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Casing is large diameter pipe that is assembled and inserted into a recently drilled section of a borehole and typically held into place with cement. ==Purpose== Casing that is cemented in place, aids the drilling process in several ways: * Prevent contamination of fresh water well zones. * Prevent unstable upper formations from caving in and sticking the drill string or forming large caverns. * Provides a strong upper foundation to use high-density drilling fluid to continue drilling deeper. * Isolates different zones, that may have different pressures or fluids - known as zonal isolation, in the drilled formations from one another. * Seals off high pressure zones from the surface, avoiding potential for a blowout * Prevents fluid loss into or contamination of production zones. * Provides a smooth internal bore for installing production equipment. A slightly different metal string, called production tubing, is often used without cement in the smallest casing of a well completion to contain production fluids and convey them to the surface from an underground reservoir. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Casing (borehole)」の詳細全文を読む スポンサード リンク
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