Page 206 - Well Logging and Formation Evaluation

196 Well Logging and Formation Evaluation

drillstring itself, although nonmagnetic collars are always used
immediately beside the tool). The tool’s intrinsic accuracy is
affected by the sensitivity of the magnetometers and accelerome-
ters (used to determine tool orientation). From published data it is
estimated that lateral borehole uncertainty arising from use of
properly calibrated magnetic survey tools is on the order of 14m
per 1,000m in a vertical well and 20m per 1,000m in a 70-degree
deviated well.
(b) Gyro survey tools. These types of tools are affected by drift in the
alignment of the gyro orientation during the survey. They are
usually run only when casing has been set in a well, so they cannot
be used for decision making during the course of drilling. From
published data it is estimated that lateral borehole uncertainty
arising from use of properly calibrated gyro survey tools is on the
order of 1.5m per 1,000m in a vertical well and 8m per 1,000m
in a 70-degree deviated well.
(c) FINDS. These tools use highly accurate accelerometers and
double-integrate the accelerations to determine absolute distance
moved by the tool during the survey. Their accuracy is estimated
at 0.5m per 1,000m irrespective of deviation.

13.2.1 The Effects of Borehole Position Uncertainty

We need to know the positions of wells accurately for the following
reasons:

1. Well safety. In the event of a well blowing out, a relief well may be
required to intercept or pass close to the well. Homing in with a relief
well (see Chapter 12) is made easier if the position of the target well
is known accurately. Also, for well collision avoidance in densely
drilled areas, the relative well positions must be known.
2. Mapping. Any geological maps are only as good as the input data.
While small uncertainty in the lateral position of wells may not be criti-
cal, uncertainty in the TVD at which a certain horizon is penetrated
may lead to serious errors in maps, and therefore reserves. In particu-
lar, if the depth of a fluid contact appears to be different in neighbor-
ing wells, it might lead one to make wrong judgments as to the position
of faults or communication/differential depletion between wells.
3. Geosteering. When drilling horizontal wells through thin horizons,
accurate measurement of TVD is of high importance.