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How Relief Wells Work

Purpose-Driven Drilling: Building a Relief Well
A worker turns a valve to transfer fuel to the equipment drilling a relief well in the Gulf of Mexico in May 2010.
A worker turns a valve to transfer fuel to the equipment drilling a relief well in the Gulf of Mexico in May 2010.
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A relief well is nearly identical to a regular well. In fact, a relief is drilled using the same equipment that's used to drill a regular well. The main difference is that, in the case of oil or natural gas relief wells, the vertical shaft is eventually drilled at an angle to intersect with the original well's pipe. But, if you're picturing a relief well going in right next to a blowout well, it's time to reframe. In reality, a relief well must be placed at a safe distance from the original well. For example, in the 2010 Gulf of Mexico oil leak, workers began drilling the first relief well about half a mile (800 meters) from the well that was gushing oil [source: BP]. Then there's the challenge of using a 1-foot (30 centimeter) wellbore to drill into earth monitored by remote mapping equipment, hoping to hit a pipe of similar diameter [source: Mowbry].

To begin, workers use a steel drill bit attached to a hollow steel pipe to delve through geographic layers [source: Bluestein and Drearen]. The drill bit is replaced several times throughout the process because it tends to dull with use; the steel pipe helps keep the drilled hole from collapsing and is later replaced by a permanent version. When it's time for the drill bit to change direction so it can intersect with the original well piping, a pipe joint is attached that causes the drill to angle -- but only to a gentle 5 degrees. As the drill reaches its target depth, the well will be "set," which means permanent tubing will be lowered into the hole and cemented [source: Earth Science Australia]. Then, as the drill bit nears the original pipe, it will be replaced with a diamond bit that can cut through the main well's existing pipe [source: Tasker].

The idea is that once the pipes intersect, the relief well can be used to do one of two things: divert the flow from the original well or block it by delivering synthetic or natural additives, such as mud, concrete or guar gum filler.

While oil relief wells are sometimes created to introduce substances into well site, a water relief well is designed to keep foreign materials out. Screens and filters are used to keep coarse materials from entering and clogging the water well. And, unlike oil relief wells, which are often directionally drilled to intersect with the original well's pipeline, water relief wells are always drilled vertically [source: U.S. Army Corps of Engineers]. As the water relief wells dispel excess water from underground, they relieve water pressure on existing wells and help keep the water table in check.

Once a drill hits its target -- whether its water, oil or natural gas -- what happens next? If you're wondering how water or fossil fuels are piped out of the ground, check out the next page.