STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 905
MAGNETOMETER
" \
Y'
H
Y
VlETER
KEY
C
Figure 4. 18. Sketch of the principle of the sensor arrangemer
tool and any magnetic directional tool. (Courtesy SPE [loll.)
in a steering
with their sensitive axis along the principal axis of the tool. Ox is oriented toward
the bent sub in the bent sub/tool axis plane. Oy is perpendicular to Ox and the
tool axis. Oz is oriented along the tool axis downward.
The arrangement of Figure 4-218 is common to all directional tools based
on the earths magnetic field for orientation: MWD tools or wireline logging tools.
The steering tools have practically been abandoned and replaced by MWD
systems, mostly because of the electric wireline. However, the high data rate of
the electric wireline (20-30 kbits/s) compared to the low data rate of the MWD
systems (1-10 bits/s) make the wireline tools still useful for scientific work.
Accelerometers. Accelerometers measure the force generated by acceleration
according to Newton's law:
F = ma (4-1 72)
where F = force in lb
m = mass in (Ib - s2)/ft (or slugs)
a = acceleration in ft/sec2