5.1 SHALY SAND ANALYSIS

Shales can cause complications for the petrophysicist because they are generally conductive and may therefore mask the high resistance characteristic of hydrocarbons. Clay crystals attract water that is adsorbed onto the surface, as well as cations (e.g., sodium) that are themselves surrounded by hydration water. This gives rise to an excess conductivity compared 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 saturations, 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 resistivity 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 successfully 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:

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 as with Archie but are derived in a different way...