Effect of Clay and Water Salinity on Electrochemical Behavior of Reservoir Rocks

Abstract

Published in Petroleum Transactions, Volume 207, 1956, pages 65–72. Abstract In quantitative interpretation of electrical logs the presence of clay minerals introduces an additional variable which further complicates an already complex problem. Although recognizing the difficulties introduced as a result of the heterogeneity of natural sediments and despite the present incomplete state of knowledge regarding electrochemical behavior of shades, disseminated clay minerals and concentrated electrolytes, it was felt that useful empirical correlations might be obtained from experimental investigation. Six typical sandstone formations, having a wide variety of petrophysical properties, were selected for the study. Approximately 45 samples from each formation were selected to satisfactorily represent the range of pore size distribution within the particular formation. As a matter of general interest, four limestone formations were also included in the investigation. Previously proposed equations relating to resistivity, SP and interrelationship of the two phenomena have, where possible, been tested with data obtained in this investigation. These equations do not satisfactorily describe experimental behavior of samples through all degrees of shaliness or throughout the range of brine solution resistivities normally encountered in logging practice. An empirical equation has been developed which quantitatively relates formation resistivity factor to saturating solution resistivity, porosity, and "effective clay content." This relation is indicated to be uniformly applicable to clean or shaly reservoir rocks. It is shown that both the SP and resistivity phenomena of shaly samples are related to the sample cation exchange capacity per unit pore volume. The independent chemical determination of this parameter is thus a means of determining the "effective clay content" of samples. Some implications regarding theory and electric log interpretation of shaly sands are discussed. Introduction The use of electrical resistivity logs as a means for estimating formation porosity is based upon the original work of Archie.