Page 46 - Well Logging and Formation Evaluation

36 Well Logging and Formation Evaluation

This has the advantage, therefore, of being directly comparable to porosi-
ties measured on core plugs, since these have had all clay-bound and free
water removed.
Having calculated the porosity, it is important to check for any zones
where washouts have resulted in erroneously high density values and thus
unrealistically high porosities. In some cases it is sufficient to just apply
a cutoff to the data whereby porosities above a certain value are capped
at a value. This recognizes the fact that zones often wash out because they
are soft and have a high porosity. However, in some cases it is necessary
to manually edit the density log using one’s best estimate of what the
density should be. Note that in water-bearing sections a good estimate of
porosity, f, may be made using true resistivity (R t ) and Archie’s equation,
which is:

R t = R w *f - m * S w - n

or

]
S w = ( [ R R w )*f m ( -1 n) .
t
where R w = formation water resistivity (measured in ohmm)
m = the cementation, or porosity, exponent
S w = water saturation
n = saturation exponent.

Alternatively, sometimes a correlation can be made between the GR
and density in non–washed-out zones and applied.
I generally favor always working in a total porosity system. The term
effective porosity is also used, although often different people take it to
mean different things. Probably the best definition is that it is the total
porosity minus the clay-bound water and water held as porosity within the
clays. It may therefore be defined as:

f eff = f total * (1 - CV )
*
sh
where C is a factor that will depend on the shale porosity and CEC (cation
exchange capacity). It may be determined from calculating the total poros-
ity in pure shales (V sh = 1) and setting f eff to zero. However, I have doubts
about the correctness of assuming that properties of the shales in non-
reservoir zones can be applied to dispersed shales within sands in the