True vertical depth
Encyclopedia
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
ly downwards from a horizontal plane
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...
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In the petroleum industry
Petroleum industry
The petroleum industry includes the global processes of exploration, extraction, refining, transporting , and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline...
, true vertical depth, abbreviated as TVD, is the measurement from the surface to the bottom of the borehole
Borehole
A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...
(or anywhere along its length) in a straight perpendicular line represented by line (a) in the image.
Line (b) is the actual borehole and its length would be considered the measured depth
Measured depth
In the oil industry measured depth , is the length of the borehole, line in the image, and is measured in feet or metres...
http://www.glossary.oilfield.slb.com/Display.cfm?Term=measured%20depth in oilfield terminology. The TVD is always equal to or less than (≤) the measured depth. If you imagine line (b) were a piece of string and pull it straight down, you would see that it would be longer than line (a). This example oilwell would be considered a directional well
Directional well
A directional well, as it is known in the oil industry, is an oil well with a borehole that deviates from a vertically straight line. This is normally an intentional decision and done purposefully, to hit several target sands, for instance.- References :...
because it deviates from a straight vertical line.