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

Abstract

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.

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

“…5 and 6) were imaged with a Supra 55-Zeiss SEM (SE2 and BSE detectors at 20 kV and working distance (WD) = 8 mm). Further details of the method are given in Klaver et al, 2012Klaver et al, , 2015Houben et al, 2013Houben et al, , 2014Hemes et al, 2013Hemes et al, , 2015and Desbois et al, 2016.…”

“…FIB and BIB), which allow imaging of mineral fabrics and porosity down to the nanometre scale in very high-quality cross sections with SEM and TEM (Lee et al, 2003;Desbois et al, 2009Desbois et al, , 2011Desbois et al, , 2013Desbois et al, , 2016Loucks et al, 2009;Curtis et al, 2010;Heath et al, 2011;Klaver et al, 2012;Keller et al, 2011Keller et al, , 2013Houben et al, 2013Houben et al, , 2014Hemes et al, 2013Hemes et al, , 2015Laurich et al, 2014;Warr et al, 2014;Song et al, 2016). Serial sectioning allows the reconstruction of microstructure in 3-D (Keller et al, 2011(Keller et al, , 2013Milliken et al, 2013;Hemes et al, 2015), and cryogenic techniques can image the pore fluid in the samples and avoid artefacts produced by drying Schmatz et al, 2015).…”

Nano-Tomography of Porous Geological Materials Using Focused Ion Beam-Scanning Electron Microscopy

“…As there is no preliminary information about the porosity, the segmentation result cannot be verified. Therefore, comparison with other techniques of porosity measurements such as mercury injection porosimetry may be an alternative in several cases to provide such information and assist in the interpretation of FIB tomography results [21,78]. Discrepancies between bulk estimations of porosity and FIB tomography can result from the random selection of the ROI in a material where the pore distribution varies in different regions, or from the invalidity of the assumptions used in the bulk estimation methods.…”

Investigation of Microstructures in Naturally and Experimentally Deformed Reference Clay Rocks Using Innovative Methods in Scanning Electron Microscopy

“…The emergence of ion-beam milling tools (i.e. BIB and FIB) has improved microstructural investigations in fine-grained geomaterials, providing unprecedented clarity of microstructures at below the micrometer scale (Loucks et al, 2009;Curtis et al, 2010;Desbois et al, 2009Desbois et al, , 2011bHeath et al, 2011;Hemes et al, 2013Hemes et al, , 2015Houben et al, 2013Houben et al, , 2014aHouben et al, , 2014bKeller et al, 2011Keller et al, , 2013aKeller et al, , 2013bKlaver et al, 2012Klaver et al, , 2015aKlaver et al, , 2015b. Cryogenic methods combined with FIB, BIB, and SEM has delivered, for the first time, direct imaging of pores filled with original pore fluids (Desbois et al, 2008(Desbois et al, , 2009.…”

“…The cross-sections produced by FIB and BIB are of such a high quality (the remaining topography is in the range of 5 nm in height; Klaver et al, 2012) that they allow direct, unambiguous investigation of the pores at the resolution (,10 nm) of state-of-the-art SEM. The direct detection of porosity within a truly planar 2D cross-section enables the statistical and stereological description of the porosity (Desbois et al, 2009;Hemes et al, 2013Hemes et al, , 2015Houben et al, 2013Houben et al, , 2014aHouben et al, , 2014bKlaver et al, 2012Klaver et al, , 2015aKlaver et al, , 2015b. On one hand, BIB cross-sectioning produces large representative areas up to 2 mm 2 for quantitative porosity analyses, but is restricted to 2D (Figure 1c -e).…”