A new method for quantifying cation exchange capacity: Method verification and application to organic-rich Mudrock formations

In this paper, a new method is proposed to quantify cation exchange capacity (CEC) in organic-rich mudrocks with complex mineralogy and in the presence of multiple types of clay minerals by integrating nitrogen adsorption-desorption, nuclear magnetic resonance (NMR), pyrolysis, and X-ray diffraction (XRD) measurements (including quantitative XRD). The CEC estimates from the new method were compared against CEC estimates from the wet chemistry method in powdered organic-rich mudrock samples and the membrane potential method in intact mudrock samples. The estimated CEC from the new method agreed with those obtained from the membrane potential method, with average relative error of less than 10%. The CEC estimates from these two methods fell within the upper and lower bounds of CEC estimates, calculated based on the reported data for pure clay minerals, but the results the wet chemistry method showed significant overestimates of the CEC evaluation. Sensitivity analysis was also carried out to evaluate the influence of CEC estimates from different methods on assessment of water saturation and demonstrated the importance of CEC evaluation and the impact of the new method on enhanced formation evaluation.