borehole (well, dug well) A hole drilled into rock, usually by rotary methods, to enable an assessment to be made of the characteristics of the rock itself and of contained fluids, e.g. groundwater, natural gas, or petroleum. The size may range from a few tens of millimetres in diameter to over 300 mm, and boreholes can be drilled at any angle to depths of several kilometres. Rock samples may be recovered from below the surface, the rock may be examined by downhole geophysical or other methods, or pumping tests may be run. Boreholes may also be used as water, gas, or oil production wells. A completed borehole is frequently termed a well. See BOREHOLE LOGGING; and WELL LOGGING, GEOPHYSICAL.
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