Directional drilling refers to drilling techniques used to create slanted wells; it thus is differentiated from traditional drilling practices, which result in vertical wells. While vertical drilling certainly has its merits, directional drilling has enabled a number of innovative uses in utility installation, mining, and oil drilling.
Directional boring, or horizontal directional drilling, is commonly used to install pipes, wires, or other buried utilities in urban or residential areas. By their nature, these areas are prone to incredible (and costly) disruption if the utility's infrastructure were to be laid using older "trench" techniques (that is, where an open trench is excavated for the infrastructure to be placed inside and reburied). "Trenchless" processes like directional boring allow for far less intrusive construction and installation, as the drill can create a cavity for the infrastructure without disrupting aboveground structures or utilities.
In coal mining, directional drilling is used to create wells for pumping out coalbed methane deposits. Surface in seam, or SIS, drilling produces wells that reach into the coal seam and pump out methane-saturated water. The process increases the safety of coal mining by removing potentially volatile gases. It also has the added benefit of producing a marketable natural gas.
By far, the most common use for directional drilling, though, is in oilfield drilling. Directional drilling can reach oilfields otherwise obstructed from vertical access, such as a field located beneath a city or a difficult geographic formation. Additionally, it can allow for multiple wellheads connected to the same oil reserve, and can allow wells to better access strategic points within the reservoir.
Another important use of directional drilling is as a means of containing an oil well blowout. Blowouts occur when pressure controls in a well fail and crude oil is uncontrollably released; the iconic image of 19th-century oil derricks spewing geysers of oil is a good illustration of this, although the best-known recent example is likely the BP oil spill in the Gulf of Mexico in 2010. In the case of a blowout, a "relief well" can be directionally drilled to intersect with the blown out well and can be used as a conduit to pump heavy liquids (mud, cement, etc.) to plug the other well. Even when using stored computer data from when the original well was dug and electromagnetic sensors to scour the rock for the metal casing from the initial well, the amount of precision needed to meet a 25-centimeter well several kilometers beneath the earth's surface is enormous.
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