Cation exchange capacity (Q_v) estimation in shaly sand reservoirs: case studies in the Junggar Basin, northwest China

Cation exchange capacity (Q_v) is a key parameter in resistivity-based water saturation models of shaly sand reservoirs, and the accuracy of Q_v calculation is crucial to the prediction of saturations of oil and gas. In this study, a theoretical expression of Q_v in terms of shaly sand permeability (K_shaly-sand), total porosity (ϕ_t), and salinity of formation water (S) is deduced based on the capillary tube model and the physics volume model. Meanwhile, the classical Schlumberger–Doll research (SDR) model has been introduced to estimate K_shaly-sand. On this basis, a novel technique to estimate Q_v from nuclear magnetic resonance (NMR) logs is proposed, and the corresponding model is also established, whose model parameters are calibrated by laboratory Q_v and NMR measurements of 15 core samples from the Toutunhe formation of the Junggar Basin, northwest China. Based on the experimental data sets, this technique can be extended to reservoir conditions to estimate continuous Q_v along the intervals. The processing results of field examples illustrate that the Q_v calculated from field NMR logs are consistent with the analyzed results, with the absolute errors within the scope of  ±0.1 mmol cm⁻³ for the majority of core samples. (paper)