A PHYSICS-BASED MODEL FOR THE DIELECTRIC RESPONSE OF SHALY SANDS

摘要

Much of the world’s oil is currently produced from shaly sand deposits. One of the primary goals of formation evaluation is to determine the hydrocarbon content of the reservoir. Interpretation of conventional resistivity measurements becomes challenging in fresh-water shaly sands where the formation resistivity is significantly influenced by clays. Determination of the formation hydrocarbon content in these reservoirs requires knowledge of the formation cation exchange capacity (CEC), or charge, on the clay, which can vary significantly depending on clay type and content. At present, the CEC is derived from core measurements or indirectly inferred. CEC measurements on core are difficult to make, are time-consuming, and provide data at only a few discrete depths. Resistivity log interpretation in shaly sands benefits from continuous and direct logging of the formation CEC.Multifrequency dielectric measurements are sensitive to the CEC of the formation, in addition 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. The formation CEC is fundamental to this model, and the polarization of the double layer of the clay particle has been developed 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 for clean formations, where the effect from the formation CEC on the dielectric response is negligible. The inversion of the multifrequency data with this model provides a continuous log ofCEC, water-filled porosity, salinity, and a water-phase tortuosity exponent, which, for fully water-saturated rocks, is analogous to the Archie m parameter.To validate the model, we present inversion results of the dielectric measurements from both core and log data. The new model provides reliable results for the determination of water-filled porosity, water salinity, water-phase tortuosity exponent, and CEC in fresh water formations. In particular, the value of the CEC from inversion agrees well with that from core measurements. The low-frequency invaded zone resistivity Rxo can also be predicted based on the inverted parameters, and agrees with independently measured shallow resistivity.