Saturation interpretations of shaly sand formations from conventional resistivity logs are known to be affected by the presence of clay minerals. As a result, saturation models that account for clay will require the value of the formation cation exchange capacity (CEC), which is obtained from core measurement or inferred indirectly from the lithological interpretation. Multifrequency dielectric logging signals are sensitive to the CEC of the formation and enable acquiring continuous logs of the CEC, water-filled porosity, water salinity, and a texture exponent associated with the water phase tortuosity (similar to the Archie m cementation parameter in the case of fully water-saturated rocks). Even if dielectric tools often measure the electromagnetic response in the flushed zone, the resulting CEC and texture parameters can be used as inputs to the saturation interpretation in the virgin zone. With this broader application in mind, it is prudent to understand the conditions in which dielectric measurements and their interpretation can reliably provide these petrophysical parameters. In addition, because the clay effect is, for simplicity, often ignored in dielectric interpretations, particularly in formations with a high brine salinity, it is crucial for the accuracy of petrophysical interpretations to determine whether the clay effect can be neglected in a given environment. Hence, the goal of this study is to address when the CEC affects the dielectric response and when it can be reliably obtained from dielectric logs.
Our study is based on the recently established dielectric shaly sand model (Freed et al., 2016, 2018) by performing a self-consistent sensitivity study to determine how reliably the CEC can be obtained from dielectric measurements. Together with analytic arguments, we found that the sensitivity of the dielectric signals to the CEC depends on both the formation factor and the brine salinity of the formation. As a result, the minimum value of formation CEC, in units of meq/(100 g), that can be reliably inverted for is identified for different values of these two parameters. For practical use, a simple criterion, which involves comparing the formation conductivity and the CEC value, was derived to provide guidance on whether the dielectric measurement is sensitive to the formation CEC. In contrast to common belief, the dielectric interpretation can be substantially affected by the presence of clay in the high brine salinity region, even though one cannot simultaneously invert for both the water salinity and the water-filled porosity in such cases. Finally, we applied the derived criterion to log data and demonstrate how our findings help to identify regions where properly accounting for the clay effect becomes essential.
