Water Saturation in Shaly Formation
Authors: Alfosail K.A. , Alkaabi A.U. 
Published: December 2007

Abstract
Electrical log interpretation is a useful method for in-situ evaluation of hydrocarbon saturated formations. The relationship for such an interpretation is based on Archie equation which relates the porosity and formation factor. The presence of clay minerals in the formation complicates the calculation of water saturation using Archie equation since the clay particles, depending on their type and distribution in the pore space, contributes to the electrical conductivity of the formation.
Numerous researches have been carried out to investigate the influence of clay on rock conductivity using theoretical and experimental approaches, and also to improve the accuracy of determination of the water saturation in oil bearing shaly formations. Based on the available theoretical and experimental data and an actual well log field data, an approach to evaluate electrical log interpretation for Saudi shaly sand is obtained.
An equation was developed to calculate water saturations in the shaly sands. The water saturation predicted by this equation was in close agreement to that calculated by the widely used Schlumberger and Indonesian equations. Also, resistivity measurements for two core samples were used for comparison purpose. It was found that the predicted water saturation was in good agreement with the measured values. Multiple salinity experiments were conducted to find the cation exchange capacity (CEC) and it was found that the values obtained for the core sample were in close agreement with that obtained by other methods.

Introduction
The resistivity log interpretation is a useful method for in-situ evaluation of hydrocarbon saturations in permeable formations. The interpretation was based on Archie's equation which relates water saturation to the resistivity of the formation water, porosity and the resistivity of the permeable formation.
The presence of clay in the formation complicates the interpretation and may give misleading results if Archie's equation is used because the clay is considered to be a conductive medium. In such case, the other equation representing the shaly formation should be used.
The effect of clay on resistivity measurements depends on the type, the distribution and the volume of clay present in the formation. In such case, Archie's equation cannot be used to calculate water saturation.
Several interpretation methods have been developed either experimentally or theoretically to represent the conductivity of such formation (a porous formation contains clay). A thorough discussion was done resulting in subdivision of the developed methods into different types as follows:
(1)
(2)
(3)
(4)
where
ct = conductivity of the shaly formation
A = represents sand term
B = represents clay term
C = represents an interactive term
Sw = water saturation
The differences between these types are:an interactive term and Sw do not appear in clay term in Type-i while it appears in both terms in Type-2, andan interactive term is present and Sw does not appear in some of the terms in Type-3 while it appears in Type-4.
In the above types, the conductivity of clay was expressed in terms of electrical logging data. P. 555^