Measurement of Cation Exchange Capacity- A Laboratory Comparative Study

Cation exchange capacity (CEC) plays a critical role in evaluating shaly formations, as it directly influences the contribution of clay minerals and clay-bound water to formation electrical conductivity. There are several laboratory methods currently available to determine CEC and results from these methods are often used interchangeably. This study aims to evaluate and compare the results of those methods, summarize their advantages and limitations, and provide guidelines for laboratory CEC evaluations. Methods of measuring CEC evaluated in this study are multi-salinity Co (core conductivity) and Cw (water conductivity), wet chemistry methylene blue dye test (MBT), and XRD mineralogy based synthetic CEC. The use of a new method based on Inductively Coupled Plasma (ICP) mass spectrometry is also explored. Based on XRD mineralogy, twelve clastic and four carbonate cores were selected for this study. The Co-Cw method measures the conductivity of samples saturated with brines of varying salinities and CEC is determined by analyzing the relationship between Co and Cw. In the MBT method, the adsorption capacity of solids using a methylene blue dye solution is measured and linked to sample CEC. The XRD mineralogy based synthetic CEC is simply a summation of CECs of individual clay minerals by using a mixing law. The ICP method chemically extracts cations from minerals using ammonium acetate, followed by measuring the concentrations of the extracted Na+, Mg++, K+, and Ca++ ions to determine CEC. Experimental results show that the CEC values determined by MBT and XRD correlate well, with a correlation coefficient R2 of about 0.85. This is important since XRD mineralogy is commonly acquired, thus abundantly available for reservoir evaluation, so that with XRD clay mineralogy, MBT equivalent CEC can be derived. Differences in CEC values obtained from using the various methods were observed and attributed to sample preparation and the type of samples tested, i.e., core plugs for Co-Cow vs crushed rock powders for MBT, ICP, and XRD methods. Sample preparation is especially important for the methods that use powder samples. Obviously, CEC from Co-Cw method is more formation representative due to the sample retaining its original distribution of minerals within the rock pore space, but the test needs to be repeated with multiple brine salinities, thus it is time consuming, especially for samples with high clay content; those rock samples with low permeabilities. Proper integration between powder and plug measurements would require additional data such as petrology to characterize the distribution of minerals. The effects of cations extracted from non-clay minerals on ICP tests are also being explored briefly. This study captures key insights into both the advantages and limitations of the CEC measurement techniques which should be valuable in quality control of CEC data for evaluating shaly formations.