Page 193 - Well Logging and Formation Evaluation

Homing-in Techniques 183

Target well

Survey well

a

Pole
Point of closest approach to pole
t
d
Point of closest approach to target well

Figure 12.1.8 Survey Well Passing a Target Well

For dipoles, the situation is more complex, and a formula can be given
for only the simplified situation of the survey and target wells being
parallel and the separation of the poles being small compared with the
separation of the wells. A dipole may be regarded as being positive if the
north pole is above the south pole, and negative if the south pole is above
the north pole. The polarity of the pole may be seen by observing AX dir
as the pole is passed. For a positive dipole, AX dir increases with depth,
reaching 180 degrees at the point of closest approach to the dipole, then
decreasing again. For a negative dipole, AX dir should decrease to zero as
the dipole is passed, and then increase again. Since AX dir is 0 or 180
degrees at the near point, HS dir cannot be defined at this point. However,
for a positive dipole, the direction to the target relative to HS is equal to
HS dir just above the near point. For a negative dipole, it is equal to HS dir
just below the near point. To find the Cartesian position of the dipole,
equations 12.1.24 to 12.1.29 may be used.
Distance may be found from one of the following methods. The half-
width of F tot is related to the distance by:

x = F tot *.453 . (12.1.30)
0