Marco Voltolini

PREFERRED ORIENTATIONS AND ANISOTROPY IN SHALES: CALLOVO-OXFORDIAN SHALE (FRANCE) AND OPALINUS CLAY (SWITZERLAND)

Anisotropy in clay-rich sedimentary rocks is receiving increasing attention. Seismic anisotropy is essential in the prospecting for petroleum deposits. Anisotropy of diffusion has become relevant for environmental contaminants, including nuclear waste. In both cases, the orientation of component minerals is a critical ingredient and, largely because of small grain size and poor crystallinity, the orientation distribution of clay minerals has been difficult to quantify. A method is demonstrated that relies on hard synchrotron X-rays to obtain diffraction images of shales and applies the crystallographic Rietveld method to deconvolute the images and extract quantitative information about phase fractions and preferred orientation that can then be used to model macroscopic physical properties. The method is applied to shales from European studies which investigate the suitability of shales as potential nuclear waste repositories (Meuse/Haute-Marne Underground Research Laboratory near Bure, France, and Benken borehole and Mont Terri Rock Laboratory, Switzerland). A Callovo-Oxfordian shale from Meuse/Haute-Marne shows a relatively weak alignment of clay minerals and a random distribution for calcite. Opalinus shales from Benken and Mont Terri show strong alignment of illite-smectite, kaolinite, chlorite, and calcite. This intrinsic contribution to anisotropy is consistent with macroscopic physical properties where anisotropy is caused both by the orientation distribution of crystallites and high-aspect-ratio pores. Polycrystal elastic properties are obtained by averaging single crystal properties over the orientation distribution and polyphase properties by averaging over all phases. From elastic properties we obtain anisotropies for p waves ranging from 7 to 22%.

Anisotropy of experimentally compressed kaolinite-illite-quartz mixtures

The anisotropy of physical properties is a well-known characteristic of many clay-bearing rocks. This anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling. Preferred orientation of clay minerals is an important factor causing anisotropy in clay-bearing rocks such as shales and mudstones that are the main cap rocksofoilreservoirs.Thepreferredorientationofclaysdepends mostly on the amount of clays and the degree of compaction. To studytheeffectoftheseparameters,wepreparedseveralsamples compressingattwoeffectiveverticalstressesamixtureofclays illite and kaolinite and quartz silt with different clay/quartz ratios. The preferred orientation of the phases was quantified with Rietveld analysis on synchrotron hard X-ray images. Pole figures for kaolinite and illite display a preferred orientation of clay platelets perpendicular to the compaction direction, increasing in strength with clay content and compaction pressure. Quartz particles have a random orientation distribution. Aggregate elastic properties can be estimated by averaging the singlecrystal properties over the orientation distribution obtained from the diffraction data analysis. Calculated P-wave velocity anisotropy ranges from 0% pure quartz sample to 44% pure clay sample, highly compacted, but calculated velocities are much higher than measured velocities. This is attributed to uncertainties about single-crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in themodel.Inthiswork,wepresentaneffectivemethodtoobtain quantitative data, helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties ofclay-bearingrocks.

Texture analysis of a turbostratically disordered Ca-montmorillonite

Abstract Turbostratic disorder, consisting in a disorder in which different layers have different rotations with respect to an axis, is commonly found in montmorillonite. The effect of this kind of disorder on diffraction profiles is significant and must be taken into account, especially in quantitative phase analysis. The effect of the turbostratic disorder in textured materials has never been investigated. In the present work, we have developed a strategy to perform quantitative texture analysis on turbostratically disordered Ca-montmorillonite aggregates that were uniaxially compressed. Synchrotron diffraction images were analyzed with a Rietveld method and disordered and ordered models are compared. The method proved to be reliable and ready for further applications.

Anisotropy in shale from Mont Terri

Anisotropy of shales is the subject of this report, and we use an example of the Jurassic Opalinus Clay from Mont Terri (Switzerland) that is being investigated in the context of radioactive waste disposal. The study is targeted at the geomechanical characterization of shale by laboratory testing. The overall aim is to improve the constitutive material laws and their application in numerical models.

In situ phase transformation and deformation of iron at high pressure and temperature

With a membrane based mechanism to allow for pressure change in a sample in a radial diffraction diamond anvil cell and simultaneous infrared laser heating, it is now possible to investigate texture changes during deformation and phase transformations over a wide range of temperature-pressure conditions. The device is used to study bcc (α), fcc (γ), and hcp (e) iron. In bcc iron, room temperature compression generates a texture characterized by (100) and (111) poles parallel to the compression direction. During the deformation induced phase transformation to hcp iron, a subset of orientations is favored to transform to the hcp structure first and generate a texture of (011¯0) at high angles to the compression direction. Upon further deformation, the remaining grains transform, resulting in a texture that obeys the Burgers relationship of (110)bcc//(0001)hcp. Contrary to these results for low temperature, at high temperature texture is developed through dominant pyramidal ⟨a+c⟩ {21¯1¯2} ⟨21¯1¯3⟩ and basal ...

A 2.5D Reactive Transport Model for Fracture Alteration Simulation.

Understanding fracture alteration resulting from geochemical reactions is critical in predicting fluid migration in the subsurface and is relevant to multiple environmental challenges. Here, we present a novel 2.5D continuum reactive transport model that captures and predicts the spatial pattern of fracture aperture change and the development of an altered layer in the near-fracture region. The model considers permeability heterogeneity in the fracture plane and updates fracture apertures and flow fields based on local reactions. It tracks the reaction front of each mineral phase and calculates the thickness of the altered layer. Given this treatment, the model is able to account for the diffusion limitation on reaction rates associated with the altered layer. The model results are in good agreement with an experimental study in which a CO2-acidified brine was injected into a fracture in the Duperow Dolomite, causing dissolution of calcite and dolomite that result in the formation of a preferential flow channel and an altered layer. With an effective diffusion coefficient consistent with the experimentally observed porosity of the altered layer, the model captures the progressive decrease in the dissolution rate of the fast-reacting mineral in the altered layer.

Evaluation of Microstructural Properties of Coffee Beans by Synchrotron X-Ray Microtomography: A Methodological Approach

UNLABELLED Synchrotron radiation microtomography was used as a nondestructive imaging technique to investigate the microstructural properties of green and roasted coffee beans. After image acquisition, 2D images have been reconstructed and 3D images of the beans have been then obtained. Qualitative and quantitative analysis of the images allow to fully characterize the morphological and structural features of the coffee beans. Roasting causes meaningful changes in the microstructure of the coffee bean tissue with the development in the entire bean of a porous structure with pores of different shape and size depending on the zone of the bean and cracks occurring mainly in the more external regions and between parenchyma and mucilage. The highly contrasted X-ray images have been analyzed to determine the pore size and its distribution in different regions of the coffee beans by selecting Volume-of-Interest (VoI). The use of phase-contrast hard X-rays imaging techniques represents an interesting tool of investigation of the internal structure, morphology, as well as the quality of whole coffee beans. Moreover, the high potentiality of 3D X-ray imaging and the approach used in this study could be applied in understanding the effects of roasting process conditions on the evolution of microstructural properties of the bean that may affect the stability as well the grinding and brewing performances. PRACTICAL APPLICATION Synchrotron radiation microtomography is an elegant nondestructive imaging technique to investigate the microstructural properties of porous cellular matrices like the green and roasted coffee beans. The quantitative analysis of the resulting 2D and 3D images allows a more comprehensive and objective characterization of the sample under investigation as a whole or of extracted Volumes-of-Interest in the bean. This imaging technique could have a major role in understanding the effects of roasting process conditions on the microstructural properties of the bean.

Coating thickness determination in highly absorbent core-shell systems

This article describes a single-shot methodology to derive an average coating thickness in multi-particle core–shell systems exhibiting high X-ray absorption. Powder composed of U–Mo alloy particles surrounded by a micrometre-thick UO2 protective layer has been used as a test sample. Combining high-energy X-ray diffraction and laser granulometry, the average shell thickness could be accurately characterized. These results have been validated by additional measurements on single particles by two techniques: X-ray nanotomography and high-energy X-ray diffraction. The presented single-shot approach gives rise to many potential applications on core–shell systems and in particular on as-fabricated heterogeneous nuclear fuels.

3D imaging of complex materials: the case of cement

Abstract Absorption-based X-ray micro-tomography (X-μCT) provides fundamental in-situ information on the 3D microstructure of complex multiphase materials such as cements. However, since the phases present in a hydrating cement paste may be characterized by similar values of the attenuation coefficient, leading to low absorption contrast between different crystalline or amorphous phases, micro-structural interpretation can be equivocal. 3D phase mapping by X-ray diffraction micro-tomography proved to be a successful technique for investigating the spatial distribution of the products in the paste during the hydration process, in a totally non-invasive mode and with enhanced phase selectivity compared to absorption tomography. Phase-selective maps, in the case of crystalline phases, can be extracted from single Bragg peaks or from the Rietveld-refined scale factor. However, even poorly crystalline and/or amorphous phases present in the cement paste, such as calcium silicate hydrates, can be successfully mapped by the use of selected portions of the measured powder data containing the relevant scattering of the phase. The reconstructed maps can be directly modeled by multifractal analysis and compared with computer-generated distributions.

Hydroxylapatite lattice preferred orientation in bone: a study of macaque, human and bovine samples

Hydroxylpatite crystallites in lamellar bone show preferred orientation. In this study, the texture (lattice preferred orientation) of the crystallites in cortical bone samples has been studied by means of synchrotron hard X-ray diffraction, performing a combined analysis with the Rietveld method to quantify fully the preferred orientation features and to obtain lattice and microstructural parameters (such as crystallite size) simultaneously. The samples were ribs from four adult female macaques of different ages, and two femurs chosen for comparison, one from a human child and one from an adult cow. The effect of the preferred orientation of the mineral component on the elastic properties is also briefly discussed. All six samples, averaging volumes of ∼0.5 mm3, show strong preferred orientation, with the hydroxylapatite c axis parallel to the bone axis. The symmetry of the texture is almost perfectly axial and clearly displays a uniform girdle of the a axis perpendicular to the bone axis. The texture strength is very similar for the four macaque rib samples, while some variation is observed in the human (weaker) and bovine (stronger) femurs. The crystallite size (8 × 30 nm) and unit-cell lattice parameters are similar in all samples. The Rietveld analysis provides for the first time a quantitative texture analysis combined with structural and microstructural hydroxylapatite analysis of the same bone samples.