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A petrographic coded correlation between interface conductivity and other pore space properties
Nina Gegenhuber, Maria Ochabauer and Christian Preuer
first break volume 32, January 2014 pp51-56

Abstract
Electrical conductivity is an important property in geoscience and petroleum engineering. It gives not only information
about the porosity and water saturation, but also about interfacial conductivity and about specific internal surface of the
pore space. Archie’s equation correlates the electrical resistivity of a water-saturated rock sample and the resistivity of the
pore water. A more detailed investigation with brine of different salinities results in the observation of interface conductivity, which is correlated to a specific internal surface.
Measurements with increasing salinity of the brine were carried out on different samples: sandstone, carbonate and magmatic rocks. Additionally, permeability and effective porosity were determined.
The resulting true formation factor and interfacial conductivity were then correlated with porosity and permeability.
Analysis of data allows the application of the interface term as a measure of specific internal surface. For the interpretation of results, the simple capillary model
a semi-empirical equation is used and delivers permeability as function of porosity, formation factor and interface conductivity.
Determined permeability out of the calculations is compared with measured permeability. The developed equation which is dependent on the porosity, interface conductivity, formation factor and covers
additionally the lithology influence can be used for a permeability calculation with a reasonable fit



Introduction
Electrical conductivity or its inverse, the specific electrical resistivity, is an important property in geoscience and petroleum engineering.
Archie’s first equation (1942) combines porosity, resistivity of the brine-saturated rock and the brine resistivity.
It is only valid for ‘clean’, clay-free rocks. If clay is present, the interface of the clay mineral with its cation exchange capacity and the formation water
creates an additional conductivity component.
A lot of work was carried out concerning these electrical properties of ‘shaly sands’ and different types of models as well as correlations were
presented (Patchett and Herrick, 1982). Many authors developed equations that include such second conductivity parts especially for shaly sands in
order to achieve a precise water saturation calculation. An overview is given by Worthington (1985). Newer approaches are found in the papers by Revil and Glover (e.g., Revil and
Glover, 1997, Revil and Glover, 1998, Revil et al., 1998, Revil et al., 1999, Walker and Glover, 2010, Glover and Walker, 2009 or Glover et al., 2000). Pape et al., (1987)
showed that for a rough pore surface, fractal geometry has to be taken into consideration for calculating the specific internal surface controlling interface conductivity also in
the case of sandstone.
Interface conductivity can be derived from measurements with different brine salinities and/or measurements of the complex electrical resistivity at different
frequencies (Börner et al., 1996, Börner and Schön, 1995, Börner and Schön, 1991).
All interface effects result in a modification of Archie’s equation with various approaches, here Glover (2009) and Glover (2010) should be cited, with a generalized Archie’s
law for 2 and n phases.
Sen et al., (1988) presented a nonlinear correlation of rock conductivity and brine conductivity for sandstones bearing clay at low and high salinities.
In 2012, Kavian et al., presented a modified resistivity model, resulting from measurements at different frequencies, saturations and salinities. They performed also two-electrode
laboratory measurements of the resistivity on unconsolidated sand packs and then adopted an empirical model for modifying a physical model, which can further describe all data
as a function of frequency and water saturation with nine parameters