Page 211 - Well Logging and Formation Evaluation

Well Deviation, Surveying, and Geosteering 201

Sand
Shale

d
Down log
GR

Up log
Depth
t
Figure 13.3.2 Example of Up/Down Response as Borehole Crosses Boundary from
Above

If the borehole deviation is f, the apparent formation dip (a) will be given
by different formulae depending on whether the wellbore leaves by the
top or bottom of the formation, and whether or not the formation dip is
broadly in the same direction or opposite to that of the borehole. The
appropriate formulae are:

Case A. Exit by bottom, formation dip opposite to borehole:

a = ( f - 90 )+ q (13.3.2)

Case B. Exit by top, formation dip opposite to borehole:

a = (90 - f)+ q (13.3.3)

Case C. Exit by bottom, formation dip same direction as borehole:

a = (90 - f) - q (13.3.4)

Case D. Exit by top, formation dip same direction as borehole:

a = (90 - f)+ q (13.3.5)

These four scenarios are illustrated in Figure 13.3.3.