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 or 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. 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.
Clusters of small-diameter boreholes equipped with heat exchangers 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. In the UK this energy capture technology is gaining popularity and is known as GHSP. There can be a few to several hundred boreholes, and in practice, depths have ranged from 150 to 1000 feet.