Page 196 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 172

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2.3.2 .Well Programming

which does not change. Magnetic north changes with time. To
compensate for this, look at a recent aeronautical or marine chart
or other suitable chart containing lines that join points of equal
magnetic variation, called isogonals. The chart will also give a date
of publication and state how quickly the variation changes in the
area. It is therefore possible to assess what the current variation for
your location will be. Variation can be east or west and this will be
stated on the chart.
If variation is west then the variation should be subtracted from the
compass survey reading; if east it should be added. There is a
rhyme which can help you remember this: “Variation west, com-
pass best. Variation east, compass least.”
6. Magnetic interference. Even if the correct configuration of Monels is
run, magnetic interference may come from various places. Monels
occasionally develop magnetic “hot spots”; these can be checked by
running a compass along the Monel collar. These hot spots tend to
be near the connections. Also, if you run a magnetic single shot on
sandline or wireline, the line itself can develop a strong enough
magnetic field to affect the survey tool. To avoid significant error
due to the line, position the survey tool over 4 m away for a line
diameter up to 5 mm and 8 m away for greater sizes. The survey tool
could also be dropped using a timer and then fished on wireline so
that the line is not attached to the tool when the survey is taken.
Close proximity to other wells, fish that have been sidetracked
past, or casings are also likely to cause interference.
7. Gyro tools do not suffer from magnetic interference. However, gyro
tools have several potential sources of error. First, the tool has to
be aligned on surface with a fixed reference that is at a known
direction from the center of the rotary table. Incorrect initial align-
ment will throw out the entire survey. It may be possible to detect
this by checking the recorded azimuths against a previous survey
at the same depths.
Gyroscopes also “precess”; that is, the gyro will slowly wander
away from its initial alignment. The rate of precession depends on
several factors, such as friction from the bearings and gimbals, the
rotation of the Earth, and small imperfections in the gyro. The rate

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