A Physics-Based Model for the Dielectric Response of Shaly Sands and Continuous CEC Logging
- Denise E. Freed (Schlumberger-Doll Research) | Nikita Seleznev (Schlumberger-Doll Research) | Chang-Yu Hou (Schlumberger-Doll Research) | Kamilla Fellah (Schlumberger-Doll Research) | Jeffrey Little (Schlumberger Data Services) | Gabriel Dumy (ESPCI Paris) | Pabitra Sen
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- June 2018
- Document Type
- Journal Paper
- 354 - 372
- 2018. Society of Petrophysicists & Well Log Analysts
- 3 in the last 30 days
- 160 since 2007
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Determining hydrocarbon content from conventional resistivity measurements in freshwater shaly sands formations is challenging because the resistivity depends on the clay formation’s cation exchange capacity (CEC). Because the CEC value depends on clay type and can vary significantly, continuous and direct logging of the formation CEC will benefit resistivity log interpretation in shaly sands. Multifrequency dielectric measurements are sensitive to the CEC of the formation, as well as to the water-filled porosity, water salinity, and texture. We introduce a new physics-based model for the dielectric response of shaly sands in the frequency range of 20 MHz to 1 GHz. To take into account the effect of the CEC, we derive the polarization of the clay particle’s double layer from first principles. The new model uses a minimal number of parameters to describe the essential macroscopic properties of shaly sands and reduces back to a widely accepted dielectric model when the effect of the formation CEC on the dielectric response is negligible.
To validate the model, we present inversion results for dielectric measurements from both core and log data. The new model provides reliable inversion results for the CEC in freshwater formations, water-filled porosity, salinity, and a water-phase tortuosity exponent, which, for fully water-saturated rocks, is analogous to the Archie m parameter. In addition, the low-frequency invaded zone resistivity Rxo can also be predicted based on the inverted parameters.
Dielectric logging tools have been used for over 30 years to distinguish fresh water from oil. The large contrast between the water permittivity and the oil and rock-matrix permittivity makes it possible to determine the water-filled porosity in situations where it would be difficult to do so from resistivity logs. These situations include freshwater formations and formations where the water salinity, the cementation exponent, or the saturation exponent is unknown.
|File Size||3 MB||Number of Pages||19|