Guillaume Desbois

Variations in the morphology of porosity in the Boom Clay Formation: insights from 2D high resolution BIB-SEM imaging and Mercury injection Porosimetry

Boom Clay is considered as one of the potential host rocks for the disposal of high level and/or long lived radioactive waste in a geological formation in Belgium (Mol study site, Mol-1 borehole) and the Netherlands. The direct characterisation of the pore space is essential to help understand the transport properties of radionuclides in argillaceous materials. This contribution aims to characterise and compare the morphology of the pore space in different Boom Clay samples, representing end-members with regard to mineralogy (i.e. clay content) and grain-size distribution of this formation. Broad ion beam (BIB) cross-sectioning is combined with SEM imaging of porosity and Mercury injection Porosimetry (MIP) to characterise the variability of the pore space in Boom Clay at the nm- to μm-scale within representative 2D areas and to relate microstructural observations to fluid flow properties of the bulk sample material. Segmented pores in 2D BIB surfaces are classified according to the mineralogy, generating representative datasets of up to 100,000 pores per cross-section. Results show total SEM-resolved porosities of 10-20% and different characteristic mineral phase internal pore morphologies and intra-phase porosities. Most of the nano-porosity resides in the clay matrix. In addition, in the silt-rich samples, larger inter-aggregate pores contribute to a major part of the resolved porosity. Pore-size distributions within the clay matrix suggest power-law behaviour of pore areas with exponents between 1.56-1.74. Mercury injection Porosimetry, with access to pore-throat diameters down to 3.6 nm, shows total interconnected porosities between 27-35 Vol.-%, and the observed hysteresis in the MIP intrusion vs. extrusion curves suggests relatively high pore-body to pore-throat ratios in Boom Clay. The difference between BIB-SEM visible and MIP measured porosities is explained by the resolution limit of the BIB-SEM method, as well as the limited size of the BIB-polished cross-section areas analysed. Compilation of the results provides a conceptual model of the pore network in fine- and coarse-grained samples of Boom Clay, where different mineral phases show characteristic internal porosities and pore morphologies and the overall pore space can be modelled based on the distribution of these mineral phases, as well as the grain-size distribution of the samples investigated.

Argon broad ion beam tomography in a cryogenic scanning electron microscope: a novel tool for the investigation of representative microstructures in sedimentary rocks containing pore fluid

The contribution describes the implementation of a broad ion beam (BIB) polisher into a scanning electron microscope (SEM) functioning at cryogenic temperature (cryo). The whole system (BIB‐cryo‐SEM) provides a first generation of a novel multibeam electron microscope that combines broad ion beam with cryogenic facilities in a conventional SEM to produce large, high‐quality cross‐sections (up to 2 mm2) at cryogenic temperature to be imaged at the state‐of‐the‐art SEM resolution. Cryogenic method allows detecting fluids in their natural environment and preserves samples against desiccation and dehydration, which may damage natural microstructures. The investigation of microstructures in the third dimension is enabled by serial cross‐sectioning, providing broad ion beam tomography with slices down to 350 nm thick.

Electrical and fluid flow properties of carbonate microporosity types from multiscale digital image analysis and mercury injection

Electrical and fluid flow properties of porous media are directly related to the morphology of pores and the connectivity of the pore network. Both are closely linked to the amount and type of intrinsic microporosity in carbonate rocks, which is not resolved by conventional techniques. Broad-ion-beam (BIB) milling produces high-quality true-two-dimensional cross sections for scanning electron microscopy (SEM) and enables accurate quantification of carbonate microporosity for the first time. The combination of BIB-SEM mosaics with optical micrographs yields a multiscale digital image analysis (MsDIA) spanning six orders of magnitude. In this paper, the pore structures of 12 different carbonate rock samples from various rock types are quantified using MsDIA. Mercury injection capillary pressure measurements are used to assess pore-throat properties. The quantified pore-structure parameters are correlated with plug measurements of electrical resistivity and permeability. Results indicate that petrophysical properties are closely linked to the type of microporosity, which is distinctive for a certain rock type. Rock types with crystalline microporosity, such as mudstone and dolomite, generally show good connectivity, in which the size of the pore-network determines if the rock favors either hydraulic or electric flow. Rock types with intercement or micromoldic microporosity, such as bindstone and travertine, show variations in connectivity due to layering and moldic micropores of biological origin. Furthermore, pore-size distributions (PSD) follow a power law in all samples, despite their depositional and diagenetic differences. The slope of the PSD correlates with the electric properties, in which samples with a steeper slope show lower cementation factors. The linearity of the power law distribution enables predictions of pore populations outside the investigated length scales.

The effect of microstructural heterogeneity on pore size distribution and permeability in Opalinus Clay (Mont Terri, Switzerland): insights from an integrated study of laboratory fluid flow and pore morphology from BIB-SEM images

Abstract Opalinus Clay (OPA) is considered as a potential host rock for the deep geological disposal of radioactive waste. One key parameter in long-term storage prediction is permeability. In this study we investigated microstructural controls on permeability for the different facies of OPA. Permeability and porosity were determined under controlled pressure conditions. In addition, the pore space was investigated by SEM, using high-quality surfaces prepared by broad ion beam (BIB) milling. Water permeability coefficients range from 1.6×10−21 to 5.6×10−20 m2; He-pycnometer porosities range between approximately 21 and 12%. The sample with the highest He porosity (shaly facies) is characterized by the lowest permeability, and vice versa (carbonate-rich sandy facies). This inverse behaviour deviates from the generally reported trend of increasing permeability with increasing porosity, indicating that parameters other than porosity affect permeability. Visible porosities from SEM images revealed that 67–95% of the total porosity resides within pores smaller than the SEM detection limit. Pore sizes follow a power-law distribution, with characteristic power-law exponents (D) differing greatly between the facies. The carbonate-rich sandy facies contains a network of much larger pores (D(shaly)≈2.4; D(carbonate-rich) c. 2.0), because of the presence of load-supporting sand grains that locally prevent clay compaction, and are responsible for a higher permeability.

Investigation of microstructures in naturally and experimentally deformed reference clay rocks using innovative methods in scanning electron microscopy

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 (BIBSEM) 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 (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.

Microstructure of the Shaly Facies of Opalinus Clay on the Mm-nm Scale

Opalinus Clay is one of the fine-grained sedimentary formations investigated as a possible geological repository for the long-term storage of radioactive waste. Porosity, pore size, pore shape and connectivity of pores will help to define sealing capacities of a host rock. Here a combination of FIB-SEM, BIB-SEM and µ-CT has been used in order to investigate the nm-mm scale microstructure of the Shaly facies of Opalinus Clay in 2D and 3D. The µ-CT measurements gave a 3D overview of the microstructure of a 2 mm core down to a 2.6 µm voxel size. Which showed cracks, larger bivalve fragments and pyrite nodules embedded in a matrix, which was investigated down to the nm scale (enabling to image the pores) in 2D using the BIB-SEM method. FIB-SEM measurements were afterwards performed on selected areas to investigate the pore shape and connectivity in 3D down to the nm scale.

Lessons Learned from Electron Microscopy of Deformed Opalinus Clay

Using a combined approach of ion-beam milling and electron microscopy, we observe, describe and quantify the microstructure of naturally and synthetically deformed Opalinus Clay (OPA) and deduce its microstructural evolution and underlying deformation mechanisms. The investigated samples derive from the so-called Main Fault, a 10 m offset fold-bend thrust fault crossing the Mont Terri Rock Laboratory in the Swiss Jura Mountains. The samples are slightly overconsolidated, experienced a burial depth of 1350 m and a maximum temperature of 55 °C. Most impact on strain is attributed to frictional sliding and rigid body rotation. However, trans-granular fracturing, dissolution-precipitation of calcite, clay particle neoformation and grain deformation by intracrystalline plasticity have a significant contribution to the fabric evolution. The long-term in-situ deformation behavior of OPA is inferred to be more viscous than measured at laboratory conditions.

Multiscale porosity estimates along the pro-and retrograde deformation path: an example from Alpine slates

Estimating the porosity of slates is of great interest for the industries dealing with sub-surface areas such as CO2 sequestration, nuclear waste disposal and shale gas but also for engineering purposes in terms of mechanical stability for underground or surface constructions. In this study, we aim at understanding estimates of the porosity of slates from the Infrahelvetic flysch units (IFUs) in the Glarus Alps (eastern Switzerland). Surface and sub-surface samples were collected along a temperature gradient from 200 to 320 C and therefore give the opportunity to link pore types along this temperature and deformation path. In addition, we indicate which porosity is the effect of surface processes and indicate the contribution of artificially induced porosity. The developed workflow consists of a combination of bulk rock measurements including helium pycnometry (He pycnometry) and mercury intrusion porosimetry (MIP) with image analysis. Image analysis was performed with high-resolution scanning electron microscopy (SEM) on broad ion beam (BIB) prepared cross sections (BIB-SEM). Different vein generations provide evidence of porosity formation at depth, as they present “paleo-porosity”. Towards peak metamorphic conditions (prograde path), porosity reduces to < 1 vol%, indicated by matrix porosity detected by BIB-SEM. During exhumation (retrograde path) porosity increases due to the formation of microfractures interpreted as the effect of unloading (open fractures). At the surface, porosity is further increased due to the formation of macro-fractures (fracture apertures up to 1 mm), which are interpreted as being either due to the effect of weathering processes such as freeze and thaw cycles or artificially induced by sample preparation. Additionally, porosity and pore morphology are strongly dependent on mineralogy, sample homogeneity and strain, which change dynamically in time and space.

Standardized Automated Multiscale Imaging Technologies to Quantify Microstructure and Petrophysical Properties in a Range of Rock Types

Rocks are heterogeneous at all scales in composition, microstructure and petrophysical properties. Starting from representative sampling and quantifying microstructure together with measurement of rock properties in the laboratory, we show that it is necessary to treat different rock types in specially tailored workflows. Our multiscale imaging technology combines the advantages of a novel virtual polarizing microscopy (ViP) with high-resolution scanning electron microscopy on broad-ion-beam sputtered sample surfaces (BIB-SEM). Automated image registration and segmentation allows a fast and reliable analysis of the microstructural features as well as the assessment of petrophysical properties. This unique combination of methods allows a comprehensive analysis of various rock types including identification of diagenetic features and the quantification of pore size distribution and effective porosity from the cm to nm- scale as shown for different carbonates and tight siliciclastic rocks. Results were used to develop new porosity models that can be used for upscaling and permeability assessment in digital rock models.

Overview of Integrated Microstructural Studies to Better Understand Mechanical and Fluid Flow Properties of Fine Grained

A comprehensive understanding of mechanical and fluid flow properties in fine-grained geo-materials like shales requires imaging the microstructure at a range of scales as the microstructures are small and the samples are heterogeneous. Broad Ion Beam (BIB) milling followed by Scanning Electron Microscopy (SEM) imaging provides access to these microstructures at nanometre resolution and large cross sections of up to several mm². Applying BIB-SEM on naturally and experimentally deformed or Wood’s Metal injected samples enables to resolve the related deformation processes and to image the pore connectivity, respectively. The pore fluid distribution, however, can be resolved by integrating BIB-SEM with cryogenic techniques. Examples of such integrated studies indicate calcite in shear fractures in Opalinus Clay and high connectivity in a fine-grained Boom Clay sample.