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A well kill is a strategy that is sometimes used to close off an active well, making it easier to contain the flow of product from the well and successfully cap the well. While there are several strategies used to accomplish this goal, many will involve the use of what is known as kill mud or fluid to effectively block the surge of other substances from the well, alleviating pressure and making it possible to move forward with successfully sealing the well bore. While a well kill may be a deliberate attempt to close a well that is damaged or that is no longer needed, this term can also refer to an accidental event that was not planned in advance.
When the well kill is intentional, the process will involve utilizing some type of substance that is heavier than the substance that was being harvested using the active well. For example, the use of kill mud may be ideal for containing the flow from an oil well. Since the mud is heavier, it can be injected into the well itself, helping to slow and eventually create a block that prevents the underground oil from escaping through the well bore and rising to the surface. Once the flow of oil is contained, the well can be sealed or capped, effectively preventing oil to use the well channel to reach the surface.
There are several different strategies used to manage a well kill. The use of mud to block the channel is among the most common and is sometimes referred to as the reverse circulation approach. A different strategy is known as bullheading. With this approach, some type of kill fluid is actively pumped into the well bore at a rate of pressure that forces the oil or other substance backward. This approach is more likely to be used when there is a need to close off a well quickly, but it does contain some risks, in that the sudden shift in pressure could cause ruptures along the shaft of the well bore.
When the well kill is a planned strategy that can be incrementally conducted over a period of time, an approach known as "lubricate and bleed" is often used. This approach involves introducing measured amounts of kill fluid into the well bore over a period of time, usually a drill pipe that is introduced into the well and allows the mud to begin settling near the bottom of the bore. Doing so makes it possible to ease any excess pressure in the bore itself, which in turn helps to minimize the possibility of a rupture. As the additional amounts of kill fluid settle in the well, the pressure continues to decline until the well can be safely capped.