A relief well is a well drilled to intersect an oil or gas well that has experienced a blowout. Specialized liquid, such as heavy (dense) drilling mud followed by cement, can then be pumped down the relief well in order to stop the flow from the reservoir in the damaged well.
The first use of a relief well was in Texas in the mid 1930s when one was drilled to pump water into an oil well that had cratered and caught on fire. [1]