Becker, J.

Giger, S. B.

Keller, Lukas

Marschall, P.

ZHAW Zürcher Hochschule für Angewandte Wissenschaften

MICROSTRUCTURAL INSIGHTS INTO THEPETROPHYSICAL CHARACTERISTICSOF INDURATED CLAYSP. MARSCHALL1,, L. KELLER2, S.B. GIGER1, and J. BECKER11National Cooperative for the Disposal of Radioactive Waste,5430 Wettingen, Switzerland2ZHAW/ICP, Wildbachstrasse 21, 8401 Winterthur, Switzerlande-mail: paul.marschall@nagra.chCore samples from the Opalinus Clay, an indurated clay formation of Jurassic age, weresubjected to comprehensive microstructural investigations: transmission electronmicroscopy, focused ion beam nanotomography, and X-ray computed tomography.Digital representations of the microstructure were derived from the tomographicimages by segmentation and 3D reconstruction of the geomaterial on a grid ofregular voxel elements with a typical edge length of as little as 10 nm and amaximum size on the order of 10003 voxels. Pertinent geotechnical properties werederived on the microscopic scale using state-of-the-art image-processing procedures.The results were compared with results from geotechnical laboratory characterizationmethods such as mercury intrusion porosimetry, nitrogen adsorption/desorptionmeasurements, grain-size analysis, X-ray diffraction, and triaxial testing. Good agree-ment was achieved between microscopic and macroscopic characterization methods.The macroscopic (poro-elastic) deformation behavior of the Opalinus Clay was alsoreproduced reasonably well by geomechanical modeling on the microscopic scale.The cross-anisotropic stiffness of the rock mass is thus caused partly by preferred orien-tation of pore structures along bedding. In addition, new insight was gained with regardto the role of small mesopores (2–10 nm), essentially controlling the macroscopicdeformation behavior of indurated clays.1. IntroductionIndurated clays, such as claystones and shales, are considered to be potential host-rockformations for the disposal of radioactive waste. Favorable features are their excellenthydraulic barrier function and their retention capacity for dissolved radionuclides,together with a significant self-sealing capacity. The peculiar petrophysical character-istics of this class of fine-grained argillaceous sediments on the macroscopic scale orig-inate essentially from their microscopic structure, which was formed by diageneticprocesses under the impact of the depositional environment, the stress experienced,and the temperature history. The mineralogical composition and bonding of thegrains, as well as the geometry and connectedness of the intergranular pore spacearound the solid skeleton, are of utmost importance for the hydro-mechanical behaviorof indurated clays at the macroscopic scale, expressed essentially in terms of porosity,hydraulic conductivity, poro-elastic rock properties (e.g. drained and undrained elastic# Copyright 2016, The Clay Minerals SocietyDOI: 10.1346/CMS-WLS-21.14The Clay Minerals Society Workshop Lectures Series, Vol. 21 (2016), Chapter 14, 191–198moduli, Biot- and Skempton-coefficient), and rock strength (drained and undrainedstrength parameters). The present study is dedicated to the derivation of scaling pro-cedures, allowing for the inference of macroscopic poro-mechanical properties from adetailed knowledge of microstructural characteristics of clay-rich geomaterials.2. Geotechnical characteristics of the Opalinus ClayOpalinus Clay in the Molasse Basin and in the Jura Fold and Thrust Belt of northern Swit-zerland has been proposed as a candidate host rock for a radioactive waste repository inSwitzerland. It is a moderately to heavily over-consolidated claystone (1.5  OCR  5)(OCR – overconsolidation ratio, i.e. maximum burial depth/recent burial depth) with aclay-mineral content of between 40 and 75%, a quartz content of between 10 and 30%,and a carbonate content of between 5 and 35% (Figure 1a). The porosity displays moder-ate depth dependence with typical values of 8–16% (corresponding to an overburden ofseveral hundred meters). Hydraulic conductivity derived from laboratory and in situtesting exhibits typical values of between 10212 and 10214 m/s and conforms well toempirical permeability correlations, drawing on measurements of porosity and clay-mineral content (Yang and Aplin, 2010). Based on grain-size distribution, the OpalinusClay is classified as a clayey (and sandy) siltstone (Figure 1b). The consistency behavior(Atterberg limits) provides additional insight into the intrinsic geomechanical propertiesof the (destructured) geomaterial, representing a qualitative indicator for the frictionangle. As shown in Figure 1c, the plasticity index plots broadly along the A-line of theplasticity chart, which separates clayey from silty soils. The values reported for liquidlimit, wL, and plasticity index, PI, range from30 to 50% and10 to 25%, respectively.Drawing on empirical correlations by Terzaghi et al. (1996) and Stark and Eid (1994),the friction angle of the Opalinus Clay can be bracketed in the range between 20 and358, which is in good agreement with findings from geomechanical testing.In terms of mechanical behavior, the Opalinus Clay displays features characteristic ofboth a (stiff) soil and a (weak) rock. One-dimensional (oedometric) consolidation testingshows a significant volume reduction in the stress range above the preconsolidationstress (20–25 MPa for core samples in northern Switzerland). Results of ‘UnconfinedCompressive Strength’ tests suggest a weak to medium-strong rock with distinct aniso-tropy (loading parallel and normal to bedding: UCSp,n ¼ 12–40 MPa; loading obliqueto bedding: UCSz ¼ 5–20 MPa, Figure 1d). Strain softening and development of dis-crete shear surfaces are typically observed in undrained triaxial testing. The (undrained)Young’s moduli were found to be stress dependent and anisotropic for values measuredparallel to and normal to bedding (Ep ¼ 5–20 GPa, En ¼ 2–10 GPa).3. Results and discussionMicrostructural investigations of Opalinus Clay samples from a deep geothermal well innortheastern Switzerland (located near the village of Schlattingen) and from the MontP. Marschall et al.192Figure 1. Classification of the Opalinus Clay in terms of mineralogy, grain size, and consistency: (a) ternary diagram of principal mineralogical components;(b) USCS classification of fine-grained sedimentary rocks; (c) plasticity chart (SN 670 004-2a); and (d) Unconfined Compressive Strength after Gens (2013).Petrophysicalcharacteristicsofinduratedclays193Terri underground laboratory (located in the Jura Fold and Thrust Belt) were conductedby Keller et al. (2013a, 2013b). The investigations consisted of multi-scale characteriz-ation of the samples by X-ray computed tomography (XCT), focused ion beam nanoto-mography (FIB-nt), and scanning transmission electron microscopy (TEM), followed bysegmentation and reconstruction of the rock fabric (Figure 2). The reconstructed digitalrepresentation of the microstructure with a maximum size on the order of 10003 voxelswas then subjected to quantitative data analyses and model simulations. The analysesincluded: estimations of the pore-size distribution, water-retention behavior, andgrain-size distribution; the derivation of representative elementary volumes (REV) ofporosity (Keller et al., 2013a), clay content (Keller, 2015), pore connectivity, capillaryproperties, permeability on the basis of numerical channel network simulations (Kelleret al., 2013a), and impact of sand fraction on solute diffusion (Keller et al., 2015); and,finally, simulations of the undrained and drained poro-elastic properties.Figure 2. Microstructural characterization of Opalinus Clay: (a) image of a core sample which containscarbonate/quartz-rich and clay-rich layers at various scales; (b) X-ray tomography data of a thin slab fromthe sample and 3D reconstruction of the mineral components (green – quartz; orange – carbonates);(c) FIB-nanotomography and visualization of connected/unconnected pore space (green – connected porespace; orange – unconnected pores); and (d) scanning transmission electron microscopy (TEM) andvisualization of pore space.P. Marschall et al.1943.1. Porosity and pore-size distributionSamples from different lithofacies of the Opalinus Clay formation (sandy, shaley, andcarbonate-rich facies) were analyzed by FIB-nt and TEM. Statistical analyses of thesegmented 3D reconstructions by simple box-counting approaches allowed for thederivation of equivalent pore-size distributions (Figure 3a). Thus, REV values of poros-ity and pore connectivity (representing an indicator of permeability) were derived, usingthe conceptual framework of the local porosity theory (Kanit et al., 2003; see also Kelleret al., 2013a). The results of this geometric analysis (drawing on image-processingprocedures) were compared with classical geotechnical methods, such as mercuryintrusion porosimetry, water-retention measurements, and nitrogen adsorption/desorption measurements (BET – Brunauer et al., 1938). The different methods reveal,in a consistent manner, the distinct differences in the pore-size distribution of the threeFigure 3. Properties extracted from FIB-nt and XCT data: (a) calculated pore-size distributions (shaley,sandy, and carbonate-rich facies) and comparison with N2 adsorption data; (b) grain-size distributions ofOpalinus Clay samples from Mont Terri (WS-H borehole), derived from XCT image analysis and grain-size analysis, respectively; (c) estimated REV for clay mineral content; and (d) ratio between bulk andsolid Young’s moduli parallel (E11 and E22) and normal (E33) to bedding derived using numerical simulations.Petrophysical characteristics of indurated clays 195facies. The shaley facies contains mainly micro- and small meso-pores (,10 nm;definitions according to IUPAC, 1997). The resolved porosity in the clay matrix ofthe sandy facies is greater and the pores are larger when compared to the shaleyfacies. Larger intergranular pores can often be observed around the quartz and carbonategrains (note that the spatial resolution of FIB-nt is on the order of 10 nm and 5 nmfor TEM).3.2. Grain-size distribution and mineralogical compositionThe methods of sieving and sedimentation (clay, silt, and sand fractions in wt.%) arestandard geotechnical procedures for measuring the grain-size distribution of finesoils. The mineralogical composition of the geomaterial in terms of principal mineralo-gical components (clay, quartz, carbonates in wt.%) is typically determined by X-raydiffraction (XRD). Quantitative analyses of microstructural investigations with XCTand FIB-nt have been used as complementary approaches for the classification (seeFigure 1) and characterization of the Opalinus Clay with respect to mineralogy and gran-ulometry. Segmented 3D reconstructions of the samples investigated were analyzedusing the aforementioned image-processing procedures, aimed at deriving grain-sizedistributions and clay-mineral content with an independent measuring approach. Theresults of the geometric analyses were compared with classical geotechnical methods.The grain-size distribution of the core sample WS-H from Mont Terri derived bygrain-size analysis (Minon et al., 2010) was compared with the results of sampleBWS from the same borehole, deduced from statistical analysis of XCT-basedsample reconstruction, in Figure 3b. The different characterization techniques providedcomparable results, classifying the samples as (sandy) siltstone according to the schemein Figure 1a. On the basis of synchrotron XCT, the analysis of the REV of clay-matrixcontent of the three core samples OPA3, BD, and BWS is shown in Figure 3 (note, theREV is defined according to Kanit et al. (2003) as the minimum volume for which thevariance of the clay-matrix content determined for one realization is sufficiently small).The REV of the clay-matrix content depends heavily on the volume fraction. For a clay-matrix content of 68 vol.% and by accepting a relative error of 10% on the referencebulk clay matrix content, the size of the REV of sample OPA3 is 100 mm (Kelleret al., 2015). The mean clay content derived on the basis of XCT images conformsvery well with the results of mineralogical analyses of the same sample carried outusing classical XRD methods (cclay,XRD ¼ 62%; see also Keller et al., 2015).3.3. Deformation behaviorSub-volume samples of the reconstructed FIB-nt data sets were used in a geomechanicalFE-model to reproduce the macroscopic (poro-elastic) deformation behavior of the Opa-linus Clay. For a typical sample from the shaley facies, the relationship between porosityand Young’s/shear moduli appears to be a well known power-law relationship(Figure 3d). In addition, a polar anisotropy in elastic properties was revealed which isrelated to a preferred orientation of pore structures parallel to bedding. Elastic propertiesare nearly isotropic within planes parallel to bedding but are substantially differentP. Marschall et al.196perpendicular to bedding. In particular, with increasing porosity, the Young’s modulusthat accounts for stiffness perpendicular to bedding decreases much more when com-pared to those that account for stiffness within the bedding plane. Drainage of waterfrom the pores enhances this effect as it reduces substantially the stiffness perpendicularto bedding.With regarding to the application of a shear strain, the material also exhibits a differ-ent response when shear stress is applied in different directions. The shear modulus isgreater when shear strain occurs due to forces acting on a plane perpendicular tobedding but is less when the shear plane is parallel to bedding. The effect of drainageon shear moduli is minor and does not increase by much the anisotropic response toshear strain. Drainage of water reduces substantially the two Poisson’s ratios whichmeasure the ratio between strain parallel and perpendicular to bedding.The effects of pores on elastic properties were determined on the basis of small samplevolumes; as a result, the question is raised regarding the finite size error related to theelastic constants determined. In other words, the question was tackled regarding thesize of the REV of the elastic constants related to the clay matrix. Again, this questionwas answered using the statistical approach of Kanit et al. (2003). In the case of acubic FIB-nt sample with an edge length of 2.8 mm, an error of 6% was expectedfor a single realization. Note that in the case of several realizations, the error decreasedsubstantially. When elastic properties are considered, the sample size related to FIB-nt is,therefore, apparently sufficient to provide relevant information.4. Summary and conclusionsStatistical analyses of 3D digital reconstructions of FIB-nt and XCT data were con-ducted to estimate pore-size distribution, porosity, pore connectivity, and mineralcontent (clay fraction, quartz, carbonates) and the associated REVs of Opalinus Claysamples from different lithofacies. The results of this geometric analysis (drawing onimage-processing procedures) compare well with classical geotechnical methods,based on mercury intrusion porosimetry, BET, and XRD. Modeling of effectiveelastic properties on the microscale using a finite element code provided insights intothe effects of pore morphology and drainage on elastic properties. The model resultsrevealed cross-anisotropic stiffness, which is related to preferred orientation of porestructures. This anisotropy of the pore space will probably contribute to the overallmacroscopic cross-anisotropic stiffness. New insight regarding the drained/undrainedporo-elastic behavior was gained, indicating that the spatial distribution and connec-tivity of the small mesopores (2–10 nm; IUPAC, 1997) are essentially controlling themacroscopic deformation behavior of indurated clays. This appears to be in line withTerzaghi’s theory of adsorbed water films, postulating that the pore water in thesmall mesopores is not free but is weakly bound to the surfaces of the mineral aggregates(diffuse double layer). The water viscosity and effective water compressibility, there-fore, start to deviate from the reference values of free water with significant impacton the drained deformation behavior of the material.Petrophysical characteristics of indurated clays 197AcknowledgmentsThe authors thank Prof. L. Laloui, Dr A. Ferrari, and V. Favero (all LMS/EPFLLausanne, Switzerland) for comprehensive geotechnical characterization of the corespecimen. This work was funded partly by the SHARC consortium, a research collab-oration between the Commonwealth Scientific and Industrial Research Organization(CSIRO), Curtin University of Technology (Australia), and NAGRA (Switzerland).Guest editor: Reiner DohrmannThe authors and editors are grateful to anonymous reviewers who offered very helpfulinput and suggestions. A list of all reviewers is given at the end of the Preface for thisvolume.ReferencesBrunauer, S.E., Emmett, P.H., and Teller, E. (1938) Adsorption of gases in multimolecular layers. Journal ofthe American Chemical Society, 60, 309–319.Gens, A. (2013) On the hydromechanical behaviour of argillaceous hard soils-weak rocks. Proceedings ofthe 15th European Conference on Soil Mechanics and Geotechnical Engineering. Geotechnics of HardSoils – Weak Rocks (Part 4).IUPAC (1997) Compendium of Chemical Terminology, The Gold Book, second edition. (A.D. McNaught andA. Wilkinson, editors). Blackwell Science, London.Kanit, T., Forest, S., Gailliet, I., Mounoury, V., and Jeulin, D. (2003) Determination of the representativevolume for random composites: statistical and numerical approach. International Journal of Solids andStructures, 40, 3647–3679.Keller, L.M., Schuetz, P., Gasser, P., and Holzer, L. (2013a) Pore-space relevant for gas permeability in Opa-linus Clay: statistical analysis of homogeneity, percolation and representative volume element (RVE).Journal of Geophysical Research, 118, 1–14.Keller, L.M., Schuetz, P., Erni, R., Rossell, M.D., Lucas, F., Gasser, P., and Holzer, L. (2013b) Characteriz-ation of multi-scale microstructural features in Opalinus Clay. Microporous and Mesoporous Materials,179, 83–94.Keller, L.M., Hilger, A., and Manke, I. (2015) Impact of sand content on solute diffusion in Opalinus Clay.Applied Clay Science, 112–113, 134–142.Keller, L.M. (2015) On the representative elementary volume of clay rocks at the mesoscale. Journal ofGeology and Mining Research, 7, 58–64.Minon, S., Salager, S., and Laloui, L. (2010) WS-H Experiment: Identification tests and rheological analysisof Opalinus Clay: Shaley facies, Sandy facies and Wet Spot material. Mont Terri Technical NoteTN 2010-61.Stark, T.D. and Eid, H.T. (1994) Drained residual strength of cohesive soils. Journal of Geotechnical Engin-eering, 120/5, 856–871.Terzaghi, K., Peck, R.B., and Mesri, G. (1996) Soil Mechanics in Engineering Practice, third edition.John Wiley & Sons, New York.Yang, Y.L. and Aplin, A.C. (2010) A permeability–porosity relationship for mudstones. Marine andPetroleum Geology, 27, 1692–1697.P. Marschall et al.198

Microstructural insights into the petrophysical characteristics of indurated clays

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Microstructural insights into the petrophysical characteristics of indurated clays

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