EXTERNAL REPORT SCK•CEN-ER-157

11/MDC/P-6

Comparative study on porosity in fine- and coarse-grained Boom Clay samples (Mol-Dessel reference site, Belgium)

Pore characterization down to the nm-scale, using BIB-SEM methods: methodology and first results

Susanne Hemes, Guillaume Desbois, Janos L. Urai, Mieke de Craen and Miroslav Honty

part of contract between RWTH Aachen University (Structural Geology, Tectonics and Geomechanics) and ONDRAF/NIRAS (Ref. PHL/SBR/bg/2010-1736) - CCHO 2010-0401/00/00 SCK•CEN Contract: CO-90-08-2214-00; NIRAS/ONDRAF contract: CCHO 2009-0940000; Research Plan Geosynthesis

February, 2011

RDD

SCK•CEN Boeretang 200 BE-2400 Mol Belgium

EXTERNAL REPORT OF THE BELGIAN NUCLEAR RESEARCH CENTRE SCK•CEN-ER-157

11/MDC/P-6

Comparative study on porosity in fine- and coarse-grained Boom Clay samples (Mol-Dessel reference site, Belgium)

Pore characterization down to the nm-scale, using BIB-SEM methods: methodology and first results

Susanne Hemes, Guillaume Desbois, Janos L. Urai, Mieke de Craen and Miroslav Honty

part of contract between RWTH Aachen University (Structural Geology, Tectonics and Geomechanics) and ONDRAF/NIRAS (Ref. PHL/SBR/bg/2010-1736) - CCHO 2010-0401/00/00 SCK•CEN Contract: CO-90-08-2214-00; NIRAS/ONDRAF contract: CCHO 2009-0940000; Research Plan Geosynthesis

February, 2011 Status: Unclassified ISSN 1782-2335

SCK•CEN Boeretang 200 BE-2400 Mol Belgium

© SCK•CEN Studiecentrum voor Kernenergie Centre d’étude de l’énergie Nucléaire Boeretang 200 BE-2400 Mol Belgium Phone +32 14 33 21 11 Fax +32 14 31 50 21 http://www.sckcen.be Contact: Knowledge Centre library@sckcen.be

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All property rights and copyright are reserved. Any communication or reproduction of this document, and any communication or use of its content without explicit authorization is prohibited. Any infringement to this rule is illegal and entitles to claim damages from the infringer, without prejudice to any other right in case of granting a patent or registration in the field of intellectual property. SCK•CEN, Studiecentrum voor Kernenergie/Centre d'Etude de l'Energie Nucléaire Stichting van Openbaar Nut – Fondation d'Utilité Publique - Foundation of Public Utility Registered Office: Avenue Herrmann Debroux 40 – BE-1160 BRUSSEL Operational Office: Boeretang 200 – BE-2400 MOL

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The porosity of clay-rich materials is well known for bulk samples, but the direct characterization of pore morphologies at nm-scale in very fine-grained samples is difficult due to sample preparation. In this study, the combination of Broad-Ion-Beam (BIB) cross-sectioning and SEM-imaging was used to produce high quality, polished cross-sections and image pores down to a few nm in size. Three samples of Boom Clay from the Mol-Dessel reference site for radioactive waste disposal (Mol-1 borehole, Belgium) were investigated and compared with regard to their porosity and grain-size.

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Abstract: The porosity of a host rock material for radioactive waste disposal is of great importance, since diffusion controlled transport of radionuclides in argillaceous materials mainly takes place in the pore space. In Belgium, the Oligocene Boom Clay is being investigated as a reference host rock formation for the disposal of high- and medium-level radioactive waste, since more than 30 years. Chemical and physical properties of bulk samples are in general well known, but still little knowledge exists on the direct characterization of porosity down to the nm-scale. This is mainly due to the lack of appropriate methods to prepare damage-free cross-sections for high-resolution SEM-imaging. Methods do exist to measure effective porosities of bulk samples (e.g. 'Mercury Porosimetry' or 'Woods Metal Injection'), but it is not so clear which pores these liquid metals reach and how the pore morphologies are affected by the injection pressures.

The aim of the present study is to investigate and compare the porosities of three different, but representative Boom Clay samples from the Mol-1 borehole in Belgium, using the combination of Broad Ion Argon-Beam cross-sectioning (BIB) and high-resolution Secondary Electron Microscope (SEM) imaging to produce high quality, damage free, polished surfaces and be able to investigate 2D-sections with pores visible down to a few nm in size. Preliminary work using this technique has been reported by Desbois et al. (2009, 2010 a+b).

The three samples investigated and compared in this study are from different horizons of the Boom Clay formation, displaying end-members of Boom Clay with regard to grain-size and mineralogical compositions.

To be able to investigate large enough sample areas to get statistically significant results and at the same time resolve microstructural features down to a few nm in size, 300 to 700 SE-images were taken of each sample at high magnification (30,000-times) and stitched together afterwards semi-automatically. Pores were segmented and mapped using ArcGIS and statistical analysis was carried out using the MATLAB extension ‘PolyLX’.

Results: At the scale of our SEM-images, all samples investigated show porosities between 15 and 21 %, but samples containing a higher amount of clay minerals and having a smaller average grain-size, show lower visible porosities, than the coarser-grained sample, which contains as well less clay minerals. These results suggest that a correlation exists between porosity, pore-size distribution and mineralogy as well as grain-size distribution of samples investigated. For the two finer grained samples, 99 % of the detected pores are within the clay-matrix or at the interface between clay and non-clay minerals, whereas in the coarser grained sample, a significant part of the visible porosity (~ 15 %) is borne by non-clay minerals - like quartz, feldspar, pyrite or mica. However, taking into account only visible pores within the clay-matrix, all three samples show similar pore-characteristics (pore-size distributions and pore morphologies). Pores within the clay-matrix follow a log-normal size-distribution, which can be fitted by a power-law, using exponents in the range between 1.7 and 1.9. The pore-size distributions exhibit a peak between 35-45 nm equivalent pore diameter (ED) and 90 % of all the pores within the clay-matrix are smaller than 250 nm (ED). The maximum pore-size of the two finer grained samples is ~ 2 μm, but in the coarser grained sample, pores can be up to ~ 20 μm in size. The preferred orientation of pores within the clay-matrix is parallel to the bedding in the fine-grained samples and around 30° from the horizontal axis in the coarse-grained sample.

Preliminary results suggest that the clay-matrix of the investigated samples can be considered as homogeneous with regard to its porosity, but considering whole samples, overall porosities seems to be controlled by the grain-size and amount of non-clay minerals. Note that largest pores preferentially develop at the interface between clay-matrix and clast grains.

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Table of Contents

1 Introduction .......................................................................................................................................... 9

1.1 Radioactive waste disposal .......................................................................................................... 9

1.2 Porosity ......................................................................................................................................... 9

2 Sample description ............................................................................................................................. 10

2.1 Sample 1, EZE55, HADES-level .................................................................................................... 10

2.2 Sample 2, EZE52, 'clay-poor', coarse-grained sample ................................................................ 10

2.3 Sample 3, EZE54, 'clay-rich', fine-grained sample ...................................................................... 11

3 Methods ............................................................................................................................................. 11

3.1 Sample preparation (cutting, drying and polishing/BIB cross-sectioning) ................................. 11

3.2 SEM-imaging ............................................................................................................................... 13

3.3 Image-processing & Analysis ...................................................................................................... 16

3.3.1 'Autopano' .......................................................................................................................... 16

3.3.2 'ArcGIS' ............................................................................................................................... 16

3.3.3 Statistical Analysis - 'PolyLX' ............................................................................................... 16

4 Results ................................................................................................................................................ 20

4.1 Mineralogical (qualitative to semi-quantitative) analysis from BSE-images and EDX-mapping 20

4.2 SE-imaging: Outline and analysis of porosities........................................................................... 20

4.2.1 Pore morphologies: "How pores look like" ........................................................................ 20

4.2.2 Statistical analysis of visible porosity ................................................................................. 27

4.2.3 Analysis of pore-sizes in clay matrix ................................................................................... 28

4.2.4 Extrapolation of pore-size distributions ............................................................................. 31

4.2.5 Analysis of pore orientations.............................................................................................. 33

5 Conclusions and outlook .................................................................................................................... 35

6 References .......................................................................................................................................... 36

Appendices

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9

1 Introduction

1.1 Radioactive waste disposal The use of atomic energy and the production of radioactive waste since more than 30 years make an adequate management and safe long-term disposal of nuclear waste indispensable.

The disposal of radioactive waste in a geological repository involves the reliance on the geological and hydrological environment to provide long-term confinement and isolation of the waste. In Belgium the principle of a multi-barrier system for disposal of high- and medium-level radioactive waste is being investigated: first several layers of engineered barriers (especially designed metal containers and concrete layers) are used to concentrate and encapsulate the waste, before a geological formation shall serve as an ultimate barrier, once the engineered barrier-system has started to fail. At that moment the host rock is the main barrier against radionuclide migration. Among its most important properties are thus the capability to isolate the radioactive waste and furthermore reduce, attenuate and delay the release of radionuclides, preventing them from reaching the atmosphere, even on a timescale of several millions of years.

Clay may fulfil these demands, since it has a very low permeability, leading to a low hydraulic conductivity and thus very limited water flow and diffusion controlled transport of radionuclides through the material. Furthermore it has good sorption capacities, providing the possibility of an efficient retardation of radionuclides and very limited migration.

In Belgium the Oligocene Boom Clay underneath the Mol-Dessel nuclear site is being investigated as a reference host rock formation for radioactive waste disposal in a geological repository since several decades and it is used as a research site for radioactive waste disposal in an underground laboratory. Boom Clay has the potential to fulfil the above described demands of good sorption capacities, low permeability and low hydraulic conductivity.

1.2 Porosity The porosity of a host rock material for radioactive waste disposal affects the diffusion-controlled transport of the radionuclides inside the material, as well as the sorption capacities of the material.

What is the porosity of a mudrock?

The volume not occupied by mineral grains or the ratio of total volume of voids to total sample volume. The total porosity of a sample is considered as the sum of its matrix porosity and its fracture porosity. In clays special attention has to be payed to inter-layers, since considerable amounts of water can be stored inside them. These inter-layer volumes are usually not included in the effective porosities, but might be accessible to some of the radionuclides.

Detailed investigation on the morphology of the pore-space in an argillaceous material are thus a key factor in understanding the sealing capacities, coupled flow and capillary processes affecting the transport of radionuclides inside the material. Important parameters to access and evaluate those properties are pore-size, -shape and connectivity of pores as well as orientation of pores.

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2 Sample description In this study three different samples of Boom Clay were investigated and compared with regard to their porosities.

All samples come from the 'Mol-1 borehole' at the Mol-Dessel reference site for radioactive waste disposal in Belgium, but from different depth and thus different layers and horizons of the Boom Clay formation. The detailed sample description is given in sections below.

Main difference between the samples investigated are their grain-size distributions (Figure 1).

Figure 1: Grain-size distributions of the samples investigated in the present study (data provided by SCK-CEN).

2.1 Sample 1, EZE55, HADES-level Sample 1 (EZE55, Core 77c122, Bed 90) is from around 225-226 meters depth (225,59-225,69 m), from the level of the HADES-research laboratory. It was chosen for representative and comparability reasons, since most investigations on Boom Clay so far have been carried out on samples originating from this layer. The horizon is part of the 'Putte Member' and the sample is very fine-grained, containing 51 % clay minerals to 49 % of non-clay minerals.

2.2 Sample 2, EZE52, 'clay-poor', coarse-grained sample Sample 2 (EZE52, Core 48c, Bed 114) stems from around 197 meters depth (197,10-197,35 m) and is therefore part of the so called 'Transition Layers'. Containing only 34 % clay minerals, it can be called 'clay-poor', compared to average Boom Clay. It contains a very high amount of non-clay minerals (66 %) and it is much more coarsely grained than the other two samples investigated in this study.

0,00

10,00

20,00

30,00

40,00

50,00

60,00

70,00

80,00

90,00

100,00

0,69 0,98 1,38 1,95 2,76 3,91 5,52 7,81 11,05 15,62 22,10 31,25 44,19 62,50 88,39

Perc

enta

ge sm

alle

r tha

n

Grain-Size (diameter) [μm]

Grain-Size Distributions of investigated samples

Sample1 (EZE55)_HADES level

Sample 2 (EZE52)_coarse-grained

Sample 3 (EZE54)_fine-grained

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2.3 Sample 3, EZE54, 'clay-rich', fine-grained sample Sample 3 (EZE54, Core 65c, Bed 100) represents the most fine-grained layer of the EZE-sample series. It contains a very high amount of clay minerals (61 %) and is therefore poor in non-clay minerals (39 %). It stems from the base of the ‘Transition Zone layers’, from a depth of around 213-214 meters (213,59-213,69 m). Samples 2 and 3 were chosen, because they represent end-members of Boom Clay, both from a mineralogical as well as grain-size point of view.

3 Methods

3.1 Sample preparation (cutting, drying and polishing/BIB cross-sectioning) The samples investigated in this study were provided by SCK-CEN in core-like pieces of 1 to 3 cm thickness and 10 cm diameter. They were not treated in any special way after coring and just kept in a fridge at a constant temperature of 8 °C.

For broad Ion-Beam cross-sectioning (BIB) sample pieces of ~ 1 cm length, 0.5 cm width and 0.2-0.3 cm thickness are needed. For cutting of these sub-samples a razorblade was used.

Before cross-sectioning the sub-samples have to be dried, because otherwise the vacuum inside the BIB would lead to a very fast removal of water inside the samples, causing damage to the samples and possibly inducing the development of drying-cracks. Drying of sub-samples is carried out for 48 hours in an oven at 60 °C. Afterwards the samples are glued onto especially designed sample holders, suitable for BIB-cross-sectionig and SEM-imaging. Furthermore, to optimize the quality of the produced BIB cross-sections, sub-samples are prepolished before BIB cross-sectioning, using carbide sand-papers of grain-sizes between 6.5 to 30 μm (average grain-size).

Figure 2: Sub-sample, mounted onto sample holder for BIB cross-sectioning and SEM-imaging.

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Figure 3: Schematic sketch of a sub-sample piece during cross-sectioning with BIB.

Figure 3 shows a schematic sketch of the principle of BIB cross-sectioning. Since BIB cross-sectioning uses a large beam, a shielding plate is placed on top of the sample surface to produce flat cross-sections. In the present study a 'JEOL SM-09010' stand-alone Argon beam cross-section polisher (Figure 4) is used, to produce high quality, damage-free surfaces for high-resolution SEM-imaging.

Figure 4: Stand alone 'JEOL SM-09010' cross-section polisher.

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All samples are cut in such a way that BIB cross-sections are oriented perpendicular to the bedding of the samples. BIB cross-sectioning is performed at 6 kV and ~ 150 μA for 7,75 hours. Resulting cross-sections typically cover areas of ~ 2 mm2 (Figure 6, 7 and 8). Before SEM-imaging the samples are coated with a gold layer to prevent them from charging.

3.2 SEM-imaging For imaging pore morphologies, a state of the art Scanning Electron Microscope (SEM, ZEISS – supra 55, Figure 5) is used. The microscope can reach a magnification of 1,000,000-times, but for investigations of pore-sizes and pore morphologies lower magnifications were chosen, thus enabling imaging of large enough sample areas to get representative and statistically significant results in a practicable period of time.

To identify mineralogical compositions of cross-sections, BSE-images are combined with EDX-maps, giving qualitative data and semi-quantitative analysis, respectively. For analysis of pore microstructures SE2 and SE in-lens detector based images are used.

Figure 6 to 8 show BSE- and SE2-overviews of the produced cross-sections under the electron microscope, using low magnifications.

Figure 5: ‘Zeiss supra55’ scanning electron microscope at RWTH Aachen University, used for imaging of

porosities in Boom Clay.

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Figure 6: BSE-overview of Sample 1 (EZE55, HADES-level).

Figure 7: SE2-overview of Sample 2 (EZE52, coarse-grained sample).

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Figure 8: SE2-overview of Sample 3 (EZE54, fine-grained sample).

For BSE-imaging a typical acceleration voltage of 20 kV is used, whereas for SE2-imaging 5 kV are sufficient; for EDX-measurement acceleration voltages between 10-12 kV were chosen. The working distance was set to ~ 7 mm for the SE2- and BSE-imaging and to ~ 9 mm for EDX-analysis.

For pore analysis, to image large enough and representative sample areas and at the same time resolve as many details as possible, SE2 and BSE-images are taken at high magnification (Chart 1) as part of mosaics with an overlap of 30 % and stitched together afterwards semi-automatically; Chart 1 illustrates the relation between magnification, resulting pixel dimension and resolution, as well as the size of the area covered by one image and the number of images needed to cover an area of 10,000 μm2, at chosen magnification; single images are stored with a resolution of 1024x768 Pixels.

As a good compromise between resolution, imaged area and processing time, a magnification of 30,000-times was chosen for SE2-imaging and 6,000-times for BSE-imaging. Smallest pores resolvable in the SE2-images were thus around 10 nm in size (1 Pixel = 9.8 nm) and the resolution was ~ 102 Pixels per μm, whereas in BSE-images the pixel dimension was ~ 50 nm (1 Pixel = 48.9 nm) and the resolution ~ 20 Pixels per μm. For statistical analysis of porosities 300-700 SE2-images were taken of each cross-section, displaying areas between 10,000 (Sample 3) and 25,000 μm2 (Samples 1 and 2). Higher magnifications (40,000-70,000-times) were chosen to image single pores with a better resolution for investigation of pore morphologies (see section Pore morphologies: "How pores look like"). For mineralogical analysis, 100-200 BSE-images were taken of each sample, imaging areas between 80,000 (Samples 1 and 3) and 160,000 μm2 (Sample 2) in size.

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Magnification Pixel

dimension [nm]

Image dimensions

[μm]

Resolution [Pixels/μm]

Area covered by one image

[μm2]

Number of images needed to cover

10,000 μm2 6000x 48.9 50.07

37.56 20 1880.52 5.32

10.000x 28.1 28.77 21.58 36 620.97 16.1

30.000x 9.8 10.04 7.53 102 75.53 132.4

80.000x 3.6 3.69 2.76 278 10.19 981.15

160.000x 1.8 1.84 1.38 556 2.55 3924.62

Chart 1: Relation between magnification, pixel dimension, resolution, area covered by one image and number of images needed to cover an area of 10,000 μm2. Single images are stored with a resolution of 1024x768 Pixels;

BSE- and SE2-imaging was performed at 6,000 and 30,000-times magnification, respectively.

3.3 Image-processing & Analysis 3.3.1 'Autopano' After SEM-imaging the set of images has to be stitched together. Stitching is performed semi-automatically using the software ‘Autopano’. Autopano can detect identical features in different images - therefore the overlap of 30 % is needed - and put the images together automatically. For areas, which lack significant characteristic features, user-intervention is necessary and images can be fit in manually.

3.3.2 'ArcGIS' For segmentation of pores, the software 'ArcGIS(10)' is used. Stitched images are loaded into ArcGIS-projects and the scale is calibrated. Segmentation of pores is done via digitizing the pore-boundaries. Segmented pores are then labeled according mineralogical phase in which they are detected. Pore areas and pore perimeters can be calculated directly in ArcGIS. Further statistical analysis is carried out using the MATLAB® open platform toolbox 'PolyLX' (Ondrej LEXA, Institute of Petrology and Structural Geology, Charles University, Prague, Czech Republic) for object-oriented grain-analysis and also Microsoft Excel.

3.3.3 Statistical Analysis - 'PolyLX' Statistical analysis of the mapped pores is performed using 'PolyLX'. The software provides a large amount of functions for the analysis as well as visualization of micro-structural data. Thus, once pores are segmented within ArcGIS, pore sizes, size distributions, orientations of pores and pore shape information can be calculated automatically. Figure 9 to 14 show segmented porosities of investigated sample areas, displayed with 'PolyLX'. Detailed description of the functions implemented in 'PolyLX' is given in the reference manual at http://petrol.natur.cuni.cz/~ondro/polylx:home.

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Figure 9: Pores in clay matrix of sample1, HADES-level, as displayed by PolyLX.

Figure 10: Pores in clay of sample2, coarse grained sample, as displayed by PolyLX.

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Figure 11: Pores in clay matrix of sample 3, fine-grained sample, as displayed by PolyLX.

Figure 12: Pores within quartz grains in sample 2, as displayed by PolyLX.

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Figure 13: Pores inside mica, sample 2, as displayed by PolyLX.

Figure 14: Pores in other ‘non-clay minerals (e.g. rutile) of sample 2, coarse-grained sample, as displayed by PolyLX.

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4 Results

4.1 Mineralogical (qualitative to semi-quantitative) analysis from BSE-images and EDX-mapping

In all samples, based on the interpretation of BSE-images and EDX-measurement, the following mineral phases were identified:

1. a very fine-grained clay matrix, 2. quartz grains of up to 50 μm in size (in the two finer grained samples, 1 and 3) and up to

80 μm size in the coarser grained sample (2), 3. feldspar grains (orthoclase and plagioclase) of 15-20 μm in size for the fine-grained

samples (1 and 3) and 30-40 μm in size for the coarser grained sample (2), 4. framboidal pyrite aggregates of 5-10 μm size for the fine-grained and 10-20 μm for the

coarse-grained sample, 5. mica-sheets of 20-50 μm length for fine-grained samples and 150-250 μm length in the

coarse-grained sample, 6. few Ca-Mg-rich grains (probably dolomite), 7. few Ti-rich grains (anatase or rutile), between 5-20 μm in size, 8. another Fe-rich grains, identified as siderite, 9. and very few apatite grains.

The BSE-overviews, stitched together from between 100 (Sample 3) to 200 (Sample 2) images taken at a magnification of 6,000-times and a resolution of ~ 20 Pixels per μm, are shown in Appendices 1-3.

4.2 SE-imaging: Outline and analysis of porosities

4.2.1 Pore morphologies: "How pores look like" Based on micro-structural observations at pore scale, all samples show qualitatively comparable pore morphologies.

Within the clay matrix pores are either elongated between similarly oriented clay-sheets or triangular to crescent-shaped in saddle-reefs of folded sheet of clay (Figure 15 and 16). Elongated pores have typically a much lower pore size (< 300 nm) than triangular or crescent-shaped ones (< 1 μm). Inside quartz- and feldspar-grains only few pores there were found; these were usually small (< 500 nm size) and often isometric (Figure 17). Pores inside fossils are typically < 500 nm in size and show rounded edges (Figure 18). Morphologies of pores inside framboidal pyrite aggregates are controlled by the pyrite grains’ arrangements. Pyrite single grains have typically short wavelength serrated edges and the boundaries of the pores are thus serrated as well, with pseudo-round to edgy shapes and sizes < 500 nm (Figure 19-21). Pores in mica are elongated with very high aspect ratios and typically < 200 nm thickness along minor axis (Figure 22+23). Typical pores inside siderite are shown in Figure 24; they have sharp edges are crescent-shaped with typical sizes in the range of a few 100 nm. Furthermore several tens of micrometers long and > 1 μm thick cracks are identified by tear-like morphology and angular-sharp edges (see Figure 25).

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At the scale of our investigations, there might exist a connection between visible porosity inside the clay-matrix to pores within pyrite and fossils, but since pores in pyrite and fossils account for only 1.34 % of the total porosity in Sample 2 and weren’t found at all in the other two samples investigated, this doesn't lead to a significant amount of interconnected pore-space in samples investigated. Visible pores within the clay-matrix, quartz, feldspar and mica weren’t found to be connected at all.

Figure 15: Typical elongated and triangle- to crescent-shaped pores within the clay matrix of Sample 1, HADES-

level.

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Figure 16: Typical elongated and triangle- to crescent-shaped pores in the clay matrix of Sample 3, fine-grained

sample.

Figure 17: Typical pore inside feldspar-grain of Sample 1, HADES-level.

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Figure 18: Typical pores inside a fossil in sample 2, coarse-grained sample.

Figure 19: Typical pores inside framboidal pyrite aggregate in sample 1, HADES-level.

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Figure 20: Zoom-in to typical pores inside framboidal pyrite aggregate in sample 1, HADES-level (detail of Figure

19).

Figure 21: Pores inside pyrite aggregate in sample 3, fine-grained sample.

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Figure 22: Typical elongated pores in mica-grain in sample 1, HADES-level.

Figure 23: Typical elongated pores inside mica-grain in sample 2, coarse-grained sample.

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Figure 24: Pores inside siderite-mineral in sample 2, coarse-grained sample.

Figure 25: Crack in sample 3, fine-grained sample.

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4.2.2 Statistical analysis of visible porosity For Sample 1 (EZE55, HADES-level) a total 4862 pores were detected, 4851 of them in the clay-matrix (99.77 %) and only 11 (0.23 %) in ‘non clay minerals’, in this case quartz. The analyzed area was 672,3 μm2 in size and pores are accounting for 112.7 μm2, thus 16.76 % of the area. Pores within the clay-matrix make up 99.95 % of the porosity and pores in ‘non-clay minerals’ only 0.05 %; because of this, pores in ‘non-clay minerals’ are considered as negligible in sample 1.

For Sample 2 (EZE52, coarse-grained sample) a total 8218 pores were detected, in an area of ~ 6775 μm2 size. 83.6 % of them (6872 pores) in the clay-matrix and 16.4 % (1346 pores) in ‘non-clay minerals’. A more detailed summary of pores outlined to date in sample 2 is given in Chart 2. The total sum of pore-areas in sample 2 is 1420 μm2 and thus accounts for 20.96 % of the analyzed area, leading to a total visible porosity of 20.96 % in Sample 2.

5871 pores were detected in Sample 3 (EZE54, fine grained sample), with 99.3 % of them (5828 pores) being in the clay matrix and only 0.7 % (43 pores) in ‘non-clay minerals’ (quartz). Thus, again, pores in non-clay minerals are negligible. In a total analyzed area of 952.5 μm2 (see appendix 9), pores are accounting for ~ 140 μm2, thus 14.7 % of the area. Pores in clay make up 99.1 % of the total pore area and pores in quart 0.9 %.

According to the to date results, sample 3 is the least porous and sample 2 the most porous one, of the samples investigated in this study.

Chart 2 gives a summary of all pores outlined to date in areas displayed in Figure 9 to 14; note that these areas are a little bit larger and different ones, than those used for analysis of total porosities, as described above.

Total number of pores outlined Percentage (%) Sample 1 (EZE55, HADES level) 5134 Clay 99.84 8 Quartz 0.16 Total number of pores (Sample 1) 5142 Sample 2 (EZE52, coarse grained sample) 10962 Clay 81.19

67 Fossils 0.5

220 Other NCM (Anatase/Rutile) 1.63

113 Pyrite 0.84 196 Feldspar 1.45 1180 Quartz 8.74 764 Mica 5.66 Sum of pores in non-clay minerals (S2) 2540 18.81

Total sum of pores (Sample 2) 13502 Sample 3 (EZE54, fine-grained sample) 9045 Clay 99.71

26 Quartz 0.29 Total sum of pores (Sample 3) 9071 Total number of pores outlined 27715 Chart 2: Summary of pores outlined to date in different investigated samples and different mineral phases.

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Appendices 4 - 9 show stitched SE-images of sample areas investigated in detail for evaluation of visible porosities. In Appendices 4 - 6 overviews of entire imaged areas are shown and in appendices 7 - 9 the areas used for pore-segmentation and determination of total visible porosities. Dimensions of analyzed areas are given in Chart 3.

Pixel Dimension [nm]

Area analyzed [Pixels] Size of analyzed Area [μm2]

Sample 1 9,8 2355x2972 672 Sample 2 9,8 8557x8244 6775 Sample 3 9,8 2791x3554 952,5

Chart 3: Size of areas analyzed for comparison of porosities in different investigated samples with a pixel dimension of 9.8 nm (SE2-imaging at a magnification of 30,000-times).

4.2.3 Analysis of pore-sizes in clay matrix To date, only pores detected in the clay-matrix were analyzed with regard to their size and size-distribution. Please notice that pore-segmentation is still ongoing and preliminary results are based on areas large with regard to individual pore-sizes, but still do not cover representative elementary area (REA), yet. Chart 4 gives a detailed summary of the results from statistical analysis of pore-sizes distributions and in Figure 26 to 28 the pore-size distributions of pores within clay-matrices are shown, for all three samples investigated.

Sample name Sample 1 (EZE55,

HADES-level)

Sample 2 (EZE52,

coarse-grained)

Sample 3 (EZE54,

fine-grained)

Number of pores outlined in clay matrix 5134 10962 9043

Maximum pore-size (equivalent diameter in nm)

1844 19009 2080

Average pore-size (nm) 112 145 107 Mode 24 24 30 Median 68 75.5 63 Standard Deviation 131 410 137 Variance 17031 168399 18875 Skewness of distribution 4 21 5

Chart 4: Results of pore-size analysis for pores within clay matrix.

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Figure 26: Size distribution of pores inside the clay matrix of Sample 1 (EZE55, HADES-level).

Figure 27: Size distribution of pores detected in the clay matrix of Sample 2 (EZE52, coarse-grained sample).

0

100

200

300

400

500

600

700

5 245

485

725

965

1205

1445

1685

Freq

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y

equivalent diameter (nm)

Sample1_HADES level

0

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400

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1400

5 245

485

725

965

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Sample2_coarse-grained

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Figure 28: Size distribution of pores found and segmented within the clay matrix of Sample 3 (EZE54, fine-

grained sample).

Pores inside the clay-matrices of all three samples investigated follow a log-normal size-distribution, with a maximum in pore-size frequencies at around 35 nm for Samples 1 and 3 and around 45 nm for Sample 2. The average pore-size (equivalent diameter) is 112 nm for Sample 1, 145 nm for Sample 2 and 107 nm for Sample 3, showing that pore-size distribution of Sample 2 is slightly shifted to larger values, compared to Samples 1 and 3. Sample 3 has the smallest average pore-size and Sample 2 the largest. Sample 1 ranges in between the other two. The median of pore-size distributions is more similar for all samples: ~ 68 nm for Sample 1, 75.5 nm for Sample 2 and ~ 63 nm for Sample 3, showing that distributions of pore-sizes in clay-matrix are similar for all samples investigated. In Samples 1 and 3 all pores within the clay-matrix are smaller than 2 μm (2.08 μm for Sample 3 respectively), whereas in Sample 2, pores of up to ~ 20 μm (equivalent diameter) were found inside the clay-matrix, affecting the average pore-size. The median is much less affected by outliers and thus much more similar for all three samples investigated. It is also noticeable that the skewness of the distribution of pore-sizes is much higher for Sample 2, than for Samples 1 and 3, describing the much wider distribution of pore-sizes within clay-matrix for Sample 2.

In all three samples 90 % of the pores detected inside the clay-matrices are smaller than 250 nm (equivalent diameter). Figure 29 gives a closer look to pore-sizes smaller than 250 nm, illustrating that pore-size distributions are quite similar for pores below 250 nm size, in all three samples investigated.

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Figure 29: Size-distributions of pores < 250 nm (equivalent diameter), detected inside clay-matrices of

investigated samples.

4.2.4 Extrapolation of pore-size distributions Power law-distribution has also been applied, to describe pore-size distributions:

Frequency = C / radius D (1) where Frequency is the number of pores < the characteristic linear dimension radius (i.e., here, the disk equivalent radius), D and C a constant of proportionality. In a log-log plot of Frequency vs. radius, equation (1) represents an equation of line (i.e. equation (2)), where D – the power-law exponent or fractal dimension – is the slope of the line and log(C) is the frequency at origin:

log (frequency) = - D * log (radius) + log (C) (2) In practice, systematic deviations from the power-law are usual and the validity-range of the power-law is defined by the length scales rmin and rmax where data form line. At low length- extremes, the data in the present study are limited by the microscope’s resolution and the picture’s noise and at high length-extremes by the fact that investigated areas are not representative and endlessly (giving rmin and rmax resprectively). For all three samples investigated in this study, pore-size data could be fitted using the above described power-law for rmin - rmax range between 30 - 100 nm. Fits give power-law exponents between 1.7 for Samples 1 and 3 and 1.9 for Sample 2. Assuming that all other non-visible pores in the clay-matrix follow the same power-law, fits can be used to extrapolate measured porosities to smaller pore-sizes – down to 1 nm in equivalent pore-radius – and to larger pore-sizes – up to the point (equivalent pore-radius) where the fit

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crosses zero for frequencies. This leads to calculated total porosities between 3.2 % for Sample 2 (pore-sizes ranging from 1-400 nm equivalent radius), 29 % for Sample 3 (pore-sizes between 1 and 540 nm equivalent radius) and 45 % for Sample 1 (pore-sizes between 1 and 630 nm equivalent radius). Figure 30-32 display log-log pore-size distribution plots, including extrapolated power-law fits for pore-size range between 30-100 nm (ER - equivalent pore radius).

Figure 30: Log-log plot of fitted pore-size distribution of Sample 1 (EZE55, HADES-level); range of fitting (rmin-rmax,

respectively): 30-100 nm (ER - equivalent pore radius); Bin-size (pore-radius) = 1nm.

Figure 31: Log-log plot of fitted pore-size distribution of Sample 2 (EZE52, coarse-grained); range of fitting (rmin-

rmax, respectively): 30-100 nm (ER - equivalent pore radius); Bin-size (pore-radius) = 1nm.

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Figure 32 Log-log plot of fitted pore-size distribution of Sample 3 (EZE54, fine-grained); range of fitting (rmin-rmax,

respectively): 30-100 nm (ER - equivalent pore radius); Bin-size (pore-radius) = 1nm.

4.2.5 Analysis of pore orientations Pore orientations were measured according to the pores longest axis, in the following (Figure 33 to 35) orientations of pores within the clay-matrix of the three samples investigated, are plotted in 'rose diagrams'. In Samples 1 and 3 most pores are oriented ~ 90° from the vertical axis, thus parallel to the bedding of samples. In Sample 2 the orientations of pores within the clay-matrix vary more, but still show a preferred orientation ~ 60° from the vertical axis, thus ~ 30° from the horizontal, which would mean an orientation parallel to the bedding of samples.

Figure 33: Orientation of pores in the clay-matrix of Sample 1 (EZE55, HADES level).

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Figure 34: Orientation of pores in the clay-matrix of Sample 2 (EZE52, coarse-grained).

Figure 35: Orientation of pores in the clay-matrix of Sample 3 (EZE54, fine-grained).

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5 Conclusions and outlook Due to the high quality surfaces prepared by BIB-milling, a combination of BIB cross-sectioning and SEM-imaging allows for unprecedented insight into pore morphologies down to the scale of a few nm in pore-size. This was not achieved by any other conventional approach before. One significant advantage of BIB-milling is the preparation of true 2D, flat cross-sections, which is an essential requirement for microstructural investigations based on stereology (Underwood, 1970).

In the course of this study, a total 27,715 pores were mapped and analyzed with regard to their size, shape and orientation. A special interest was addressed to the pore space borne by the clay matrix. It appears that pore characteristics of pores within the clay-matrix and < 250 nm equivalent pore diameter (ED), are quite similar for all three samples investigated. In the range between 60-200 nm (ED) pores follow a power-law size distribution with similar power-law exponents, in good agreement with Desbois et al. (2009). The power-law exponents are 1.7 for the two finer grained samples (1 and 3) and a little bit higher (~ 1.9) for the coarser grained sample (Sample 2). This suggests that Boom Clay is microstructurally homogeneous for pores within the clay matrix and < 250 nm (ED).

From our microstructural observations, the inter-phase connectivity is low and we assume in the following that the effective porosity is mainly borne by the clay matrix. Therefore resulting effective and visible porosities of investigated samples are similar (17 % for Sample 1, ~ 18 % for Sample 2 and ~ 15 % for Sample 3). Comparing these values to results from literature, it shows that porosities measured in this study are lower than expected. In ‘Boisson’ (2005) and for comparable samples, water content porosity measurements give ~ 36 % porosity and values from ‘Mercury porosimetry’ range between 35 - 40 %. Explanations for this discrepancy are at first, the magnification chosen in this study for SE2-imaging, since it fixes the resolution of pores to 10 nm (minimum resolvable pore-size size), whereas it is known from TEM (transmission electron microscopy) measurements that pores smaller than this do exist in Boom Clay in large amounts. Furthermore the limited area of investigation leads to an underestimation of bigger pores in the analyzed sample areas.

Assuming that calculated power-law distributions hold true for pore-sizes outside the range of 60-200 nm (ED), total effective porosities could be estimated. This approach was used to predict the contribution of both non-visible, small pores, as well as large pores, not measured in statistically representative amounts, due to the too small analyzed sample areas. Unfortunately, this approach is not giving satisfying results: 45 % porosity for Sample 1 (HADES-level), only 3.2 % for Sample 2 (coarse-grained sample) and ~ 29 % porosity for Sample 3 (fine-grained sample). Whilst the results for Samples 1 and 3 are close to the expected range, the result for Sample 2 is far away from it. One explanation for this is the much higher amount of large pores (>> 250 nm (ED)), present in Sample 2. These big pores preferentially develop at the interface between clay-matrix and clast-grains and are the bigger, the larger the grain-size of the 'non-clay minerals' is.

Sample 2 contains a much higher amount of 'non-clay minerals' than the other two samples investigated and furthermore has a much larger average grain-size. Since large pore-sizes are not reached by the extrapolation of fits to a range between 30 and 100 nm (ER), the 'fitting and extrapolation' procedure fails for Sample 2. To get hold of this problem, much larger sample areas would have to be investigated and at best investigations would have to be carried out on different scales, to get input data over a wider scale-range of pore-sizes. To create a more complete picture of the porosity in Boom Clay in general, many more samples, from different Boom Clay horizons, mineral compositions and grain-size distributions would have to be investigated and again investigations would have to be carried out on representative elementary

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areas (REA). Furthermore a mathematical description has to be found to significantly describe pore shapes and enable a systematic, statistical analysis of pore morphologies. Moreover a more clear differentiation between inter and intra-phase porosity and porosity in different minerals is needed.

Another still unclear factor, to be investigated in the future, is the impact of the sample drying procedure on the clays microstructure and the pore morphologies. So far it is not clear, if big cracks, found in all samples investigated, develop due to the drying of samples or if they are the result of mechanical exposure of samples. To check this, samples should be investigated in ‘in-situ’, wet (saturated) conditions, using cryo-BIB-SEM. At last, as well anisotropy of sample porosity characteristics should be checked, by production of cross-sections in all 3 dimensions, oriented perpendicular to each other.

6 References

[1] Boisson, J.Y. (2005) Clay Club Catalogue of Characteristics of Argillaceous Rocks, OECD/NEA/RWMC/IGSC (Working Group on Measurement and Physical Understanding of Groundwater Flow through Argillaceous Media). Report NEA No. 4436. OECD/NEA Paris, France (p. 72).

[2] Desbois, G., Urai, J.L. and de Craen, M. (2010) In-situ and direct characterization of porosity in Boom Clay (Mol site, Belgium) by using novel combination of ion beam cross-sectioning, SEM and cryogenic methods - Motivations, first results and perspectives. External report (SCK-CEN-ER-124) 10/MDC/P-26, (SCK-CEN), Boeretang, Antwerpen/Mol, Belgium.

[3] Desbois, G., Urai, J.L., Houben, M.E. and Sholokhova Y. (2010) Typology, morphology and connectivity of pore space in claystones from reference site for research using BIB, FIB and cryo-SEM methods. EPJ Web of Conferences, 6, 2205. DOI: 10.1051/epjconf/2010062205.

[4] Desbois, G., Urai, J.L., Kukla, P.A. (2009) Morphology of the pore space in claystones - evidence from BIB/FIB ion beam sectioning and cryo-SEM observations eEarth, 4, 15-22.

[5] Underwood, E.E. (1970) Quantitative Stereology. Addison-Wesley, Massachusetts, U.S.A.

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Appendices 1: BSE-image of Sample 1 (EZE55, HADES-level) 2: BSE-image of Sample 2 (EZE52, coarse-grained sample) 3: BSE-image of Sample 3 (EZE54, fine-grained sample) 4: SE2-overview of Sample 1; area imaged: 54 x 69 μm = 3840 μm2 5: SE2-overview of Sample 2; area imaged: 166 x 151 μm = ~ 25,000 μm2 6: SE2-overview of Sample 3; area imaged: ~ 124 x 84.3 μm = ~ 10,445 μm2 7: Sample 1 - after outline of porosity; area analyzed: 23.1 x 29.1 μm = 672.3 μm2 8: Sample 2 - after outline of porosity; area analyzed: ~ 84 x 81 μm = ~ 6775 μm2 9: Sample 3 - after outline of porosity; area analyzed: 27.4 x 34.8 μm = 952.5 μm2

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Appendix 1: BSE-image of Sample 1 (EZE55, HADES-level). Rectangles: Areas used for SE2-imaging and outlines of porosities, pore segmentation (Appendices 4 and 7) respectively.

Appendix 2: BSE-image of Sample 2 (EZE52, coarse-grained sample). In rectangle is shown the area, used for SE2-imaging, as well as outline and analysis of porosities (see Appendices 5 and 8 respectively) (black areas are due to missing images from SE2-imaging).

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Appendix 3: BSE-image of Sample 3 (EZE54, fine-grained sample). In rectangles are shown the areas used for SE2-imaging (Appendix 6) and pore segmentation as well as statistical analysis of porosities (Appendix 9).

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Appendix 4: SE2-overview of Sample 1, displaying an area of ~ 3840 μm2 in size. The image has been stitched together from ~ 110 high magnification (30,000-times) images. In rectangle is shown the area, which was used for outline of porosities (Appendix 7).

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Appendix 5: SE2-overview of Sample 2, stitched together from ~ 700 high magnification (30,000-times) images, displaying an area of ~ 25,000 μm2 in size. The rectangle shows the area, which was used for detailed analysis and outline of porosities (Appendix 8) (black areas are due to missing images from SE2-imaging).

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Appendix 6: SE2-image of Sample 3, stitched together from ~ 300 high resolution (magnification of 30,000-times) images and displaying an area of ~ 10,445 μm2. In the black rectangle is shown the area, which was used for detailed analysis of porosities (see Appendix 9) (black areas are due to missing images from SE2-imaging).

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Appendix 7: Outline of porosity in Sample 1 (HADES-level). 4862 pores were outlined in this area of 672.3 μm2 size, showing an apparent porosity of 16.76 %, with 99.95 % of the pores being within the clay-matrix.

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Appendix 8: Outline of porosity in Sample 2 (coarse-grained sample). 8218 pores were outlined in this area of 6775 μm2 size. ~ 84 % of them were found inside the clay-matrix, ~ 9 % in quartz, ~ 4 % in mica, ~ 1 % in feldspar and ~ 2.5 % in other 'non-clay minerals'. The total sum of pore-areas lead to an apparent porosity of 20.96 % for this sample (black areas are due to missing images from SE2-imaging).

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Appendix 9: Apparent porosity in Sample 3 (fine-grained sample). The analyzed area is 952.5 μm2

in size and 5871 pores were outlined and analyzed to date in this area. ~ 99 % of them in clay, leading to a total apparent porosity of 14.7 % for Sample 3.