A top kill is a procedure used as a means of regaining control over an oil well

Oil well

An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...

 that is experiencing an uncontrolled eruption of crude oil or natural gas

Natural gas

Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

 (blowout

Blowout

-Science and technology:*Blowout , a sudden release of oil and gas from a well*Blowout , a sandy depression caused by the removal of sediment by wind*Blowout grass is a grass that can be found on sand dunes...

). The process involves pumping heavyweight drilling mud

Drilling fluid

In geotechnical engineering, drilling fluid is a fluid used to aid the drilling of boreholes into the earth. Often 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. Liquid drilling fluid...

 into the well. This procedure is expected to stop the flow of oil

Oil

An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

 and gas

Gas

Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 from the well.
A further step could be sealing the well completely, often with cement

Cement

In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

.

In use

The top kill procedure was used to plug flaming oil wells

Kuwaiti oil fires

The Kuwaiti oil fires were caused by Iraqi military forces setting fire to 700 oil wells as part of a scorched earth policy while retreating from Kuwait in 1991 after invading the country but being driven out by Coalition military forces...

, blown up by retreating Iraqi forces, in 1991, during the Gulf War

Gulf War

The Persian Gulf War , commonly referred to as simply the Gulf War, was a war waged by a U.N.-authorized coalition force from 34 nations led by the United States, against Iraq in response to Iraq's invasion and annexation of Kuwait.The war is also known under other names, such as the First Gulf...

.

This technique came to prominence during the 2010 Deepwater Horizon oil spill

Deepwater Horizon oil spill

The Deepwater Horizon oil spill is an oil spill in the Gulf of Mexico which flowed unabated for three months in 2010, and continues to leak fresh oil. It is the largest accidental marine oil spill in the history of the petroleum industry...

 when it was used in an attempt to seal a seafloor oil well after the failure of the blowout preventer

Blowout preventer

A blowout preventer is a large, specialized valve used to seal, control and monitor oil and gas wells. Blowout preventers were developed to cope with extreme erratic pressures and uncontrolled flow emanating from a well reservoir during drilling. Kicks can lead to a potentially catastrophic...

. However, it failed to block the flow of oil.