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A drilling rig is a machine which creates holes (usually called boreholes) and/or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells or they can be small enough to be moved manually by one person. They sample sub-surface mineral deposits, test rock, soil and groundwater physical properties, and to install sub-surface fabrications, such as underground utilities, instrumentation, tunnels or wells. Drilling rigs can be mobile equipment mounted on trucks, tracks or trailers, or more permanent land or marine-based structures (such as oil platforms, commonly called 'offshore oil rigs'). The term "rig" therefore generally refers to the complex of equipment that is used to penetrate the surface of the earth's crust.
Drilling rigs can be:
* Small and portable, such as those used in mineral exploration drilling, water wells and environmental investigations.
* Huge, capable of drilling through thousands of meters of the Earth's crust. Large "mud pumps" circulate drilling mud (slurry) through the drill bit and the casing, for cooling and removing the "cuttings" while a well is drilled. Hoists in the rig can lift hundreds of tons of pipe. Other equipment can force acid or sand into reservoirs to facilitate extraction of the oil or mineral sample; and permanent living accommodation and catering for crews which may be more than a hundred. Marine rigs may operate many hundreds of miles or kilometres offshore with infrequent crew rotation.
Oil and Natural Gas drilling rigs can be used not only to identify geologic reservoirs but also to create holes that allow the extraction of oil or natural gas from those reservoirs. Primarily in on shore oil and gas fields once a well has been drilled, the drilling rig will be moved off of the well and a service rig ( a smaller rig ) that is purpose built for completions will be moved on to the well to get the well on line. This frees up the drilling rig to drill another hole and streamlines the operation as well as allowing for specialization of certain services, ie. completions vs. drilling.
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