Structural modifications of Callovo-Oxfordian argillite under hydration/dehydration conditions (original) (raw)
Related papers
Irreversible deformation and damage in argillaceous rocks induced by wetting/drying
Journal of Applied Geophysics, 2014
Some irreversible phenomena of argillaceous rocks during wetting and drying processes are experimentally investigated at the micrometric scale, by combining environmental scanning electron microscope (ESEM) imaging and digital image correlation (DIC) techniques. According to previous macroscopic experimental results, argillaceous rocks under a free wetting-drying cycle usually exhibit a reversible deformation; however, the microscopic observation of this work evidences several irreversible phenomena. Some irreversible deformations located in clay matrix are observed and their mechanisms are identified. Some local damage phenomena, in the form of a network of micro-cracks typically with~1 μm openings, are also observed in both the wetting and drying cases: the former locates in the bulk of clay matrix and/or at inclusion-matrix interfaces, whereas the latter is mostly found in the bulk of clay matrix. The two types of micro-cracks are activated by different mechanisms, and their morphology and extension have been evidenced to be strongly dependent on the wetting or drying rate. All these irreversible phenomena are localized and sometimes counteracted each other, leading the macroscopic deformation to be apparently reversible. Nevertheless, they are of crucial importance for the reliability of long-term storage.
Mémoires de la Société …, 2007
The texture and porosity of Callovo-Oxfordian argillites, coming from the Meuse/Haute-Marne region, have been studied at different scales using borehole and laboratory measurements to more spot-like qualitative and quantitative analyses and descriptions of minerals and of the porous space. Due to their limitations and to their resolution, each of the experimental methods used explores differently and often complementarily the porous environment of argillites. The texture is characterised by two-dimensional imaging techniques (optical microscopy, scanning electron microscopy and autoradiography) that provide a view of the material from 0.1 micrometres to a few millimetres or even centimetres. Since borehole measurements are more integrative and representative of the rock in its natural state, they were used to analyse the spatial variability of the porosity. The measurements taken on laboratory samples served to characterise more finely the porous network by providing relevant geometric parameters in order to improve the understanding of the transfer and reaction processes occurring at the interface between fluids and minerals. In the end, a conceptual model for the spatial organisation of the porosity and of the minerals was developed for argillites on the basis of the overall information collected at each step. The reference values for the connected porosity of argillites are obtained by combining the mercury porosity and the calculated porosity in the domain still free from mercury as determined by gas-adsorption techniques. The total connected porosity then varies between 14% for carbonated levels and 19.5% for more argillaceous levels. Those values match those measured in boreholes. In percentage of the total porosity, the macroporosity characterised by mercury porosimetry ranges from 20 to 40%, whereas the mesoporosity resulting from the coupling of mercury porosimetry with nitrogen adsorption oscillates between 60 and 80%. The microporosity determined by nitrogen adsorption is lower than 2%. The relationship between the structure of the porosity and the mineralogy is described by considering 1) the prevailing calcic pole in the upper part of the series around the facies containing argillaceous-limestone beds (lithofacies C2d and C2c, and base of C2b) and 2) the argillaceous pole (lithofacies C2b and C2a). The properties of the porous network are discussed afterwards by reviewing water-saturation effects. Assemblage minéralogique, texture et porosité des argilites du Callovo-Oxfordien de Meuse/Haute-Marne (est du bassin de Paris) Mots-clés.-Injection de mercure, Adsorption d'azote, Minéralogie, Texture, Porosité, Argilites Résumé.-La texture et la porosité des argilites du Callovo-Oxfordien de Meuse/Haute-Marne ont été étudiées à différentes échelles depuis les mesures en forage et en laboratoire jusqu'aux analyses et descriptions plus ponctuelles qualitatives et quantitatives des minéraux et de l'espace poreux. De par leurs limitations et résolution, chaque méthode expérimentale utilisée explore de manière différente et souvent complémentaire le milieu poreux des argilites. La texture a été caractérisée à partir de techniques d'imagerie bidimensionnelles (microscopie optique, microscopie électronique à balayage, autoradiographie), qui apportent une vision du matériau de quelques 0,1 µm à quelques millimètres voir centimètres. Les mesures en forage, plus intégratives et plus représentatives de la roche dans son état naturel, ont permis d'analyser la variabilité spatiale de la porosité. Les mesures conduites sur échantillons en laboratoire ont permis de caractériser plus finement l'espace poreux en fournissant des paramètres géométriques pertinents pour la compréhension des processus de transfert et de réaction à l'interface entre les fluides et les minéraux. In fine, un modèle conceptuel d'organisation spatiale de la porosité et des minéraux a été développé pour les argilites, en cohérence avec la totalité des informations recueillies à chaque étape. Les valeurs de référence de la porosité connectée des argilites sont obtenues en combinant la porosité mercure et la porosité calculée dans le domaine non envahi par le mercure, déterminée par des techniques d'adsorption de gaz. Ainsi, la porosité totale connectée varie entre 14% pour les niveaux carbonatés et 19,5% pour les niveaux les plus argileux. Ces valeurs concordent avec celles mesurées en forage. En pourcentage de la porosité totale, la macroporosité, caractérisée par porosimétrie au mercure, est comprise entre 20 et 40%, la mésoporosité, obtenue en couplant porosimétrie au mercure et adsorption d'azote, est comprise entre 60 et 80% de la porosité alors que la microporosité obtenue uniquement via l'adsorption d'azote est inférieure à 2%. Le lien entre
Filling the Gaps – from Microscopic Pore Structures to Transport Properties in Shales
The application of ion-beam milling techniques to clays allows investigation of the porosity at nm resolution using scanning electron microscopy (SEM). Imaging of pores by SEM of surfaces prepared by broad ion beam (BIB) gives both qualitative and quantitative insights into the porosity and mineral fabrics in 2D representative cross-sections. The combination of cryogenic techniques with ion-beam milling preparation (BIB and FIB, focused ion beam) allows the study of pore fluids in preserved clay-rich samples. Characterization of the pore network is achieved, first, using X-ray computed tomography to provide insights into the largest pore bodies only, which are generally not connected at the resolution achieved. Secondly, access to 3D pore connectivity is achieved by FIB-SEM tomography and the results are compared with 2D porosity analysis (BIB-SEM) and correlated with bulk porosity measurements (e.g. mercury injection porosimetry, MIP). Effective pore connectivity was investigated with an analog of MIP based on Wood's metal (WM), which is solid at room temperature and allows microstructural investigation of WM-filled pores with BIB-SEM after injection. Combination of these microstructural investigations at scales of ,1 mm with conventional stressstrain data, and strain localization characterized by strain-fields measurement (DICdigital image correlation) on the same sample offers a unique opportunity to answer the fundamental questions: (1) when, (2) where, and (3) how the sample was deformed in the laboratory. All the methods above were combined to study the microstructures in naturally and experimentally deformed argillites. Preliminary results are promising and leading toward better understanding of the deformation behavior displayed by argillites in the transition between rocks and soils.
Poroelastic parameters of Meuse/Haute Marne argillites: effect of loading and saturation states
Applied Clay Science, 2004
The aim of this paper is to summarize the experimental work carried out by IFP on rock samples taken from the Meuse/ Haute Marne underground research laboratory host formation. The behavior of this clayey rock, called ''argillite,'' has been studied within the framework of Biot's mechanics of fluid saturated porous solids. Drained and undrained oedometric tests (i.e., uniaxial strain tests) have been performed to determine the poroelastic parameters for different stress levels. As the provided samples were not fully water-saturated, particular care has been taken over the definition of a preliminary resaturation phase and the estimation of each sample's final saturation level.
Microstructural insight into the nonlinear swelling of argillaceous rocks
Engineering Geology, 2015
Argillaceous rocks are chosen as possible host rocks for underground radioactive nuclear waste disposal. These rocks exhibit complex coupled thermo-hydro-chemo-mechanical behavior, the description of which would strongly benefit from an improved experimental insight on micro-scale. In this work we present some recent observations of the evolution of these rocks upon swelling on the scale of their composite microstructure, essentially made of a clay matrix with embedded grains of calcite and quartz with sizes ranging from a few to several hundreds of micrometers. The micro-scale experimental investigation was based on the combination of high definition and high resolution imaging in an environmental scanning electron microscope (ESEM) and digital image correlation techniques. Samples were held at a constant temperature of 2°C while the vapor pressure in the ESEM chamber was varied from a few to several hundreds of Pascals, generating a relative humidity (RH) ranging from about 10% up to 99%. Results on micro-scale showed strongly heterogeneous deformation fields, which result from complex hydromechanical interactions between different components of argillaceous rocks. The swelling of argillaceous rocks is moderate at low RH but becomes significant at high RH. The observations demonstrated that the nonlinearity is related not only to the micro-cracking upon wetting, but also to the nonlinear swelling of the clay matrix itself that is governed by different mechanisms.
Anhydrite–gypsum transition in the argillites of flooded salt workings in eastern France
2009
This study aims at understanding the physicochemical interactions between the saturated brine and the rocks enclosing the underground salt workings in Lorraine (eastern France). These anhydrite-rich and argillaceous rocks were characterized in terms of mineralogy, microtexture and connected porosity. Then, the two main lithofacies, massive anhydrite and anhydrite-rich argillite, were immersed in brine during more than 1 year. During this batch experiment, the argillites were affected by macroscopic splitting, contrarily to the massive anhydrite. Micro-texture and brine chemical analyses clearly show the swelling due to the hydration of anhydrite into gypsum inside the argillites, whereas hydration occurs superficially on the massive anhydrite, due to its very low permeability. Anhydrite-gypsum transformation is promoted by the presence of dissolved strontium and potassium in saturated brine. The low activity of water in saturated brine does not allow the clay fraction to swell significantly during the experiment. Thus, the expansion resulting from the hydration of anhydrite into gypsum might be responsible of the splitting of argillite in a saturated brine environment. The superficial anhydrite hydration on massive anhydrite can be explained by the low amount of connected porosity (less than 1%).
International Workshop on Geomechanics and Energy, 2013
Digital image correlation was used for the first time in an underground gallery to monitor the argillaceous rock deformations during an annual climatic cycle. This experimental in situ investigation was carried out on a study area of 34.4×27.5 cm², located on the East96 gallery front at the Tournemire experimental station, during which the relative humidity (RH) and temperature (T) were continuously measured for more than one year and fluctuate naturally.Our results demonstrate the ability of the non-invasive DIC method to monitor clay-rock strains and the opening and closure of desiccation cracks. Moreover, our study provides the following results. First, the hydric strains were anisotropic; the strains perpendicular to the desiccation cracks were almost homogeneous and much larger than those parallel to the same cracks. Second, the changes in crack apertures calculated from the displacement fields and the strain fields were clearly correlated and concomitant with changes in RH and T. Third, contrary to direct measurements acquired at the Mont-Terri site, the crack apertures of the desiccation cracks were reversible after one year of data acquisition. Moreover, although the main desiccation cracks were sub-horizontal and associated with the direction of bedding planes, our work demonstrated the existence of sub-vertical cracks.
Study of the pore water chemistry through an argillaceous formation: a paleohydrochemical approach
Applied Geochemistry, 2003
The spatial and temporal changes of the pore water along an argillaceous formation were studied on the basis of the chemistries of the fluids collected through the system. The study is based on chemical characterisation results of the pore water, which requires careful sampling and 4 years monitoring. The concept was applied to the water sampled from several boreholes drilled through the Opalinus Clay formation in the anticline at Mont Terri, Canton Jura, Switzerland. Asymmetrical gradients from the clay formation toward the limestone overburden are observed. In this upper formation, recharge with low mineralised water has caused the diffusion of species from the saline pore waters in the clay formation and their depletion. These species form concentration profiles from the underlying Jurensis Marl formation containing hydrophobic organic material toward the upper Lower Dogger Limestone formation. Nonsorbing species such as chlorides, bromides, iodides and sulphates as well as Na as a weakly sorbing cation form increasing concentration profiles from the Lower Dogger Limestone into the Opalinus Clay formation. Heavy water isotopes display similar profiles. The pH increases slightly downward through the system (from about 7 to 8) together with the total organic C (TOC) concentration, while the total inorganic C (TIC) concentration decreases from the Lower Dogger Limestone through the Opalinus Clay formation. A similar profile is observed for pe which decreases from the limestone groundwater (+2.5) toward the underlying marl water (À2.5). The water composition is discussed taking into account in-situ pore water dilution from recharge water by mean of a diffusion mechanism. Quantification of this transport process and of the consequent concentration profile is carried out through the 165 m thick Opalinus Clay layer, which was impregnated by fossil pore water derived from Tethys Ocean, considering a diffusion process that started about 10 Ma ago. The apparent diffusion coefficients estimated in the Opalinus Clay for Cl À , Br À , Iand Na + , on one hand, and, for 2 H and 18 O on the other are 2.6 AE0.8 and 5.2 AE 1.5. 10 À11 m 2 s À1 , respectively. These values are compared with data gained from other argillaceous systems. The Opalinus Clay formation is likely to have acted as a geological nanoporous barrier for 10 Ma.
Active porosity in swelling shales: insight from the Callovo-Oxfordian claystone
Géotechnique Letters, 2018
In swelling shales, significant clay–water interactions take place along the faces of the smectite minerals contained in the clay fraction, giving rise to the distinction between free water and adsorbed water. Further insight was recently gained by means of microstructure investigations, showing that the hydration mechanisms of pure or compacted smectites also explain the hydration and swelling behaviour of the Callovo-Oxfordian claystone, a possible host rock for deep radioactive waste disposal in France. In this rock, the proportion of water molecules adsorbed in intra-platelet pores was estimated to be around 25% of the total porosity, with 75% of the porosity containing free water. On the basis of these findings, the data of high-precision poroelastic measurements conducted in an isotropic compression cell showed that the porosity to account for a proper calculation of Skempton's B coefficient is not the total porosity, but the 75% proportion of the porosity corresponding to...