Page 60 - Well Logging and Formation Evaluation

50 Well Logging and Formation Evaluation

In order to match the net sand footage calculated from the logs with
that seen on the core, the shale volume (V sh ) cutoff may be varied. Core
photographs will be taken under both normal and UV light, which can
also assist in the determination of net reservoir. Once the conventional
core analyses have been completed, one will have regular measurements
of core porosity, grain density, and permeability.
If measurements at overburden conditions have been performed, then
the conversion factors to convert porosity and permeability to in-situ con-
ditions should be established. If they are not available, one should assume
values based on regional data until special core analyses (SCALs) are
completed.
In-situ porosity vs. logarithm of permeability should be plotted, if nec-
essary dividing the data according to facies and/or formation such that a
single line can be fitted to the data with reasonable accuracy. This yields
the so-called poroperm relationship, which is usually of the form (in
millidarcies [md], porosity as fraction):

k = 10 Ÿ ( k a + k b *f ) (3.1.1)

where k = permeability of the reservoir. Typical values of k a and k b are
-2 and 20, respectively.
Using the V sh cutoff chosen, it should be the case that the net sands
should not contain porosities much below a level corresponding to 1md
permeability in oil zones and 0.1md in gas zones. If this is not true, then
it may be necessary to apply an additional porosity cutoff to exclude tight
zones, which are not picked up purely by a V sh cutoff.
Where core data are not available, it is sometimes helpful to plot the
gamma ray (GR) vs. the density log to help to establish the best point to
discriminate net from non-net from the GR log. Typically the plot will
show a behavior as shown in Figure 3.1.1.
As shale becomes dispersed in the pore space (increasing GR), the
density will rise until the point at which the pore space available for free
fluids becomes zero. Beyond this point, the amount of shale may still
increase until the formation becomes 100% shale, but the density will
change only slightly (depending on variation in density between quartz
and shale). The correct cutoff point is therefore the point at which the
gradient changes, corresponding to zero effective porosity.
If radioactive minerals are present in the sands, deriving V sh from the
GR alone will not be appropriate. In such formations it is recommended
to use purely a porosity cutoff. In the case of thinly laminated sands, it is