Relief wells are drilled to regain control in the event of a blowout, either by relieving pressure or blocking the well. They can also be used for soil stabilization. The development of safety measures such as blowout preventers has reduced the occurrence of blowouts.
When a well experiences a blowout, which is a sudden, uncontrolled ejection of natural gas or crude oil, relief wells are one way to regain control. The vent well is typically drilled so that it intersects with the blowout, both to relieve pressure and to block the tank. If the goal is to relieve pressure, the new drain pan can be connected to the original pipeline. Otherwise, heavy drilling mud, concrete, or other materials may be pumped into the reservoir through the new wellbore. These materials can essentially block the original well, effectively bringing it back under control.
In the early days of hydrocarbon exploration, it was quite common to tap into a pressurized oil field. When this happened, oil or gas would gush out of the ground. This is where the term gusher originated, and the iconic image of a tower with a plume of crude oil spraying from above is still often associated with the industry. These gushes were both wasteful and bad for the environment, so technology, such as the blowout preventer (BOP), was developed to avoid such situations. When these safety measures fail, a blowout can occur and pressurized natural gas or crude oil can be released into the environment.
Relief wells are a primary means of defense against blowouts if the BOP and other safety measures have failed. In some cases it is possible to drill a relief well before any trouble occurs, although this practice is not usually mandatory. Most relief wells tend to be drilled in response to a catastrophic blowout and must be aimed perfectly to connect to the original well. If the target is all wrong, another relief well typically has to be drilled.
A relief well can address either the underground well pipe or the tank. If the drain hole faces the tank, it is sometimes used to divert some pressure from the damaged original well. In other cases, water, drilling mud, concrete, and other materials can be pumped in to stop the blowout. The exact method used can vary from situation to situation.
Relief wells can also be used in soil stabilization, which has nothing to do with oil or gas. Unlike oil and gas wells, these are usually drilled as a precautionary measure. When the ground becomes saturated with water, wells can be used to draw some of it. This can have a stabilizing effect which is sometimes used to help prevent levees from subsidence.
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