Water‐Filled Porosity 2D Imaging—Part 1: Theoretical Workflow

ABSTRACT Heterogeneous reservoirs, especially shaly formations, are challenging in reservoir characterization. Porosity and fluid distribution vary in both vertical and lateral directions. High‐resolution oil‐based mud (OBM) conductivity images are commonly acquired to help to reveal formation conductivity heterogeneities, azimuthally and vertically. With calibrated formation conductivity from the conductivity images, water‐filled porosity () 2D images may be obtained by using Archie type models, provided formation shaliness, water salinity and Archie parameters are given. In megahertz electromagnetic (EM) wave frequencies where the OBM imager is operating, porous rock interfacial polarization is very complex. Factors such as water‐filled porosity (), water salinity, water phase tortuosity () and the connection of the pores interact among themselves non‐linearly and all contribute to the overall responses of rock conductivity and dielectric permittivity. For example, for the same , higher will result in a decrease in rock conductivity. This can be confusing with a decrease in if rock dielectric permittivity is not taken into consideration. This non‐linear interaction among the rock parameters is not captured by the Archie model properly, especially in complex pore systems with high interfacial polarization such as in carbonate and shaly rock types. The latest generation of OBM imager can derive both conductivity and permittivity images. However, due to the lack of quantitative characterization, use of those images is greatly limited to relative image feature identification and extraction. In this article, for the first time, we provide a multi‐kernel electrical image quantification scheme and develop a workflow to derive and images, in 2D, independent of the Archie model. The workflow takes advantage of a high‐resolution pad‐based conductivity and permittivity imager, which operates at tens of megahertz, and a multifrequency dielectric tool, which operates at a wide range of frequencies ranging from tens of megahertz up to gigahertz. The OBM imager can provide high‐resolution conductivity and dielectric permittivity 2D images through advanced inversion processing, once the following challenges below are resolved: Understand rock responses at tens of megahertz and identify key constraint rock parameter configurations for deriving 2D images of and . Characterize the OBM imager processing bias and its impact on the dielectric rock model inversion. Evaluate compatibility between the OBM imager and the multifrequency dielectric tool data that provide the key rock parameters. To quantify the OBM imager processing, a multi‐kernel method is used on synthetic rocks covering a wide range of rock parameters. Exploratory data analysis is applied to better understand the relationships between rock parameters and OBM imager processing bias and its impact on dielectric rock model inversion. Uncertainty study is used to identify key rock parameters in deriving and images at the imager's frequency. The synthetic data results are further verified on the laboratory core plugs. This workflow is generic in clean formation and can apply to all the rock resistivity ranges, including resistivity below 1 ohm m.