Downhole Characterization of Clays and Formation Water Salinity Using Low Frequency Permittivity Dispersion

Abstract

AbstractClays in reservoir rocks have a significant impact on formation evaluation. An important property of clay minerals is their ability to adsorb ions on their exposed surface, which is measured by its cation exchange capacity (CEC). This property can affect saturation calculation if not properly accounted for. Many techniques have been developed for clay characterization, but it remains a focus of frontline research to find an accurate method for quantifying clays in situ and is the main objective of this study. Extensive numerical studies have been conducted and the results have indicated that for a shaly sand formation, strong dielectric dispersions were observed for all the simulated models at frequencies below 1MHz, which allowed an equation to be established that can be used to calculate clay volume through derivatives of the permittivity dispersive curves. In deriving this equation, parameters used for the studies include water-filled porosity, formation water salinity, grain size, clay type and volume, and Archie cementation exponent.The developed model contains two calibration constants, a and b, which need to be predefined. Results indicate that, for the simulated models, b is a constant while a depends on water-filled porosity, formation water salinity, grain size, clay content, and Archie cementation exponent. Data shows that there is no simple way to define constant a, due to its complex relationship with the various parameters. When focusing on relationships between constant a and clay volume and formation water salinity, by assuming other parameters can be estimated either by conventional petrophysical interpretations or laboratory measurements, then the current developed workflows can be used to estimate either clay volume or formation water salinity: Workflow 1: knowing formation water salinity, computing constant a, then estimating clay volume and Workflow 2: knowing clay volume, computing constant a, then estimating formation water salinity. To our knowledge, such an equation is the first model that has never been published before. The correlation works well in the synthetic data. It has a potential to provide a new method for characterizing formation clays and water salinity from low frequency resistivity measurements such as induction logs that is routinely run in almost every well drilled. Thus, it provides a powerful economical tool for enhanced reservoir characterization. Field testing is being conducted.