USE OF CEC-DEPENDENT CEMENTATION AND SATURATION EXPONENTS IN SHALY SAND RESISTIVITY MODELS

摘要

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.