Page 199 - Drilling Technology in Nontechnical Language

190 Drilling Technology in Nontechnical Language Second Edition

Wellbore surveying calculations

The well has had several surveys taken, and the measured depth of
each is known. Now these results are input into a set of calculations that
then show the path of the well.
The surface location is known. What is needed is the vertical depth and
N/S-E/W coordinates for each survey point. These can then be plotted to
show the view from above and the view from the side from a predetermined

direction (see fig. 8–2).
While historically several different calculation methods have been
used, one standard used by all eventually evolved. This is called minimum
curvature. Imagine that two surveys are connected by a perfect arc of
known length. The arc length is the difference in the measured depths
of the two surveys. How the top and bottom are aligned is shown by the
inclination and azimuth of each survey.
This is best explained with an example. Imagine that the surface
coordinates are 0, 0 (that is, zero displacement east/west and zero
displacement north/south). The inclination is 0° (vertical), and the MD and
TVD are both 0. Call this survey number 0.
The next survey, number 1, is at 250 ft and is also 0° inclination.

The profile of the well will be relative to north, so although there is no
azimuth if the survey is vertical, the input to the calculations will be 000°
azimuth. Clearly, it is not necessary to actually calculate this out to know
the following:
▪ MD = 250 ft

▪ TVD = 250 ft
▪ E/W coordinate = 0 ft
▪ N/S coordinate = 0 ft
Survey 2 is at 500 ft measured depth. Inclination is now 3° and azimuth
is 000°. This is easy to visualize—a nice curve taking the well north. The
calculations show that the position of the well at this survey point is given
as follows:

▪ MD = 500 ft
▪ TVD = 499.89 ft
▪ E/W coordinate = 0 ft

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