True vertical depth is the measurement of a straight line perpendicularly downwards from a horizontal plane.
In the petroleum industry, true vertical depth, abbreviated as TVD, is the measurement from the surface to the bottom of the borehole (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 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 because it deviates from a straight vertical line.
Famous quotes containing the words true, vertical and/or depth:
“All that stock of arguments [the skeptics] produce to depreciate our faculties, and make mankind appear ignorant and low, are drawn principally from this head, to wit, that we are under an invincible blindness as to the true and real nature of things.”
—George Berkeley (1685–1753)
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—Elizabeth Barrett Browning (1806–1861)
“It is true, that a little Philosophy inclineth Mans Minde to Atheisme; But depth in Philosophy, bringeth Mens Mindes about to Religion.”
—Francis Bacon (1560–1626)