Abstract Understanding streaming potential generation in porous media is of high interest for hydrological and reservoir studies as it allows to relate water fluxes to measurable electrical potential distributions. This streaming potential generation results from an electrokinetic coupling due to the presence of an electrical double layer developing at the interface between minerals and pore water. Therefore, the pore sizes of the porous medium are expected to play an important role in the streaming potential generation. In this work we use 2‐D pore network simulations to study the effect of the pore size distribution upon this electrokinetic mechanism. Our simulations allow a detailed study of the influence of a large range of permeabilities (from 10 −16 to 10 −10 m 2 ) for different ionic concentrations (from 10 −4 to 1 mol/L). We then use and compare two different approaches that have been used over the last decades to model and interpret the streaming potential generation: the classical coupling coefficient or the effective excess charge density, which has been defined recently. Our results show that the four pore size distributions tested in the present work have a restricted influence on the coupling coefficient for ionic concentration smaller than 10 −3 mol/L while it completely drives the behavior of the effective excess charge density over orders of magnitude. Then, we use these simulation results to test an analytical model based on a fractal pore size distributions. This model predicts well the effective excess charge density for all pore size distributions under the thin double layer assumption.
Summary Clays are very abundant minerals in the Earth's crust and express a high conductivity response that can be observed at the scale of the geological formation by electrical and electromagnetic methods. However, these minerals have a complex microstructure that renders difficult the quantitative petrophysical interpretation of the electrical field measurements. In this study, we developed a new approach to interpret spectral induced polarisation (SIP) signals measured on clay materials in terms of microstructural and electrical double layer (EDL) properties, including surface conductivity, using the physical model recently developed by Leroy et al. (2024). With a restrained set of optimised physical-chemical parameters, i.e., the fraction of the counter-charge and cation effective ion mobility in the Stern layer, number of stacked sheets per montmorillonite particle, and clay aggregate effective shape and size distribution, our model well reproduces the measured laboratory SIP spectra in the mHz to kHz frequency range on kaolinite, illite and montmorillonite muds at different NaCl concentrations. Our results suggest that most of the EDL counter-charge controlling SIP spectra is located in the Stern layer on the basal surfaces. First Archie's law explains the in-phase conductivity measurements provided that the contribution of the diffuse layer to water conductivity is properly included. The ratio of imaginary surface conductivity to real surface conductivity is highly dependent on frequency and slightly dependent on salinity. Finally the measured quadrature conductivity is proportional but not necessarily equal to the imaginary surface conductivity. Our study is a step forward to better understand the complex electrical conductivity of clays.
Macro-scale transport properties (e.g., electrical conductivity, effective excess charge density and hydraulic conductivity) can be conceptualized as capillary bundle models, in which the pore structure of porous medium is viewed as a bundle of capillary tubes of varying sizes. This approach can be used to understand and address the relationship between the petrophysical properties and the geometry of soil phases. When the temperature of porous medium decreases below the freezing temperature, the soil physical properties (transport properties) change drastically. This is attributed to the complexity of the heterogeneous formation of ice in the porous medium. Therefore, understanding better pore ice formation from microscale insights is crucial to describe the evolution of electrical conductivity with temperature in frozen porous medium. In this study, we consider that capillary radius and tortuous length follow fractal distributions, and that total conductance at the microscale scale is determined by the Gibbs-Thomson and Young-Laplace effects as well as by the surface complexation model. A new capillary bundle model is then proposed using an upscaling procedure, which considers the effects of both bulk and surface conductions. Based primarily on an electrical resistance apparatus and the NMR method, a series of laboratory experiments are carried out to study the influence of initial water saturation and salinity on electrical conductivity under unfrozen and frozen conditions. Additionally, the rationality and validity of the proposed model were successfully verified with published data in the literature and experimental data of this study. Our new physically-based model for electrical conductivity opens up new possibilities to interpret electrical and electromagnetic monitoring to easily infer changes in key variables such as liquid water content and moisture gradients.
SUMMARY Clays are ubiquitously located in the Earth’s near surface and have a high impact on the subsurface permeability. Most geo-electrical characterizations of clays do not take into account the heterogeneous nature of clay geological media. We want to better understand the influence of heterogeneities on the geo-electrical signature, thus we collected a data set of spectral induced polarization (SIP) of artificial heterogeneous non-consolidated clay samples. The samples are made of illite and red montmorillonite in a parallel and perpendicular disposition (with respect to the applied electric field). Another sample is a homogeneous mixture composed of the same volumetric fraction of illite and red montmorillonite. For all the samples, the polarization is dominated by the red montmorillonite, given by the shape of the spectra (presence or lack of a peak at a particular frequency). We compared the experimental data with classical mixing laws and complex conductance network models to test how to better predict the SIP signature of such mixtures when the SIP spectra of the two components are known. The real conductivity is better predicted by the mixing laws, but the shape of the spectra (presence of polarization peaks at particular frequencies) is best predicted by the conductance network models. This study is a step forward towards a better characterization of heterogeneous clay systems using SIP.
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Solid Earth. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v2]Spectral induced polarization characterization of non-consolidated clays for varying salinities - an experimental studyAuthorsAidaMendietaiDDamienJougnotiDPhilippeLeroyAlexisMaineultSee all authors Aida MendietaiDCorresponding Author• Submitting AuthorSorbonne UniversitéiDhttps://orcid.org/0000-0002-8396-3237view email addressThe email was not providedcopy email addressDamien JougnotiDSorbonne UniversitéiDhttps://orcid.org/0000-0003-4950-5766view email addressThe email was not providedcopy email addressPhilippe LeroyBRGMview email addressThe email was not providedcopy email addressAlexis MaineultSorbonne Universitéview email addressThe email was not providedcopy email address
This dataset contains measurements of spectral induced polarization from mixtures of a natural soil and biochar. The mixtures were done at different biochar contents (wt. %): 0%, 0.1%, 1%, 5%, and 10%. There is a README file explaining the structure of the files.
