Page 64 - Well Logging and Formation Evaluation

54 Well Logging and Formation Evaluation

oil leg are not representative. However, it may also be the case that the
R w value is not robust for reasons highlighted in Chapter 2 (Section 2.5).
With respect to the value of R t to be used, one needs to decide whether
invasion or shoulder-bed effects are significantly affecting the deepest
reading resistivity tool. For a well drilled with OBM (oil-based mud) that
encounters thick sands, I would recommend simply using the deep resis-
tivity as it is. Where there are significant effects of invasion or shoulders,
then generally I would always recommend going with a saturation/height
approach in favor of Archie.
If it is decided to still try to correct the resistivity for such effects, then
the contractor’s chart books may be used for making the appropriate cor-
rections, or computer-based algorithms may be applied. Remember that
such corrections apply equally in the water leg if one is using a Pickett
plot to determine m and R w.
3.4 PERMEABILITY

For the final evaluation, a permeability log, as well as zonal averages,
will usually be required for input to the static and dynamic models. Using
the poroperm relationship described in Section 3.1, it is relatively simple
to derive a permeability log using the porosity log. However, once the log
has been derived, it is important to scrutinize it for any intervals for which
the permeability goes to an anomalously high value. Most sandstones do
not exceed about 1500md, although top-quality sands with porosities
above 35% may have permeabilities up to about 4000md. If necessary,
apply a cutoff to cap the permeability at a level that is supportable by the
core data. In the nonreservoir sections, the permeability should usually be
set to a very low value (e.g., 0.001md). Permeabilities calculated should
be roughly in line with those calculated from other sources, such as a for-
mation pressure tool, NMR (nuclear magnetic resonance), or production
tests.
For making zonal averages of the permeability, it should be noted that
three types of average are possible: arithmetic, geometric, and harmonic.
The arithmetic average is given by:

k arith = k h i  h i (3.4.1)
*
i
Hence, if, say, the zone were 50ft, comprising 100k values at 0.5-ft
spacing, the average would simply be the sum of the 100 values divided
by 100. This average is appropriate to use if the flow in the reservoir is