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Related to Borehole: Borehole mining
(well), a circular mining excavation more than 5 m deep and usually between 75 and 300 mm in diameter, made by a drilling rig. Boreholes are drilled from the earth’s surface and from underground mining excavations at any angle relative to the horizon. Three parts are distinguished: the head (mouth), bottom (face), and shaft. Borehole depths range from a few to 9 km and more. During exploratory drilling for solid minerals, borehole diameters are usually 59 and 76 mm, whereas petroleum and gas wells range from 100 to 400 mm in diameter.
Depending on their purpose, boreholes are classified as exploratory boreholes, operating wells, auxiliary wells, special-purpose boreholes, and shot holes. Exploratory boreholes are used for geological purposes, geological-engineering and hydrologic surveying, the study of structures, geophysical work, and exploration and analysis of useful minerals. Operating wells are used to extract petroleum and gas, subterranean water, and mineral salts. Auxiliary wells include pressure, observation, piezometric, ventilation, water-removal, and de-gasification wells. Special purpose boreholes are used for refrigeration, plugging, and drainage, and shot holes are used for the placement of explosive charges.
Boreholes are made by drilling and removal of the drilled rock and, where necessary, reinforcement of the borehole walls against caving. The drilled rock is removed by a flushing liquid, gas, or mechanical devices. Exploratory boreholes drilled through unstable upper rocks are cased with thin piping; in stable rocks, no reinforcement is used. In heavily fissured rocks and absorption zones, quick-setting mixtures are used for plugging. Operational wells and deep exploratory boreholes are reinforced with metal casing and cemented. The casing is screwed or welded together; in shallow water wells, plastic, asbestos cement, and other types of piping are used.
Reinforcing a petroleum and gas well involves setting the first casing column, usually up to 20 m in length and called a guide, into the mouth of the well. A second column of casing, which is called the conductor and ranges from dozens to hundreds of meters in length, is sunk both to ensure that the subsequent shaft of the well is vertical or has the proper inclination and to seal out gas and water flows. A cement solution is pumped through the conductor into the open space between the walls of the well and the conductor by means of a flushing or special fluid. After drilling has reached the planned depth and geophysical work to determine the presence of petroleum, gas, and other producing horizons has been completed, the operating column of casing is lowered into the well. The well space outside the operating column is also filled with a cement solution to prevent oil or gas seepage into overlying horizons and to keep water out of producing strata. Under complex geological conditions (water-bearing, absorbent horizons), when it is impossible to drill the well without additional reinforcement, an intermediate column is lowered between the conductor and the operating column. If only an operating column is sunk after the conductor, the well is called a single-column well; when there are one or two intermediate columns, the well design is called two- or three-column.
There are different methods of opening and rigging up the face to extract liquid or gas minerals from the strata. In most cases a series of holes is shot in the wall of the casing and in the cement shell in the lower cemented part of the operating column, which is in the producing stratum. In stable rocks, the well zone near the face is equipped with various types of filters and not cemented or the casting is lowered to the roof of the producing stratum; the stratum is drilled and worked without reinforcement of the well shaft. Depending on its purpose, the well head may have various kinds of fittings, such as a column head, shutoff valves, or crosses.
REFERENCESBurenie neftianykh i gazovykh skvazhin. Moscow, 1961.
Kulichikhin, N. I., and B. I. Vozdvizhenskii. Razvedochnoe burenie, 2nd ed. Moscow, 1973.
S. N. UDIANSKII