Borehole

A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid (such as petroleum) or gases (such as natural gas), as part of a geotechnical investigation, environmental site assessment, mineral exploration, temperature measurement or as a pilot hole for installing piers or underground utilities. Also boreholes can be made for geothermal installations. As well as pumping petroleum from an underground well through a borehole, liquid or gas can be pumped into it, for that process, or for underground storage of unwanted substances., e.g. in Carbon capture and storage.

Engineers and environmental consultants use the term 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.

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.

The world’s deepest borehole was the Kola Superdeep Borehole for many years. From 2011 until August 2012 the record was held by the 12,345 metres (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.

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.