Page 182 - Well Logging and Formation Evaluation

172 Well Logging and Formation Evaluation

such disturbances may be detected (typically <15m) and the uncertain
nature of the magnetization.
Since standard surveying tools, such as MWD (measurement while
drilling), now possess accurate magnetometers, there is no need for
dedicated tools for magnetostatic homing in. However, interpretation
from such tools is not straightforward. It is hoped that the following will
provide some useful techniques, as well as a better understanding of mag-
netic surveying in general.

12.1.1 Magnetization of Steel Casing/Drillstrings

As a result of the manufacturing process, or from subsequent magnetic
inspection or mechanical shocks, the steel used in making casing,
drillpipe, or accessories (e.g., jars, drill collars, bits) usually acquires a
degree of magnetization. The simplest model for describing the field due
to such a magnetization is in terms of a superposition of magnetic point
monopoles of either north or south polarity. A north monopole is defined
as one for which the lines of magnetic flux flow symmetrically and spher-
ically toward the pole; for a south pole, the lines of flux flow similarly
away from the pole. Although free monopoles are never observed in
nature, in the vicinity of a pole, of which the opposite pole is far removed,
the field will be dominated by that of the near pole, and the far pole may
be ignored. When two or more poles are a similar distance from the point
of measurement, the field is a linear superposition of the monopole field
due to each of the individual poles. The magnetic field due to a monopole
in a Cartesian reference system is as follows:

3
r M x ( * *
F x = m * * 4 p r ) (12.1.1)
3
F y = m * * 4 p r ) (12.1.2)
r M y ( * *
3
F z = m * * 4 p r ) (12.1.3)
r M z ( * *
where:
r = ( x 2 + y 2 + ) , in m
z 2
M = the pole strength, in Webers (Wb)
F x, F y, F z are the field strengths, in Tesla (T)
mr is the relative magnetic permeability of the medium
x, y, z are the distances, in m, from the monopole to the measuring point
along the x, y, and z axes.