casing string OCTG casing pipes.jpg
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.

Contents

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 installation Casing & Temporary Home.JPG
Casing arranged on a rack at a drilling rig in preparation for installation

A 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

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.

Intervals

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+38 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.

Cementing

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

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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<span class="mw-page-title-main">Oil well</span> Well drilled to extract crude oil and/or gas

An oil well is a drillhole boring in Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released as associated petroleum gas along with the oil. A well that is designed to produce only gas may be termed a gas well. Wells are created by drilling down into an oil or gas reserve and if necessary equipped with extraction devices such as pumpjacks. Creating the wells can be an expensive process, costing at least hundreds of thousands of dollars, and costing much more when in difficult-to-access locations, e.g., offshore. The process of modern drilling for wells first started in the 19th century but was made more efficient with advances to oil drilling rigs and technology during the 20th century.

<span class="mw-page-title-main">Casing hanger</span>

In petroleum production, the casing hanger is that portion of a wellhead assembly which provides support for the casing string when it is lowered into the wellbore. It serves to ensure that the casing is properly located. When the casing string has been run into the wellbore it is hung off, or suspended, by a casing hanger, which rests on a landing shoulder inside the casing spool. Casing hangers must be designed to take the full weight of the casing, and provide a seal between the casing hanger and the spool.

Well control is the technique used in oil and gas operations such as drilling, well workover and well completion for maintaining the hydrostatic pressure and formation pressure to prevent the influx of formation fluids into the wellbore. This technique involves the estimation of formation fluid pressures, the strength of the subsurface formations and the use of casing and mud density to offset those pressures in a predictable fashion. Understanding pressure and pressure relationships is important in well control.

Production tubing is a tube used in a wellbore through which production fluids are produced (travel).

<span class="mw-page-title-main">Wellhead</span> Component at the surface of a well that provides the structural and pressure-containing interface

A wellhead is the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment.

<span class="mw-page-title-main">Drilling fluid</span> Aid for drilling boreholes into the ground

In geotechnical engineering, drilling fluid, also known as drilling mud, is used to aid the drilling of boreholes into the earth. Used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells.

The annulus of an oil well or water well is any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept. The presence of an annulus gives the ability to circulate fluid in the well, provided that excess drill cuttings have not accumulated in the annulus, preventing fluid movement and possibly sticking the pipe in the borehole.

<span class="mw-page-title-main">Blowout preventer</span> Specialized valve

A blowout preventer (BOP) is a specialized valve or similar mechanical device, used to seal, control and monitor oil and gas wells to prevent blowouts, the uncontrolled release of crude oil or natural gas from a well. They are usually installed in stacks of other valves.

In drilling technology, casing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented. The purpose of the casing pipe is as follows:

<span class="mw-page-title-main">Coiled tubing</span> Long metal pipe used in oil and gas wells

In the oil and gas industry, coiled tubing refers to a long metal pipe, normally 1 to 3.25 in in diameter which is supplied spooled on a large reel. It is used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells. Coiled tubing is often used to carry out operations similar to wirelining. The main benefits over wireline are the ability to pump chemicals through the coil and the ability to push it into the hole rather than relying on gravity. Pumping can be fairly self-contained, almost a closed system, since the tube is continuous instead of jointed pipe. For offshore operations, the 'footprint' for a coiled tubing operation is generally larger than a wireline spread, which can limit the number of installations where coiled tubing can be performed and make the operation more costly. A coiled tubing operation is normally performed through the drilling derrick on the oil platform, which is used to support the surface equipment, although on platforms with no drilling facilities a self-supporting tower can be used instead. For coiled tubing operations on sub-sea wells a mobile offshore drilling unit (MODU) e.g. semi-submersible, drillship etc. has to be utilized to support all the surface equipment and personnel, whereas wireline can be carried out from a smaller and cheaper intervention vessel. Onshore, they can be run using smaller service rigs, and for light operations a mobile self-contained coiled tubing rig can be used.

A production packer is a standard component of the completion hardware of oil or gas wells used to provide a seal between the outside of the production tubing and the inside of the casing, liner, or wellbore wall.

A well kill is the operation of placing a column of special fluids of the required density into a well bore in order to prevent the flow of reservoir fluids without the need for pressure control equipment at the surface. It works on the principle that the hydrostatic head of the "kill fluid" or "kill mud" will be enough to suppress the pressure of the formation fluids. Well kills may be planned in the case of advanced interventions such as workovers, or be contingency operations. The situation calling for a well kill will dictate the method taken.

<span class="mw-page-title-main">Completion (oil and gas wells)</span> Last operation for oil and gas wells

Well completion is the process of making a well ready for production after drilling operations. This principally involves preparing the bottom of the hole to the required specifications, running in the production tubing and its associated down hole tools as well as perforating and stimulating as required. Sometimes, the process of running in and cementing the casing is also included. After a well has been drilled, should the drilling fluids be removed, the well would eventually close in upon itself. Casing ensures that this will not happen while also protecting the wellstream from outside incumbents, like water or sand.

Oilfield terminology refers to the jargon used by those working in fields within and related to the upstream segment of the petroleum industry. It includes words and phrases describing professions, equipment, and procedures specific to the industry. It may also include slang terms used by oilfield workers to describe the same.

Squeeze job, or squeeze cementing is a term often used in the oilfield to describe the process of injecting cement slurry into a zone, generally for pressure-isolation purposes.

Oil well cementing equipment are essential for oil and gas exploration and are required oilfield equipment when drilling a well.

<span class="mw-page-title-main">Offshore oil spill prevention and response</span>

Offshore oil spill prevention and response is the study and practice of reducing the number of offshore incidents that release oil or hazardous substances into the environment and limiting the amount released during those incidents.

<span class="mw-page-title-main">Oil well control</span> The management of oil wells

Oil well control is the management of the dangerous effects caused by the unexpected release of formation fluid, such as natural gas and/or crude oil, upon surface equipment of oil or gas drilling rigs and escaping into the atmosphere. Technically, oil well control involves preventing the formation gas or fluid (hydrocarbons), usually referred to as kick, from entering into the wellbore during drilling or well interventions.

References

  1. "How Does Casing Work?". www.rigzone.com. Retrieved July 5, 2018.
  2. 1 2 3 Rabia, Hussain (1986). Oil Well Drilling Engineering. springer. pp. 185–243. ISBN   0860106616.
  3. Fontenot, Kyle R.; Strickler, Bob; Warren, T. (2005). "Using Casing to Drill Directional Wells". Oilfield Review. S2CID   16241819.
  4. Wagner, R. R.; Warling, D. J.; Halal, A. S. (January 1, 1996). Minimum Cost Casing Design. Society of Petroleum Engineers. doi:10.2118/36448-MS. ISBN   9781555634230.
  5. 1 2 3 4 Petroleum Engineering Handbook, Volume II: Drilling Engineering. Society of Petroleum Engineers. 2007. pp. 287–288. ISBN   978-1-55563-114-7.
  6. USpatent 2012174581A1,"Closed-Loop Systems and Methods for Geothermal Electricity Generation"