Page 195 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 171

2.3.2
Directional Planning [ ]

The magnetic compass instrument should be placed 40% up from
the bottom of the Monels.
Note that Monel lengths are minimum for wells drilled north or
south and maximum for wells drilled east or west.
For example, for a wellbore inclination of 30˚ and a wellbore mag-
netic azimuth of 70˚, start at the 30˚ arc (bottom axis). Follow the arc
until it intersects the 70˚ radial. Read off the minimum length of mon-
els required, in this case 40 m (131 ft). The compass should be placed
about 16 m (53 ft) from the bottom.
Potential sources of survey errors. It is important to understand
the potential errors that can arise in the various surveying instru-
ments and methods of surveying. The main sources of error are sum-
marized below.

1. Long intervals between surveys. The calculations give some sort of
averaging between surveys and so if the interval is long, the aver-
aged path may be significantly different from the actual path. The
more the wellbore path may have changed during the interval, the
more significant the potential error. A deviated well would give rise
to more inaccuracy than a vertical well for the same depth interval
between surveys.
2. Inaccurate measured depths for the survey locations. This error can
arise from a drillstring tally mistake, faulty depth counters on wire-
line units, or a simple error in writing down the information.
3. Reading errors. When surveys are recorded on photographic film
(single and multi-shot surveys), there is always the chance that the
surveyor may make a mistake. It is good practice for two people to
independently read the film and compare their readings.
4. Instrument calibration errors. All measuring instruments should be
calibrated regularly and a record made of the calibration date and
results. The drilling program could include a cautionary note to
check the calibration certificate, which should be kept with the
tool. If the tool has not been recently calibrated or if the data is not
available, the tool should be replaced.
5. Forgetting to account for magnetic variation. At most places on the
Earth’s surface there will be a difference between true and magnet-
ic north. Most survey calculations are done relative to true north,

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