A New CEC-Measurement Proxy Using High-Frequency Dielectric Analysis of Crushed Rock

Measuring the cation exchange capacity (CEC) of clay-bearing rocks is a useful tool to estimate smectite content, or amount of swelling clay in the rock, and is referenced in many aspects of oil and gas exploration. Measuring the CEC of a rock, however, is laborious and depending on the method used requires saturation and extraction steps, the use of multiple chemicals, titration, and spectroscopic analysis. This study builds on the established petrophysical link between clays and relative permittivity (ɛ'_r) and outlines a workflow and set of equations that allow for bulk rock CEC to be calculated from permittivity measurements of crushed rock using a handheld dielectric probe. A series of quartz-smectite mineral mixtures were prepared and high-frequency (80 MHz to 1.4 GHz) dielectric measurements collected at six relative humidity (RH) conditions ranging from 8 to 75%. For each RH data set, a strong linear relationship (R² ≥ 0.98) exists between permittivity values at 120 MHz and the laboratory-measured CEC of the mineral mixtures. The equations from these calibration curves were used to derive three RH-dependent equations and were the basis for developing a relationship between RH, ɛ'_r, and CEC. The method was validated on a variety of crushed sedimentary rocks and differences between the calculated values from this study and the laboratory-measured CEC values range ± 6 meq/100g. These results demonstrate that dielectric permittivity measurements on crushed rock can be used as a CEC-proxy and is a fast and flexible alternative to laboratory-based CEC analysis.