If the drilling company has lost control of a well and it has blown out (uncontrolled release of pressure subsurface fluids) a relief well can be drilled from a nearby location to intersect the pressured reservoir horizon close to the original borehole and to allow the pumping of high density mud to kill the original blow-out.

Relief well is to counter the problem of the flow of oil or gas from a ruptured well which needs to be filled from the bottom. The relief well is drilled at an angle to intersect the damaged well just above the oil reservoir and is supposed to intercept the failed well where it ends, such as in the case of the BP well in the Gulf of Mexico at about 18,000 feet underground. Once the wells intersect, BP will inject massive amounts of heavy mud — much heavier than the oil — from the new well into the leaking well. The same pressure that is currently pushing out the oil will also force the mud up the well hole. Once there’s enough mud in the failed hole, the leak should stop.