AAPG Annual Convention and Exhibition

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Evaluation of Clay Conductivity From CEC Measurement Using Destructive and Non-Destructive Techniques

Abstract

CEC (cation exchange capacity) is an inherent property of clay due to the negatively charged sites on its surface. This is an important parameter in determining the clay conductivity, which is used for the evaluation of water and oil saturations that can significantly affect well logging data. The accurate determination of CEC is required to characterize and understand shale reservoirs. Three different techniques are discussed, methylene blue absorption, conductometric titration, and membrane potential measurements. The methylene blue technique measures the change in color of the methylene blue solution due to the adsorption of methylene blue cations on the clay surface using absorbance colorimetry techniques. This technique is the simplest experimentally. The conductometric titration technique measures the amount of Mg ions required to replace all the Ba ions in the Ba-saturated clay by monitoring conductivity during titration. It takes longer than the methylene blue technique, but directly measures the replacing of exchange ions on the clay surface. The previous two techniques require ground samples and are destructive. The main advantage of the membrane potential technique is that the CEC of an intact sample is measured. The sample, which plays the role of a membrane, is placed between two brines of different concentrations. The difference in ionic concentration between the two fluids generates an emf. Clay minerals have an excess of negatively charged sites that lower the activity of negative ions and therefore suppress the magnitude of this emf. The CEC is determined from the magnitude of the reduction of the measured potential. We will discuss the relative merits of these techniques and make recommendations for their use.