Page 201 - Well Logging and Formation Evaluation

Homing-in Techniques 191

At x = 0, HS dir is also zero, which means that the field direction is along
the HS direction. When HS dir = 45 degrees and x = X 45 , rearranging
equation 12.2.2. yields:

X 45 = D*cotan ( ).a (12.2.3)

Hence, if we plot HS dir vs. x and measure the width D over which HS dir
varies from -45 to +45 degrees from its value at x = 0, we can say:

D = tan ( )*a D . 2 (12.2.4)

If the target well is not vertical, there will be a static shift in HS dir , and
a needs to be taken as the intersection angle between the survey well and
target well. Intensity data may also be used for triangulation purposes if
it can be assumed that the current on the target well is approximately con-
stant as the sensor passes the target well. It is also necessary that any
background signal be removed. This is done as follows. The separation in
measured depth (D) between points for which the intensity curve has fallen
to half its maximum height is given by:

D= (D sin ( ))* *a 2 (y + (y 2 + ) ) (12.2.5)
3
2
2
where y = (2 - cos (a)) / cos (a) and D is the distance of closest approach.
Exercise 12.2. Interpretation of Electromagnetic
Homing-in Data

A survey well was drilled at an inclination of 50 degrees past a vertical
target well. After correction for background effects, an electromagnetic
tool measured the data given in Table 12.2.1.

With the quicklook methods described above, measure the distance at
the point of closest approach, using both the directional and intensity data.