Page 198 - Well Logging and Formation Evaluation

188 Well Logging and Formation Evaluation

Target well
Survey well

Electrode

Current on target well

Magnetometers in the sensor plane

Field due to current

Figure 12.2.1 Electromagnetic Homing-in Principles

3. The current in the target is influenced by the thickness of the
casing/drillpipe (which may vary with depth) and quality of the con-
ducting path between steel and the formation, which is also variable.

In some field cases there have been attempts to overcome some of
these limitations by injecting current directly into the target well at
surface. While this has some advantages, it is nevertheless virtually
impossible to accurately predict the current as a function of depth in the
target well. During my time in research, I was involved in developing a
series of Fortran programs that performed a full mathematical simulation
of the tool response as a function of depth for deviated well paths, incor-
porating the effect of noninfinite target casing. While these programs are
still available, it is beyond the scope of this book to explain the algorithms
in detail.
In Figure 12.2.2, a typical response of the total field strength and HS dir
of the field as a survey well passes a target well are shown.
Note that the direction of the field (unlike that from a monopole) varies
with depth as the survey well passes the target well. This means that while