A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water, other liquids (such as petroleum) or gases (such as natural gas), as part of a geotechnical investigation, environmental site assessment, mineral exploration, temperature measurement, as a pilot hole for installing piers or underground utilities, for geothermal installations, or for underground storage of unwanted substances, e.g. in Carbon capture and storage.
Engineers and environmental consultants use the term borehole to collectively describe all of the various types of holes drilled as part of a geotechnical investigation or environmental site assessment (a so-called Phase II ESA). This includes holes advanced to collect soil samples, water samples or rock cores, to advance in situ sampling equipment, or to install monitoring wells or piezometers. Samples collected from boreholes are often tested in a laboratory to determine their physical properties, or to assess levels of various chemical constituents or contaminants.
Typically, a borehole used as a water well is completed by installing a vertical pipe (casing) and well screen to keep the borehole from caving. This also helps prevent surface contaminants from entering the borehole and protects any installed pump from drawing in sand and sediment. Oil and natural gas wells are completed in a similar, albeit usually more complex, manner.
As detailed in proxy (climate), borehole temperature measurements at a series of different depths can be effectively "inverted" (a mathematical formula to solve a matrix equation) to help estimate historic surface temperatures.
Clusters of small-diameter boreholes equipped with heat exhangers made of plastic PEX pipe can be used to store heat or cold between opposing seasons in a mass of native rock. The technique is called seasonal thermal energy storage. Mediums that can be used for this technique range from gravel to bedrock. There can be a few to several hundred boreholes, and in practice, depths have ranged from 150 to 1000 feet.
Borehole drilling has a long history. Han Dynasty China (202 BC – 220 AD) used deep borehole drilling for mining and other projects. Chinese borehole sites could reach as deep as 600 m (2000 ft). The practice of well logging in boreholes dates to 1927, for the French Pechelbronn oil field.
For many years, the world’s deepest borehole was the Kola Superdeep Borehole. From 2011 until August 2012 the record was held by the 12,345-metre (40,502 ft) long Sakhalin-I Odoptu OP-11 Well, offshore the Russian island Sakhalin. The Chayvo Z-44 extended-reach well took the title of the world's longest borehole on 27 August 2012. Z-44's total measured depth is 12,376 m (40,604 ft). However, ERD wells are more shallow than Kola Superdeep Borehole, owing to a large horizontal displacement.
Drillers may sink a borehole using a drilling rig or a hand-operated rig. The machinery and techniques to advance a borehole vary considerably according to manufacturer, geological conditions, and the intended purpose. For offshore drilling floating units or platforms supported by the seafloor are used for the drilling rig.
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- Sakhalin-1 Project Drills World's Longest Extended-Reach Well
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