STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 955
M
- J
H
Figure 4-258. Solid geometry sketch of the planes defining the azimuth angle.
empirically corrected with the single-shot instruments. Since the three components
of H are measured, the magnitude of the error vector can be calculated if the
module of the nonperturbed earth magnetic vector is known. The corrected dip
angle vector can also be computed and compared to the non-perturbed dip angle.
The computation should match; if it does not, then a nonaxial perturbation is
present. This perturbation may be due to “hot spots,” points in the nonmagnetic
drill collar that have developed some magnetism, or to external factors such as a
casing in the vicinity. Correction techniques have been introduced for the hot spots.
External magnetism due to casing or steel in the well vicinity is used in passive
ranging tools for blowout well detection from a relief well.
The accuracy of MWD directional measurements is generally much better than
the single- or multishot-type measurements since the sensors are more advanced
and the measurements more numerous. The azimuth measurement is made with
the three components of the earth magnetic field vector and only with the
horizontal component in the case of the single shot or multishot. The accelerometer
measurements of the inclination are also more accurate whatever the
value of the inclination. The average error in the horizontal position varies from