Page 200 - Drilling Technology in Nontechnical Language

Chapter 8 – DIRECTIONAL AND HORIZONTAL DRILLING 191

▪ N/S coordinate = 6.54 ft
As would be expected, the vertical depth is now slightly less than
the depth along the hole, and there is a small displacement of the well to
the north.
Survey 3 is at 750 ft depth. Inclination is now 5° and azimuth is 020°.
The well has built some angle and has turned slightly towards the east. The
calculated position of this survey point is given as follows:

▪ MD = 750 ft
▪ TVD = 749.28 ft
▪ E/W coordinate = 3.73 ft
▪ N/S coordinate = 23.33 ft
The assumption behind the calculations is that the well follows a perfect
curve between each survey. In reality, this is not the case, and so there is an
error between each survey. This error grows as the well is deepened. This
error creates a cone-shaped well path, where the well can be anywhere
within the cone. This is called the cone of uncertainty. If two wells are
drilled close together and these cones intersect, a possibility of collision
arises. The cone does not necessarily have a round shape if cut through,
as the instrument that measures inclination has a different accuracy to the
instrument measuring the azimuth. Thus the cone would actually have an
elliptical cross section.
There are ways to reduce this error so that the well path is known more
accurately. One way to do this is to take more surveys. This shortens the
distance between each survey and so will more closely measure the actual
well path. Another way is to use gyro-based tools rather than magnetic-
based tools to measure azimuth. In practice, MWD tools are used while
drilling a section of hole, then casing is run, and a gyro is run inside the
casing to give a corrected (and more accurate) path. Gyros centralized
inside casings can take readings while moving at very small intervals and
so will give a much more accurate survey than MWD tools, which have
magnetic sensors and thus in-built azimuth errors, not all of which can be
compensated for. MWD tools run in the open hole will not perfectly align
the sensors with the centerline of the wellbore (as a hole is not a perfectly

even cylinder of fixed diameter). Thus MWD survey measurements will

have a significant error that cannot be compensated for. In an inclined hole,
the BHA will sag towards the low side of the hole in between stabilizers,

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