Page 185 - Well Logging and Formation Evaluation

Homing-in Techniques 175

depths in the target well, the magnetic field, in the highside reference
system of the survey well magnetometer, is modeled. This involves
using equations 12.1.1 to 12.1.3, together with survey data from both
wells. In order to match the measured and modeled sets of data as func-
tions of depth, it is necessary to move the assumed position of the
target well with respect to the relief well and to vary the assumed dis-
tribution of poles on the target well until a best fit has been found.

Weaknesses in the above approach are as follows:

1. Effect of smearing. The point monopole model may not apply if the
magnetization has become smeared along the axis of the pipe. The
effect of this is that estimates of the distance made will be too large.
2. Poorly defined HS. If the relief well is nearly vertical, the HS direc-
tion ceases to be defined, and it is not possible to make the correct sub-
traction of the Earth’s magnetic field in the sensor plane. This problem
can be avoided by ensuring that the survey well is drilled with a few
degrees deviation.
3. Nonlinear well paths. It is highly recommended that the survey well
be drilled on a constant deviation and azimuth when passing close to
the target. This is because normal surveying of the well path is not pos-
sible while the magnetometers are affected by the disturbance. While
this problem could be solved through running a gyro in the survey well,
this is not normally practical in open hole. Also, visualization of the
survey and target wells is much harder if the highside reference system
does not remain fairly constant.

12.1.3 The Earth’s Field

It will now be shown how the components of the Earth’s magnetic field
in the highside reference system may be derived. Let the Earth’s magnetic
field be defined by:

• E h = horizontal component of Earth’s field
• E v = vertical component of Earth’s field
• Declination = angle clockwise from true north to magnetic north.

The Cartesian components of the Earth’s field vector are given by:

E x = E h *cos (declination ) (12.1.9)