Page 98 - Well Logging and Formation Evaluation

88 Well Logging and Formation Evaluation

serious problems, provided that any variations average over an interval
used for evaluation.
Problems arise where the variations initiate a nonlinear response in the
tool used to evaluate them. The most common example of this is the effect
of thin shale beds on the resistivity log. For beds that are perpendicular
to the borehole, the resistivity may be approximated by:

1 R t = 1 R shale + 1 R sand

Hence a small amount of conductive shale may significantly lower the
R t seen by the tool. When saturations are calculated, they may make the
zone appear predominantly water bearing and not perforated, when in fact
the water saturations in the individual sand beds are very low and the zone
is capable of producing dry oil.
Note that if the laminae are sufficiently wide, so that they are still
resolved by the density log (~1-ft resolution), the problem can easily be
overcome by using a J function approach, as described in Chapter 3. If
the laminae are on the millimeter scale, then they will not be resolved by
the density log and other approaches should be adopted.
First of all it is necessary to identify the laminated zones and to deter-
mine the proportion of sand to shale. The most reliable way to identify
laminated sand is through direct inspection of the core. Measurements
should be made to determine the relative thicknesses of the sand and shale
layers as a function of depth. If this is done, then it is recommended to
assume common properties for the sand, with the porosity taken from a
core average and the saturation derived using a saturation/height function
from core capillary pressure measurements. If core is not available, then
common ways to identify laminated sands are:

• Use of borehole images derived from either resistivity or ultrasonic-
based tools
• Inspection of the microresistivity, which may show rapidly varying
behavior (although this may also be due simply to variations in the
borehole wall)
• Measurement of resistivity anisotropy, either from comparison of
induction with laterolog types of devices or by running special induc-
tion tools with perpendicular coils, which may be indicative of lami-
nated sequences
• The presence of strong shows while drilling in zones appearing to be
nonreservoir on the logs