Page 194 - Well Logging and Formation Evaluation

184 Well Logging and Formation Evaluation

The width of the AX dir curve at the points where AX dir is 90 degrees is
related to distance by:

x =DAX dir 2 . (12.1.31)

The points on the F ax curve where the gradient is zero are such that the
separation between the first zero and the last zero, on either side of the
central zero, is related to distance by:

x =D F ax 6 . (12.1.32)

When the far field approximation no longer applies and the wells are
not parallel, the situation becomes more complex, and it is not possible
to use quicklook methods. However, it should be noted that at the point
of closest distance from one of the monopoles forming a dipole, the field
behavior of F tot , F xy , and F ax will be dominated by the monopole, and
monopole quicklook methods may be applied.

Exercise 12.1. Worked Field Example of Magnetostatic
Homing In

Consider the following case. A vertical exploration well has blown out,
and a relief well is required for homing in and intersection of the target
well. The best survey data available for the target well are as follows:
Total depth: 2,200ft TVDss
Easting: 5,340ft
Northing: 6,898ft
A relief well has been drilled to pass close to the blowout well, with
the survey data in Table 12.1.2.
The Earth’s magnetic field has the following components:

-6
.
E v =-775 10 T
*
-6
E h = 40 9 10 T .
.*
Magnetic declination (the angle clockwise from grid north to magnetic
north) is zero.
In the relief well, a survey package consisting of three magnetometers
and three accelerometers has been run. The results are shown in Table
12.1.3 (note that depths/distances are in feet and not meters).