Ionic Double-Layer Conductivity in Reservoir Rock

The abnormal conductivity found in shaly reservoir rocks containing anelectrolyte is shown to be a consequence of the electrical double layer in thesolution adjacent to charged clay surfaces. This increased conductivity resultsfrom a higher concentration of ions in the double layer than in the solution inequilibrium with the double layer. It is shown that the magnitude of theincreased conductivity of a shaly reservoir material is influenced by theconcentration and type of ions in the equilibrium solution as well as by thecolloidal nature of the rock. Introduction An important factor in the quantitative interpretation of the electric logis the resistivity factor of reservoir rock. The resistivity factor is definedas the resistivity of the rock when completely saturated with an electrolytedivided by the resistivity of the electrolyte itself. In the normal case, theresistivity factor for a particular rock sample is independent of theresistivity of the electrolyte and reflects the pore geometry of the rock. In 1949, Patnode and Wyllie showed that in some cases the resistivity factoris not constant, but instead varies with the resistivity of the electrolyticsolution. It was shown that clean sands behave normally - i.e., that theresistivity factor does not vary with resistivity of the electrolyte - but thatsands con taining shale and clay may exhibit abnormally low resistivity factors when thesolution used to saturate them is of fairly high resistivity. At lowresistivities of the electrolyte, the resistivity factor for a shaly sandappeared to approach a normal behavior. It is evident that the resistivityfactor of a shaly sand is dependent upon factors other than pore geometry whenthe solution used to saturate it is of high resistivity. Patnode and Wyllie, and later de Witte, assumed that the conductivity of ashaly sand saturated with an electrolyte could be represented as the sum of twoquantities. One of these was assumed, in effect, to be the conductivity whichwould be expected if the sample were a clean sand, and the other was assumed tobe a conductivity inherent in the sample itself. The latter was supposed to beconstant for a given rock sample regardless of the solution used to saturateit. The source of this added conductivity was ascribed to "conductivesolids." In actual practice, the effect of the abnormal behavior of shaly sands is ofminor importance except when the resistivity of a formation such as a sand ishigh. Thus the effect is more important when the sand is saturated with adilute brine than when it contains a more concentrated electrolyte. The effectis also more important when the sample contains oil or gas than when itcontains only saline water. T.P. 3565