Summary
Because of their sensitivity to ionic content and surface texture, wide-band electromagnetic (WBEM) measurements of saturated rocks exhibit frequency dispersions of electrical conductivity and dielectric constant that are influenced by a variety of petrophysical properties. Factors as diverse as fluid saturation, porosity, pore morphology, thin wetting films, and electrically charged clays affect the WBEM response of rocks. Traditional dielectric mixing laws fail to quantitatively and practically integrate these factors to quantify petrophysical information from WBEM measurements. This paper advances a numerical proof of concept for useful petrophysical WBEM measurements. A comprehensive pore-scale numerical framework is introduced that incorporates explicit geometrical distributions of grains, fluids and clays constructed from core pictures, and that reproduces the WBEM saturated-rock response on the entire kHz-GHz frequency range. WBEM measurements are verified to be primarily sensitive (a) in the kHz range to clay amounts and wettability; (b) in the MHz range to pore morphology (i.e., connectivity and eccentricity), fluid distribution, salinity, and clay presence; and (c) in the GHz range to porosity, pore morphology and fluid saturation. Our simulations emphasize the need to measure dielectric dispersion in the entire frequency spectrum to capture the complexity of the different polarization effects. In particular, it is crucial to accurately quantify the phenomena occurring in the MHz range where pore connectivity effects are confounded with clay polarization and pore/grain shape effects usually considered in dielectric phenomena. These different sensitivities suggest a strong complementarity between WBEM and NMR measurements for improved assessments of pore-size distribution, hydraulic permeability, wettability, and fluid saturation.
Introduction
A number of experimental and theoretical studies suggest the measurable sensitivity of WBEM to various petrophysical factors, including porosity, brine salinity, fluid saturation and wettability, clay content, surface roughness, and even pore surface-to-volume ratio. Given the complexity of the different phenomena under consideration, practical models are designed to fit measured dielectric dispersions to ad-hoc models whose parameters are marginally supported by quantitative petrophysical concepts.
Therefore, to assess whether accurate and reliable petrophysical interpretations are possible with WBEM measurements requires an analysis that (a) incorporates pore structure, pore connectivity, multiphase saturation and electrochemical effects; and (b) quantifies the contributions of each factor in the measured WBEM dispersions. However, extracting explicit petrophysical information from WBEM responses is a difficult task. Myers (1991), for instance, illustrated the non-uniqueness of WBEM measurements when a decrease of water saturation, porosity, or brine salinity yielded similar responses. Recent advances in NMR logging and interpretation (Freedman et al. 1990) can eliminate some of these ambiguities with adequate experimental conditions, and if rock wettability is known. Conversely, WBEM measurements could provide independent wettability assessment in the cases where NMR measurements alone reach their limits of sensitivity [for instance, the impact of fluid saturation history on wettability determination was studied by Toumelin et al. (2006)]. Likewise, the interpretation of NMR measurements can be biased by unaccounted rock morphology (Ramakrishnan et al. 1999) or by internal magnetic fields in shaly or iron-rich sands (Zhang et al. 2003), whereas WBEM measurements provide independent information on overall rock morphology. It is therefore timely to consider integrating both technologies for improving petrophysical analysis.
The objectives of this paper are twofold:Review existing results on the extraction of petrophysical information from rock WBEM measurements, andestablish a proof of concept for the necessity to integrate electromagnetic measurements on the wide-frequency band from the kHz range to the GHz range, and study how WBEM techniques may yield petrophysical information unavailable from other in-situ measurements.
To reach the second objective, we introduce a generalized pore-scale simulation framework that allows incorporating arbitrary rock morphology and multiphase fluid distribution.
Influence of clay fraction on pore-scale properties and hydraulic conductivity of experimentally compacted mudstones