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A top kill is a procedure where an oil and gas company halts production at a well by pumping in very dense material to reduce pressure, forcing the contents of the well to remain in place. This may be done for routine or emergency reasons and requires careful coordination to kill the well effectively and maintain safety for workers involved in the process. If a top kill does not achieve the desired effect, the company will have to work with engineers on another plan for shutting down production.
In a controlled top kill, a company has plans to stop a well for scheduled maintenance, repairs, and other activities. All of the systems at the well should be functioning properly, allowing the company to pump heavy drilling mud in through the top of the well. Normally this dense fluid acts as a lubricant. When delivered into the well at a very high rate, it will neutralize the pressure. With no pressure buildup, oil and gas cannot move to the surface, and the well stops producing. This allows personnel to work on it safely.
Emergencies may call for a top kill if a well experiences what is known as a blowout, where it starts to release material in an uncontrolled fashion. Pressure valves known as blowout preventers are, as the name implies, supposed to prevent this from happening, but sometimes they are not effective. To stop a spill, the company can attempt to reverse the movement of material in the well by adding drilling mud until the pressure drops and the well stops pumping out oil and gas.
Top kills in emergency situations can be dangerous. The well can pose a significant health hazard. Gas may flame as it erupts from the well, endangering people on site, and crude oil contains hydrocarbons known to be dangerous to human health. Workers exposed to the material need to take precautions to clean off at the end of the workday, and must be monitored for early warning signs of health complications like skin irritation from exposure to crude oil.
Performing a top kill requires securing a supply of drilling mud large enough to meet the need, and pumping it consistently to force the well to stop producing. In cases where blowout preventers have failed, the company may also pump debris into the well, in the hopes that it will clog the valve and prevent the additional release of oil or gas. This type of top kill is irreversible and may be followed with cement to seal the well, unlike a routine well kill where the company can pump the drilling mud out to start production again.