Experimental features of cation exchange capacity determination in organic-rich mudstones

Abstract

To the petroleum industry, the cation exchange capacity (CEC) is one of the critical petrophysical parameters of the rock-gas-fluid system and the response of well logging tools. The widely used laboratory protocols for CEC determination (wet chemistry methods) came mainly from soil science, but with slight or even without modifications are being applied to organic-rich mudstones (shales). However, using such laboratory practice leads to large CEC errors in the presence of carbonate and gypsum minerals, high-salinity pore water, etc.The paper attempts to find optimal in terms of accuracy and reliability methods for measuring both CEC and exchangeable cations concentrations of organic-rich mudstones, along with sample treatment procedures including drying, extraction, and fraction size due to crushing. The paper presents a CEC data set obtained using both hexaamminecobalt(III) chloride (CoHex) and modified (NH4Cl)Alc methods for a representative collection of core samples of the Bazhenov shale formation (West Siberia, Russian Federation). The results show a) CEC growth inversely versus fraction size, and b) stable CEC measurements within fraction size 0.25–1.0 mm. The BET specific surface area (SSA) spans in the range of 0.25–6 m2/g. SSA follows the CEC trend in terms of fraction size with a growth ratio up to 3 times. The modified (NH4Cl)Alc method outperforms the CoHex method and reliably determines the concentrations of the exchangeable cations for a carbonate content of more than 5 wt%. Both direct [Co(NH3)6]3+ measurements in the CoHex method and the sum of exchangeable cation concentrations in the modified (NH4Cl)Alc method provide a fair estimate of the total CEC. Careful sample preparation, including drying or solvent extraction, is required for accurate CEC determinations. The measured CEC for the target rock samples varies from 2.87 to 6.43 cmol/kg and driven by the total clay content.