The term workover is used to refer to any kind of oil well intervention involving invasive techniques, such as wireline,

coiled tubing In the oil and gas industries, coiled tubing refers to a very long metal pipe, normally 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 ...

snubbing Snubbing is a type of heavy well intervention performed on oil and gas wells. It involves running the BHA on a pipe string using a hydraulic workover rig. Unlike wireline or coiled tubing, the pipe is not spooled off a drum but made up ...

. More specifically, a workover refers to the expensive process of pulling and replacing completion or

production Production may refer to: Economics and business * Production (economics) * Production, the act of manufacturing goods * Production, in the outline of industrial organization, the act of making products (goods and services) * Production as a stati ...

hardware in order to extend the life of the well.

Reason to perform a workover

Workovers rank among the most complex, difficult and expensive types of wellwork. They are only performed if the completion of a well is terminally unsuitable for the job at hand. The

production tubing Production tubing is a tube used in a wellbore through which production fluids are produced (travel). Background Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore cas ...

may have become damaged due to operational factors like corrosion to the point where well integrity is threatened. Downhole components such as tubing, retrievable

downhole safety valve A downhole safety valve refers to a component on an oil and gas well, which acts as a failsafe to prevent the uncontrolled release of reservoir fluids in the event of a worst-case-scenario surface disaster. It is almost always installed as a v ...

s, or

electrical submersible pump A submersible pump (or electric submersible pump (ESP)) is a device which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to be pumped. The main advantage of this type of pump is that i ...

s may have malfunctioned, needing replacement. Workover Rig doing a Snub Job

In other circumstances, the reason for a workover may not be that the completion itself is in a bad condition, but that changing

reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...

conditions make the former completion unsuitable. For example, a high productivity well may have been completed with 5½" tubing to allow high flow rates because a narrower tubing would have unnecessarily choked the flow, but declining productivity could lead to stable flow being unsupportable through such a wide bore.

Operation

Before any workover, the well must first be killed. Since workovers are long planned in advance, there would be much time to plan the

well kill A well kill is the operation of placing a column of heavy fluid 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 o ...

and so the reverse circulation would be common. The intense nature of this operation often requires no less than the capabilities of a

drilling rig A drilling rig is an integrated system that drills wells, such as oil or water wells, or holes for piling and other construction purposes, into the earth's subsurface. Drilling rigs can be massive structures housing equipment used to drill wa ...

. The workover begins by killing the well then removing the wellhead and possibly the flow line, then installing a B.O.P commonly known as a

blowout preventer A blowout preventer (BOP) (pronounced B-O-P, not "bop") is a specialized valve or similar mechanical device, used to seal, control and monitor oil well, oil and gas wells to prevent Blowout (well drilling), blowouts, the uncontrolled release of ...

, then lifting the

tubing hanger A tubing hanger is a component used in the completion of oil and gas production wells. It is set in the tree or the wellhead and suspends the production tubing and/or casing. Sometimes it provides porting to allow the communication of hydraulic ...

from the casing head, thus beginning to pull the completion out of the well. The string will almost always be fixed in place by at least one

production packer 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. Based on their primary use, pa ...

. If the packer is retrievable it can be released easily enough and pulled out with the completion string. If it is permanent, then it is common to cut the tubing just above it and pull out the upper portion of the string. If necessary, the packer and the tubing left in hole can be milled out, though more commonly, the new completion will make use of it by setting a new packer just above it and running new tubing down to the top of the old.

Workovers on casing

Although less exposed to wellbore fluids, casing strings too have been known to lose integrity. On occasion, it may be deemed economical to pull and replace it. Because casing strings are cemented in place, this is significantly more difficult and expensive than replacing the completion string. If in some instances the casing cannot be removed from the well, it may be necessary to sidetrack the offending area and recomplete, also an expensive process. For all but the most productive well, replacing casing would never be economical.

References

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External links

Schlumberger oilfield glossary
Oil wells Petroleum production