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In the oil industry measured depth (commonly referred to as MD, or just the depth) is the length of the drilled borehole. [1] In conventional vertical wells, this coincides with the true vertical depth , but in directional or horizontal wells, especially those using extended reach drilling , the two can deviate greatly.
Nested wells, also referred to as nested monitoring wells, are composed of multiple tubes or pipes, typically terminating with short screened intervals (2–3 ft), installed in single boreholes. Sand packs must be installed at the screen depths and seals in the borehole are constructed between the sand packs.
The Kola Superdeep Borehole SG-3 (Russian: Кольская сверхглубокая скважина СГ-3, romanized: Kol'skaya sverkhglubokaya skvazhina SG-3) is the deepest human-made hole on Earth (since 1979), which attained maximum true vertical depth of 12,262 metres (40,230 ft; 7.619 mi) in 1989. [1]
Borehole drilling has a long history. Han dynasty China (202 BC – 220 AD) used deep borehole drilling for mining and other projects. Chinese borehole sites could reach as deep as 600 m (2,000 ft). [1]
The British sinologist and historian Michael Loewe states that borehole sites could reach as deep as 600 metres (2,000 ft). [3] K.S. Tom describes the drilling process: "The Chinese method of deep drilling was accomplished by a team of men jumping on and off a beam to impact the drilling bit while the boring tool was rotated by buffalo and oxen."
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For example, if the tool fails at 1,500 ft (460 m) while drilling with a triple rig (able to trip 3 joints of pipe, or about 90 ft (30 m) feet, at a time), then it would generally be faster to pull the tool out of the hole than it would be to rig up wireline and retrieve the tool, especially if the wireline unit must be transported to the rig.
Well logging, also known as borehole logging is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole.The log may be based either on visual inspection of samples brought to the surface (geological logs) or on physical measurements made by instruments lowered into the hole (geophysical logs).