Modelling the induced polarization of bentonite-sand mixtures

International audienceSpectral induced polarization (SIP) has become an increasingly popular geophysical method for hydrogeological and environmental applications. These applications include for instance the non-intrusive characterization of the textural and interfacial physicochemical properties of bentonites used as permeability barriers in landfills or to store various types of contaminants including radioactive wastes. Bentonites are mainly constituted of smectites, which have very high specific surface areas (SSA) and cation exchange capacities (CEC). Therefore, these minerals have very high electromigration and polarization current densities responsible for very high in phase and quadrature conductivities, respectively. In addition, in diluted water, the diffuse layer of smectites occupies a large fraction of the pore space and may be therefore considered as part of the pore space. In our approach [1], complex electrical conductivities of saturated unconsolidated bentonite and bentonite-sand mixtures are modeled at different salinities (NaCl) of the bulk pore water using a Donnan equilibrium model coupled to the revisited SIP model of Leroy and Revil [2]. Our complex surface conductivity model considers the DC contribution of the diffuse and Stern layers as well as the electrochemical polarization of the Stern layer coating the grains with different sizes. The macroscopic SIP model is based on the differential effective medium theory and considers the complex surface conductivity of the sand and smectite grains and the complex conductivity of the pore space. In our model, the diffuse layer of quartz sands occupies a small fraction of the pore space and is considered therefore as part of the surface of the grains. Our SIP model predicts very well the low frequency (0.1 Hz - kHz) complex electrical conductivities of bentonite and bentonite-sand mixtures, except for very low frequencies (< 0.1 Hz) where membrane polarization may occur (Figure 1). The in phase conductivity of the sample with a high clay content (20 % in volume) increases slowly with salinity because of the very high DC surface conductivity of smectite. The observed large increase of the in phase and quadrature conductivity of the samples with the clay content (1, 20 and 100% in volume) is also predicted by our model. The quadrature conductivity of the samples with a high clay content is fairly independent on the pore fluid salinity because it is strongly connected with the SSA, CEC and Stern layer of smectite (Figure 1). The in phase conductivity of the sample with a low clay content (1% in volume) increases quickly with the salinity because of its low DC surface conductivity. Its quadrature conductivity also increases quickly with salinity because of the formation of the Stern layer at the surface of quartz sand. Nevertheless, our SIP model can't predict the quadrature conductivity spectra observed at very low frequencies (< 10-1 Hz). The missing polarization mechanism may correspond to membrane polarization and there is an effort to be done to incorporate this contribution in a unified model

Induced polarization of clay-sand mixtures: experiments and modelling

Frequency-domain induced polarization (IP) measurements consist of imposing an alternative sinusoidal electrical current (AC) at a given frequency and measuring the resulting electrical potential difference between two other non-polarizing electrodes. The magnitude of the conductivity and the phase lag between the current and the difference of potential can be expressed into a complex conductivity with the in-phase representing electromigration and a quadrature conductivity representing the reversible storage of electrical charges (capacitive effect) of the porous material. Induced polarization has become an increasingly popular geophysical method for hydrogeological and environmental applications [1]. These applications include for instance the characterization of clay materials used as permeability barriers in landfills or to contain various types of contaminants including radioactive wastes [2]. The goal of our study is to get a better understanding of the influence of the clay content, clay mineralogy, and pore water salinity upon complex conductivity measurements of saturated clay-sand mixtures in the frequency range ~ 1 mHz-12 kHz. The complex conductivity of saturated unconsolidated sand-clay mixtures was experimentally investigated using two types of clay minerals, kaolinite and smectite in the frequency range 1.4 mHz - 12 kHz. Four different types of sample were used, two containing mainly kaolinite (80% of the mass, the remaining containing 15% of smectite and 5% of illite/muscovite; 95% of kaolinite, 5% of illite/muscovite), and the two others containing mainly Na-smectite or Na-Ca-smectite (95% of the mass; bentonite). The experiments were performed with various clay contents (1, 5, 20, and 100 % in volume of the sand-clay mixture) and salinities (distilled water, 0.1 g/L, 1 g/L, and 10 g/L NaCl solution). In total, 44 saturated clay or clay-sand mixtures were prepared. Induced polarization measurements were performed with a cylindrical four-electrode sample-holder (cylinder made of PVC with 30 cm in length and 19 cm in diameter) associated with a SIP-Fuchs II impedance meter and non-polarizing Cu/CuSO4 electrodes (Figure 1). These electrodes were installed at 10 cm from the base of the sample holder and regularly spaced (each 90 degree). The results illustrate the strong impact of the Cationic Exchange Capacity (CEC) of the clay minerals upon the complex conductivity. The amplitude of the in-phase conductivity of the kaolinite-clay samples is strongly dependent to saturating fluid salinity (Figure 2) for all volumetric clay fractions, whereas the in-phase conductivity of the smectite-clay samples is quite independent on the salinity, except at the low clay content (5% and 1% of clay in volume). This is due to the strong and constant surface conductivity of smectite associated with its very high CEC. The quadrature conductivity increases steadily with the CEC and the clay content. We observe that the dependence on frequency of the quadrature conductivity of sand-kaolinite mixtures is more important than for sand-bentonite mixtures. For both types of clay, the quadrature conductivity seems to be fairly independent on the pore fluid salinity (Figure 2) except at very low clay contents (1% kaolinite-clay in volume). This is due to the constant surface site density of Na counter-ions in the Stern layer of clay materials [3]. At the lowest clay content (1%), the magnitude of the quadrature conductivity increases with the salinity, as expected for silica sands. In this case, the surface site density of Na counter-ions in the Stern layer increases with salinity [4]. The experimental data show good agreement with predicted values given by our Spectral Induced Polarization (SIP) model [4]. This complex conductivity model considers the electrochemical polarization of the Stern layer coating the clay particles and the Maxwell-Wagner polarization. We use the differential effective medium theory to calculate the complex conductivity of the porous medium constituted of the grains and the electrolyte. The SIP model includes also the effect of the grain size distribution upon the complex conductivity spectra. Interfacial parameters are estimated using the TLM of Sverjensky [5] for silica and the Donnan model of Tournassat and Appelo [6] for smectite

Étude par couplage CIN-MEB de l'influence de la microstructure sur les déformations par dessiccation de l'argilite de Tournemire

La relation entre les déformations par dessiccation de l’argilite de Tournemire et les propriétés texturales de la roche a été étudiée par un nouveau montage expérimental. Celui-ci se déroule en deux phases : la première utilise la méthode de corrélation d’images numériques (CIN) et la seconde la microscopie électronique à balayage (MEB). L’intérêt de cette manipulation est de pouvoir quantifier l’influence de la microstructure de l’argilite sur des déformations provoquées par des variations d’humidités. Dans une enceinte étanche, un échantillon provenant du forage FD90 du tunnel de Tournemire a été soumis à cycle de saturation-désaturation entre 98 et 33% d’humidité (98-33-98-33). L’échantillon se présente sous la forme d’un cube de 20x20x20mm3 et est filmé par deux caméras. La première caméra filme un champ large sur une face entière (25x20mm²) tandis que la seconde filme un champ zoomé (5.5x4.1mm²) sur une autre face. Le champ zoomé est ensuite analysé au MEB afin d’établir une cartographie des minéraux en électrons rétrodiffusés, qui sera comparée aux champs de déformations obtenus par CIN à partir des images des caméras. Ce montage expérimental permet d’étudier les champs de déformations dans deux directions (approximation du phénomène en 3D) à deux échelles différentes (millimétrique à micrométrique) avec la microstructure et la teneur en eau de la roche

Flights and landings in the existential windows of Environmental Education

[Resumo] Este artigo é um ensaio vivido na existência eloquente num momento de pandemia. Das vagas horas de solidão, do adiamento do abraço e por vezes do desespero em testemunhar a ruína planetária perante o desastre que a própria humanidade criou, acreditando que o capitalismo fosse a grande solução. Para além do mercado, as vidas não têm preço, independentemente das cores que possuam. E é enorme a contribuição da educação ambiental à ressignificação de uma nova humanidade[Abstract] This article is an essay lived in eloquent existence in a pandemic moment. From the vague hours of loneliness, the postponement of the embrace and sometimes the desperation to witness the planetary ruin in the face of the disaster that humanity itself created, believing that capitalism was the great solution. Much more than market, lives are priceless, regardless of the colours they have. And the contribution of environmental education to the resignification of a new humanity is enormous

Indirect determination of soil water content

This manuscript reviews the recent developments in indirect methods used to measure soil watent content (SWC). It focuses on three types of methodological approaches: (i) high-frequency electromagnetic methodologies (dielectric methods and Ground Penetrating Radar), (ii) low-frequency electromagnetic methods (resistivity method and spectral induced polarization-SIP) and (iii) emerging methods (laboratory and ground-based nuclear magnetic resonance-NMR techniques and methods based on fiber optic temperature sensing systems). For each method the physical principles and the corresponding methodology are briefly described first, followed by the recent methodological advances. These recent advances address the following issues: (a) integration of different types of sensors to design new multi-functional devices, (b) miniaturization of low-cost and easily-installed electromagnetic sensors, (c) fusion and assimilation of data acquired from multiple modalities with different sampling rates and spatial resolutions, (d) physical understanding of spectra and distributed parameters measured by SIP and NMR methods, (e) improvement of geophysical inversion techniques combined with fast data acquisitions (4D monitoring of SWC)

Sur les couplages entre comportement mécanique et processus de transfert de masse dans le sel gemme

The results obtained from an in situ test at the Alsace potash mines (MDPA) show that rock salt is permeable to gas and brine, even far from underground structures (four radii from the gallery). Unlike gas (nitrogen) volume flow rates, brine injection flow rates are fairly well reproduced by Darcy's law. In addition, after percolation of brine into the massif, the effect of capillary pressure reduces the gas injection rate by an order of magnitude. In triaxial tests carried out under high confinement pressures of up to sixty MPa, the MDPA salt samples in direct contact with the confining liquid (oil and brine) showed earlier damage and lower compressive strength than the lined samples. These effects induced by the permeability of the salt are accentuated when the confining liquid is brine. In terms of modelling, a macroscopic theoretical framework is proposed and used to discuss the role of the dissolution/crystallisation phenomenon of the brine trapped in the interstices. We have been able to establish that the clogging process accompanied by viscoplastic deformation of the grains can explain the impermeability of rock salt over geological time scales. In terms of the safety of storage in saline formations, we have been able to show that very low permeability, although not measurable by current permeametry devices, is likely to have a significant influence on the long-term evolution of storage.Les résultats obtenus a partir d'un essai in situ aux mines de potasse d'alsace (MDPA) permettent d'affirmer que le sel gemme est perméable au gaz et a la saumure, même loin des ouvrages souterrains (quatre rayons de la galerie). Contrairement aux débits volumiques de gaz (azote), les débits d'injection de saumure sont assez bien reproduits par la loi de darcy. En outre, après percolation de saumure dans le massif, l'effet de la pression capillaire fait diminuer d'un ordre de grandeur le débit d'injection de gaz. Au cours d'essais triaxiaux réalises sous des pressions de confinement élevées jusqu'à soixante MPa, les échantillons de sel des MDPA en contact direct avec le liquide de confinement (pétrole et saumure), montrent un endommagement plus précoce et une résistance a la compression plus faible que les échantillons gaines. Ces effets induits par la perméabilité du sel sont accentués lorsque le liquide de confinement est la saumure. Sur le plan de la modélisation, un cadre théorique macroscopique est proposé et utilise pour discuter le rôle du phénomène de dissolution/cristallisation de la saumure piégée dans les interstices. Nous avons pu établir que le processus de colmatage accompagne d'une déformation viscoplastique des grains peut expliquer l'imperméabilité du sel gemme sur des échelles de temps géologiques. En matière de sureté des stockages en formations salines, nous avons pu montrer qu'une très faible perméabilité, bien que non mesurable par les dispositifs de perméamétrie actuels, est de nature à influencer de manière sensible l'évolution à long terme du stockage

Comment on ''Generalized effective-medium theory of induced polarization'' (Michael Zhdanov, 2008, GEOPHYSICS, 73, F197-F211)

International audienceZhdanov (2008) developed recently a self-consistent effective-medium approach to model induced polarization of porous media with the goal to "provide a unified mathematical model of heterogeneity, multiphase structure, and the polarizability of rocks" (Zhdanov, 2008, p. F197). In this discussion paper, we claim that whereas the work of Zhdanov is mathematically correct, it does not incorporate most of the fundamental mechanisms involved in induced polarization. In short, his model accounts for a special form of polarization mechanism (e.g., associated with the discontinuity of the electrical current displacement at the interface between different phases or electron transfer at the electrolyte/metallic particle interface in the linear asymptotic limit of the Butler-Volmer equation; see Bockris and Reddy, 1970)

Numéro spécial « Microstructure des matériaux argileux − conséquences pour l’ingénieur ». Texte introductif par Pierre Delage et Philippe Cosenza

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