Abstract 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 stress-strain data, and strain localization characterized by strain-fields measurement (DIC–digital 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.

Abstract Low-permeability (unconventional) reservoirs exhibit heterogeneities at multiple scales that affect flow. While macro-scale heterogeneities are typically evaluated using well-test or production-analysis techniques, core-scale (and finer) analyses are relied upon to evaluate micro-and nano-scale heterogeneities. Pore structure is a known control on flow at the core scale; for unconventional reservoirs, however, pore-size distributions, pore accessibility, and connectivity are challenging to evaluate. In the present study, a combination of neutron-scattering methods, fluid invasion, and imaging techniques were used to evaluate the degree of pore accessibility and connectivity in a tight oil reservoir within the Cardium Fm. of western Canada. In previous work, cm- to sub-cm scale variations in lithology (elementary lithological components, or ELCs) have been shown to affect flow at the core scale significantly and that reservoir quality varies with the ELCs. The fundamental controls on flow within these ELCs ( i.e. at the pore scale), however, are poorly understood. The aim of the present study was to gain insight into these controls. Small-angle and ultra-small angle neutron scattering (SANS/USANS) has revealed that while the Cardium samples exhibit a wide range in pore-size distribution, the accessibility of pores varies significantly with pore size. In particular, the large pore fraction seems to be less accessible than the small pore fraction, which is counter-intuitive. Mercury intrusion data partially support this finding. High-resolution scanning electron microscopy (SEM) imaging suggests that pores within mineral grains may indeed be disconnected/isolated. The SANS/USANS interpretations are based on a simple 2-component (pores + average mineral phase) model, however; detailed mineral mapping reveals that several components may affect significantly the scattering behavior which leads to the conclusion that a multi-phase model may be more appropriate, and that use of the conventional 2-phase model could lead to errors in calculated pore-size distributions and the percentage of disconnected porosity.

Abstract Grains within siliciclastic muds are deposited either as flocs, in which grains are generally <~10µm, or as single grains: “sortable silt,” generally > 10µm. When clay-size (<2µm diameter) particles form >30% of mudstones, pore-size distributions are controlled mainly by the interaction of phyllosilicates; these materials are ‘matrix-supported.’ Pores associated with clay-size particles are typically <20 nm, even at shallow burial. When clay-size particles comprise <~30% of the grain-size distribution, a second, much larger pore system is observed, controlled by the amount and size of sortable silt; these mudstones are ‘framework-supported.’ Compaction of these silt-rich materials occurs mainly by the loss of the largest pores, but large pores still exist up to high effective stresses in the absence of chemical compaction. Mercury injection porosimetry (MIP) gives information about pore-throat size and pore connectivity and thus provides useful data with which to estimate permeability. Models based on generally flat pore shapes can estimate the permeability of homogenous mudstones to ± a factor of 3 of the true value, but cannot be used for heterogeneous, laminated mudstones, which exhibit highly anisotropic permeabilities. As MIP gives information about pore throats and microscopy gives information about pore bodies, the two techniques generate different results. Both are required, along with other techniques such as small-angle neutron scattering and low-pressure gas sorption, in order to fully appreciate the complexity of mudstone pore systems.

Abstract Soils are highly heterogeneous entities in which organic and inorganic as well as living and non-living building blocks interact to form biogeochemical interfaces. While processes at these interfaces occur at the micrometer or submicrometer scale, they are thought to influence the behavior of soils at the global scale, e.g. soils as carbon sinks. Analytical methodologies with a high resolution are, therefore, required in order to investigate these processes with the final goal to understand biogeochemical-interface formation mechanistically. In the present study, sorption experiments of water-extractable organic matter on model minerals, such as boehmite and illite, were performed. Adsorption of organic matter on the minerals was quantified by conventional bulk-scale methods and compared with data from nanoSIMS measurements. From the data obtained, scaling factors have been developed which permit the quantification of organic matter in the secondary ion images provided by nanoSiMS.

Abstract The present study describes new instrumentation developments at the GSECARS 13-ID-E hard X-ray microprobe beamline at the Advanced Photon Source that allows for high-speed, coupled micro-beam X-ray diffractions/X-ray fluorescence/X-ray absorption fine structure mapping. These new methodologies provide Earth and environmental scientists with unique coupled tools for evaluating microscale mineralogical and chemical heterogeneities in fine-grained sediments, soils, shales, and mine tailings and associated secondary precipitates. In particular, new technologies and approaches for integrating fast µXRD mapping into routine X-ray microprobe beamline operations are described and several real-world examples are given of how this approach provides unique insights with regards to micrometer-scale heterogeneities in mineralogy and chemistry that are difficult to obtain by other methods. Examples described here include waste streams associated with mine tailings from the McClean Lake mining facility located on the eastern edge of the Athabasca Basin in northern Saskatchewan, Canada, and from mine-drainage waters from the epithermal Au-Ag-Cu deposits of the Lagunas Norte mine in the Peruvian Andes.

Abstract The development of complementary imaging techniques at beamline I18 at Diamond Light Source (Didcot, UK) to investigate the microstructure of inorganic materials is described. In particular, the use of X-ray micro-imaging techniques to understand the effect of alpha radiation on phyllosilicates, and the nature of individual catalytic particles are reported. Micro X-ray diffraction (µXRD) studies of the former materials have shown structural changes that will affect their adsorption properties, while the chemistry of the catalyst particles has been investigated using micro X-ray fluorescence, µXRD and µX-ray absorption near-edge structure mapping. The distribution of a Mopromoted Pt nitrobenzene hydrogenation catalyst has shown that some of the Pt penetrated to the core of the particle and has the same chemistry as the bulk of the Pt located on the outside of the particle. The phase distribution in an as-prepared Re-Ti-promoted Co Fischer-Tropsch catalyst is reported.

Abstract From current and previous work by the present authors, experimental results showing the connectivity and the water mobility in natural clay systems are emphasized. Using various low-field Nuclear Magnetic Resonance techniques, a large range of time- and length scales, from nm to cm, can be explored. The amount of water in the interlayer space of smectites at different relative humidities can be detected and quantified. Simple criteria such as unimodal or multimodal relaxation time distributions give a simple and quick indication of connectivity between different porosity compartments. A new technique has allowed the orientation of water in the interlayer space of smectites to be determined; from 39 to 66% of the water molecules are noted as being ‘oriented.’

Abstract Transport properties in shales were investigated using experimental and computer-simulation methods. First, an experimental method based on a transient pressure-decay technique was further developed and used instead of classical Darcy core-flood methods. This has allowed measurement of the permeability of tight shale samples on much shorter time scales than by conventional methods. Second, molecular dynamics (MD) computer simulations were used to measure the diffusion coefficients of water and cations in a model clay sample. Measurements of the self-diffusion coefficient showed that the values increased with increasing water content. The results for Na-, Li-, and K-smectites are in satisfactory agreement with experimental and with other simulation results in the literature indicating that the clay interlayer space is an important route of transport for ions and water. The results also lend credibility to the correctness of the diffusion coefficients obtained from the current MD simulations.

Abstract Tight clay formations are frequently employed as natural or engineered barrier systems in the context of safe disposal of toxic waste. To evaluate long-term barrier efficiency, understanding the spreading and transport of contaminants in these porous media is of critical importance. Tight clay formations exhibit pronounced physical and chemical heterogeneities at various length scales. These heterogeneities potentially dictate the reactive transport characteristics. Modern micro-analytical techniques such as synchrotron-based micro X-ray fluorescence, X-ray spectromicroscopy or X-ray tomographic microscopy, and neutron imaging techniques, as well as laboratory-based microprobe techniques, can be employed to gain new insights into diffusion processes of reactive chemicals occurring in such multi-domain, micro-structured porous media. In addition to structural information, detailed chemical information can be obtained. Most importantly, these modern methods are capable of providing information from within the porous medium directly illustrating the heterogeneous distribution of chemical properties and their inter-relations. Consequently, combined with the capability to image the reactive transport pattern in up to full three dimensions, heterogeneity-reactivity relationships can be derived. Based on the illustrative example of cesium (Cs) migration in Opalinus Clay rock, multi-dimensional and multi-modal imaging of reactive transport phenomena have demonstrated unequivocally that physical and chemical heterogeneities are indeed transport relevant.

Abstract A method for the numerical determination of the steady-state response of complex charged porous media to pressure, salt concentration, and electric potential gradients is presented here. The Pore Network Model (PNM), describing the porosity as a network of pores connected by channels, is extended to capture electrokinetic couplings which arise at charged solid−liquid interfaces and allows us to compute the macroscopic fluxes of solvent, salt, and charge across a numerical sample submitted to macroscopic gradients. On the channel scale, the microscopic transport coefficients are obtained by solving analytically (in simple cases) or numerically the Poisson-Nernst-Planck and Stokes equations. The PNM approach then allows us to upscale these transport properties to the sample scale, accounting for the complex pore structure of the material via the distribution of channel diameters. The Onsager relations between macroscopic transport coefficients are preserved, as expected. Electrokinetic couplings, combined with the sample heterogeneity, result in qualitative differences with respect to their microscopic counterparts for some macroscopic transport coefficients, however ( e.g. permeability or electro-osmotic coefficient). This underlines the care that should be taken when accounting for transport properties based on a single channel of average diameter.

Abstract The sensitivity of ion-concentration distribution models to three key model assumptions, the pore-size distribution of clay media, the ‘distance of closest approach’ of ions to the clay surface, and the accessibility of sub-nanometer-wide clay mineral interlayer spaces to anions, was explored by solving the Poisson-Boltzmann equation for swelling and nonswelling clay materials. The calculations show that all three model assumptions impact significantly on values predicted for the anion-accessible porosity. As a consequence, macroscopic measurements of anion exclusion in clay media cannot be used to test any of the three model assumptions independently of the other two. Information gained at the nanoscale, a detailed characterization of pore-size distribution in particular, is necessary to develop accurate predictive models of the anion accessible porosity of clay media.