Susanne Hemes

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.

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.

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.