Researchers at Louisiana State University (LSU) have introduced several petrophysical models expressing the electric properties of shaly sands. These models, to be used for hydrocarbon detection, are based on the Waxman and Smits (W-S) concept of supplementing the water conductivity with clay counter-ions conductivity. The models also utilize the Dual Water (D-W) concept, which incorporates a free and a bound water each occupying a specific volume of the total pore space. However, unlike W-S and D-W models, the LSU models treat the counter-ion conductivity as that of a hypothetical sodium chloride solution. The latest version of the LSU models considers that the electric current follows two distinct paths, one through the free electrolyte and the other through the bound water. Hence, two different formation factors, expressed using two different cementation exponents, were used to express the free water and the bound water term. The two cementation exponents are estimated in the adjacent clean sand and pure shale. The results of extensive laboratory experiments, described in this paper, show that the use of two cementation exponents to characterize electrical behavior in shaly sand is substantially better than the conventional use of one cementation exponent value. In addition, the experimental results show that a correlation between saturation exponent value, n and CEC exists. Also, brine saturation of 15,000 ppm was found to be the upper limit of "low" salinity range in which extra care is needed for shaly sand evaluation.
The effect of clay mineral on electric conductivity of rocks which is represented by the cation exchange capacity (CEC) has been widely investigated based on core measurement (Waxman and Smits, 1968; clavier et al., 1984). CEC is a measure of both clay volume and clay type. In practice, quantifying this effect usually requires a CEC value determined from core measurements. Earlier researchers at LSU overcame this limitation by treating the counter-ion conductivity as that of a hypothetical sodium chloride solution (Silva and Bassiouni, 1985; Silva, 1986). The latest LSU model by Ipek and Bassiouni (2002) introduced the use of two cementation exponents. A cementation exponent (mf) in the free water term and another cementation exponent (mc) in the clay bound water term. The two exponents can be estimated from log data in clean sand and pure shale, respectively. The path of ions in free water and clay bound water is defined by the degree of the rock tortuosity. The cementation exponent, m of a formation is used to indicate its tortuosity (Archie, 1942). This model yielded a better estimation of water saturation in shaly sand reservoirs. However, the use of two formation resistivity factors lacks experimental validation. To further validate this model; sets of experimental work were conducted using actual reservoir rocks from a South Louisiana. In addition to the validation of the use of two cementation exponents, the experimental work will investigate the definition of the saturation exponent (n) in shaly sand evaluations. In clean formations this value is controlled by the distribution of the conducting brine in the pore space. Hence, it depends on the rock texture, wetting properties and saturation history caused by capillary effects (Schon, 1996) In shaly formations it is expected that n will also depends on the degree of shaliness and type of clay minerals.
