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CHAPTER 5
ADVANCED LOG
INTERPRETATION
TECHNIQUES
5.1 SHALY SAND ANALYSIS
Shales can cause complications for the petrophysicist because they are
generally conductive and may therefore mask the high resistance char-
acteristic of hydrocarbons. Clay crystals attract water that is adsorbed
onto the surface, as well as cations (e.g., sodium) that are themselves sur-
rounded by hydration water. This gives rise to an excess conductivity com-
pared with rock, in which clay crystals are not present and this space might
otherwise be filled with hydrocarbon.
Using Archie’s equation in shaly sands results in values of water satu-
rations, S w, that are too high, and may lead to potentially hydrocarbon
bearing zones being missed. Many equations have been proposed in the
past for accounting for the excess conductivity resulting from dispersed
clays in the formation, which can have the effect of suppressing the resis-
tivity and making S w calculated using Archie too pessimistic. While these
equations will be given, I propose to work only one method through in
detail, namely a modification to the Waxman-Smits approach. I have suc-
cessfully used this method in a number of fields, and it has the advantage
of not necessarily relying on additional core analyses for calibration
(although these data may be included in the model).
Waxman-Smit’s equation may be stated as follows:
- n* = ( [ m* R BQ S w )]
S w R R w )*f ( * 1 + w v (5.1.1)
t
where B is a constant related to temperature, and Q v = cation exchange
capacity per unit pore volume. Here m* and n* have a similar definition
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