Page 187 - Well Logging and Formation Evaluation

Homing-in Techniques 177

Table 12.1.1
Determining HSTF from accelerometer data
HSTF
A x A y
+ + 180 -q
+ - 180 -q
- + - q
- - - q

The arctan function will normally return a value of q between -90 and
+90 degrees, and we are interested in the clockwise angle (between 0
and 360 degrees) between the HS and the toolface, denoted by HSTF.
To derive HSTF from q, the transformation must be applied as shown in
Table 12.1.1.
B hs and B hsr are given by the following equations:

B hs = B x *cos (HSTF ) - B y *sin (HSTF ) (12.1.18)

B hsr = B x *sin (HSTF ) + B y *cos (HSTF ). (12.1.19)

The field due to a monopole, as measured in the highside reference
system of a survey well, may likewise be modeled by replacing the compo-
nents E x , E y , and E z in the above equations with F x , F y , and F z , as given by
equations 12.1.1 to 12.1.3. In terms of the measurements made of F tot , F xy ,
F z , DF z , and AX dir , the behavior will be as shown in Figures 12.1.2 to 12.1.4.
Similarly for a dipole, consisting of a north and a south pole of equal
strength, the behavior will be as shown in Figures 12.1.5 to 12.1.7.
Note that in this example the axis of the dipole is parallel with that of
the survey hole. If the survey hole passes closer to one pole than to the
other, or if the poles are of different strengths, the behavior of the files
will not be symmetric. In general, where more than one pole is present,
it is necessary to model the field for different configurations and try to
match with the measured data. This may be done by an automated
procedure with a computer program.

12.1.5 Quicklook Interpretation Methods

Where the field is dominated by one pole, quicklook methods may
be applied to estimate the shortest distance and direction to the pole.