Seismic and mechanical properties of Opalinus Clay: comparison between sandy and shaly facies from Mont Terri (Switzerland)

For the safe disposal of high-level radioactive waste, different host rocks are currently being considered. The favorable properties of clay are low permeability, some retention capacity concerning radionuclides, and the ability to self-seal cracks and fissures, e.g. by swelling or time-dependent compaction creep. In Switzerland, the Jurassic Opalinus Clay is envisaged as a potential host rock which—at Mont Terri—is subdivided into the sandy, shaly, and carbonate-rich facies, the latter being less abundant. For long-term safety assessments, the understanding of the relations of properties (e.g. mineralogical composition and microstructure) and performance (e.g. mechanical behavior) of clays and claystones is essential. In the case of the sandy Opalinus Clay, the mechanical strength increases with increasing carbonate content, because carbonates form the matrix. The mineralogical investigation of a set of sandy facies samples proved a significantly larger carbonate content (20–40 mass %) when compared to the shaly facies (10–20 mass %). The carbonates of the shaly Opalinus Clay, on the other hand, are mostly localized fossils aligned parallel to the bedding, acting as predetermined breaking points. Image analysis of SEM images of polished sections proved the determined microstructural differences. In addition, carbonate particles of the sandy facies are mostly isometric, whereas carbonate particles of the shaly facies cover a greater range of shapes. The mechanical tests were accompanied by investigations of the p- and s-wave velocities, which revealed that the anisotropy of the sandy facies is less pronounced than sedimentological analyses would suggest. The mechanical strength, which, for the first time, presents results of real triaxial tests of the sandy facies. The samples of the sandy facies exhibit a failure strength of σ _eff,B, approximately twice as high as was found for the shaly facies considering the deformation axis parallel to the bedding. Similar values were obtained when measuring perpendicularly to the bedding.     

Ageing effect on the mineral and chemical composition of Opalinus Clays (Mont Terri,Switzerland) after excavation and surface storage

Samples of Opalinus Clays from Mont Terri Underground Research Laboratory in the Swiss Jura were analyzed repetitively relative to the duration of their exposure to atmosphere. The objective was the evaluation of such a progressive exposure on the chemical composition of whole-rock samples, and on the chemical and Sr isotopic compositions of leachates obtained by leaching the rock powders with dilute acetic acid. This chemical study was complemented by scanning electron microscope observations to identify the related mineral alterations. The chemical data for the rock powders remained quite constant whatever the duration of the storage, whereas significant changes were observed for the leachates. Similar changes were observed in the leachates of samples collected progressively closer to the walls of a previously excavated niche. When storage time or distance to gallery wall increases, the main variations are: (1) a progressive decrease of the element contents and of the ⁸⁷Sr/⁸⁶Sr ratio in the leachates, (2) an alteration of pyrite followed by precipitation of Ca-sulfate in the rocks, both observed by SEM, and (3) a probable precipitation of new mineral phases in the rocks, such as Fe-oxyhydroxides and/or jarosite that could not be visualized by electron microscopic observation because of their very limited amount and very small grain size. The modifications call for an oxidation of pyrite that probably induced also an oxidation of the organic matter. It could also be shown that the reactions were enhanced by temperature increase, and that they were less pronounced in the samples behind the gallery wall than in the cores stored in laboratory conditions.     

The Mont Terri rock laboratory, a new international research project in a Mesozoic shale formation, in Switzerland

In many countries, argillaceous formations are being considered as potential host rocks for repositories of radioactive waste. Therefore, in 1995 several organisations decided to start an international research project in the reconnaissance gallery of the Mont Terri motorway tunnel, in north-western Switzerland, in a Mesozoic shale formation, the Opalinus Clay (Aalenian).

This project is under the patronage of the Swiss National Hydrological and Geological Survey. The following organisations are also partners in the project: ANDRA and IPSN (France), BGR (Germany), ENRESA (Spain), Nagra (Switzerland), JNC and Obayashi (Japan), SCKCEN (Belgium).

The aims of the project are to analyse the hydrogeological, geochemical and rock mechanical properties of an argillaceous formation, the changes of these properties induced by the excavation of galleries and to evaluate and improve appropriate investigation techniques. In January, 1996, eight niches were excavated for the project and 15 experiments were started in boreholes of no more than 30 m in length. In the winter of 1997–1998 a new research gallery was excavated to host further experiments. Different excavation, drilling and investigation techniques have been tested and improved. Significant results of the different experiments are already available. Some of those dealing with hydrochemistry, porewater pressure measurements and the evolution of the excavation disturbed zone (EDZ) are reported in this paper.     

Solute transport in formations of very low permeability: profiles of stable isotope and dissolved noble gas contents of pore water in the Opalinus Clay, Mont Terri, Switzerland

Pore water profiles of water, stable isotope, and dissolved noble gas content have been determined across the Opalinus Clay and adjacent formations at the rock laboratory at Mont Terri. We have found enhanced helium contents (up to [⁴He] = 1 × 10⁻⁴ cubic centimeters at standard pressure and temperature per gram of pore water) and argon isotope ratios (⁴⁰Ar/³⁶Ar ratios up to 334) due to accumulation of ⁴He and ⁴⁰Ar produced in situ. The helium profile was found to be in steady state with respect to in situ production and diffusive loss into the adjacent limestones where groundwater circulates. From this profile a representative mean value of the apparent diffusion coefficient for helium in the pore water of the whole formation was derived for the first time to be D_a = 3.5 × 10⁻¹¹ m² · s⁻¹, which is more than two orders of magnitude lower than the diffusion coefficient D₀ in free water. The stable isotope profile, however, indicates a component of fossil marine pore water, which has not yet been replaced by molecular diffusion of meteoric water from the adjacent limestone and shale formations over the past 10 million years.     

Large-scale tracer profiles in a deep claystone formation (Opalinus Clay at Mont Russelin,Switzerland): Implications for solute transport processes and transport properties of the rock

Natural tracers (Cl^? and stable water isotopes) in pore water of the Opalinus Clay and adjacent formations were studied in the motorway tunnel at Mont Russelin, Switzerland. The Opalinus Clay occurs in the core of an anticline which is cut by a complex system of thrust faults. Concentration profiles of natural tracers were taken from 17 boreholes along a 363 m long section. Pore waters of drillcore samples were analysed with indirect and direct methods. The Cl^? and stable water isotope distribution in the pore water shows a regular and well defined profile, with a conspicuous decrease towards the overlying Dogger limestone aquifer. The highest Cl^? values (approximately 23,000 mg/L) are found in the core of the anticline in Liassic claystones underlying the Opalinus Clay. To quantify the large-scale transport properties of the Opalinus Clay formation, a 2D transport model was constructed and used to reproduce the observed concentration profiles. The calculations indicate that the observed tracer distributions are consistent with diffusion as the dominant transport process. Groundwater flow in the overlying Dogger aquifer was initiated about 2–4 Ma ago, which is long after the folding of the Jura Mountains and probably coincides with the exposure of the aquifer to freshwater recharge following continued erosion of the anticline. The calculations suggest that tracer distributions are controlled by 1) the timing of freshwater recharge in the overlying limestone aquifer, 2) the shape of the anticline and 3) the magnitude and the anisotropy ratio of diffusion coefficients.     

Faults and fractures in the Gallery 04 of the Mont Terri rock laboratory: Characterization,simulation and application

This paper deals with the reconstruction of a fracture network observed in an underground gallery of a tunnel buried in clay stones in Switzerland, below Mont Terri. The trace maps of the Gallery 04 and of the EZ-G niche of this site have first been digitized and used in various ways to characterize the data. The traces have been divided into two groups, i.e., the pre-existing faults in the gallery and the fractures of the excavated damaged zone (EDZ) in the niche. Each group has been analyzed individually. The number of data for the faults has turned out to be statistically significant, while it was very limited for the EDZ fractures. For both cases, the major statistical characteristics of the traces have been extracted and a reconstruction procedure developed and tested. A full example has been worked out; a gallery immersed in a reconstructed fractured porous medium has been meshed, and the electrical field created by a dipole at the wall calculated by solving the three dimensional Laplace equations in the fractures and in the porous medium.     

Biogeochemical processes in a clay formation in situ experiment: Part E - Equilibrium controls on chemistry of pore water from the Opalinus Clay, Mont Terri Underground Research Laboratory, Switzerland

The chemistry of pore water (particularly pH and ionic strength) is an important property of clay rocks being considered as host rocks for long-term storage of radioactive waste. Pore waters in clay-rich rocks generally cannot be sampled directly. Instead, their chemistry must be found using laboratory-measured properties of core samples and geochemical modelling. Many such measurements have been made on samples from the Opalinus Clay from the Mont Terri Underground Research Laboratory (URL). Several boreholes in that URL yielded water samples against which pore water models have been calibrated. Following a first synthesis report published in 2003, this paper presents the evolution of the modelling approaches developed within Mont Terri URL scientific programs through the last decade (1997-2009). Models are compared to the composition of waters sampled during dedicated borehole experiments. Reanalysis of the models, parameters and database enabled the principal shortcomings of the previous modelling efforts to be overcome. The inability to model the K concentrations correctly with the measured cation exchange properties was found to be due to the use of an inappropriate selectivity coefficient for Na-K exchange; the inability to reproduce the measured carbonate chemistry and pH of the pore waters using mineral-water reactions alone was corrected by considering clay mineral equilibria. Re-examination of the measured Ca/Mg activity ratios and consideration of the mineralogical composition of the Opalinus Clay suggested that Ca/Mg cation exchange rather than dolomite saturation may control the ratio of these ions in solution. This re-examination also suggests that the Ca/Mg ratio decreases with increasing pore-water salinity. Several possible reasons for this are proposed. Moreover, it is demonstrated that feldspar equilibria must not be included in Opalinus Clay modelling because feldspars are present only in very small quantities in the formation and because Na/K ratios measured in pore water samples are inconsistent with feldspar saturation. The principal need to improve future modelling is additional or better data on rock properties, in particular: (i) a more detailed identification of phases in the Opalinus Clay that include redox-sensitive elements together with evaluation of their thermodynamic properties; (ii) an improved understanding of the distribution of celestite throughout the Opalinus Clay for Sr/SO₄ concentrations control; (iii) improvements in analytic and thermodynamic data for Ca-Mg rock cation exchange and mineral chemical properties and (iv) the measurement of composition and stability constants of clay minerals actually present in the formation.     

Biogeochemical processes in a clay formation in situ experiment: Part A - Overview, experimental design and water data of an experiment in the Opalinus Clay at the Mont Terri Underground Research Laboratory, Switzerland

An in situ test in the Opalinus Clay formation, termed porewater chemistry (PC) experiment, was carried out for a period of 5 years. It was based on the concept of diffusive equilibration whereby a traced water with a composition close to that expected in the formation was continuously circulated and monitored in a packed-off borehole. The main original focus was to obtain reliable data on the pH/pCO₂ conditions of the porewater, but because of unexpected microbiologically-induced redox reactions, the objective was extended to elucidate the biogeochemical processes occurring in the borehole and to understand their impact on pH/pCO₂ and porewater chemistry in the low permeability clay formation.The behaviour of the conservative tracers ²H and Br⁻ could be explained by diffusive dilution in the clay and moreover the results showed that diffusive equilibration between the borehole water and the formation occurred within about 3 year’s time. However, the composition and pH/pCO₂ conditions differed considerably from those of the in situ porewater. Thus, pH was lower and pCO₂ was higher than indicated by complementary laboratory investigations. The noted differences are explained by microbiologically-induced redox reactions occurring in the borehole and in the interfacial wall area which were caused by an organic source released from the equipment material. The degradation of this source was accompanied by sulfate reduction and - to a lesser extent - by methane generation, which induced a high rate of acetogenic reactions corresponding to very high acetate concentrations for the first 600 days. Concomitantly with the anaerobic degradation of an organic source, carbonate dissolution occurred and these processes resulted in high pCO₂ and alkalinities as well as drop in pH. Afterwards, the microbial regime changed and, in parallel to ongoing sulfate reduction, acetate was consumed, leading to a strong decrease in TOC which reached background levels after about 1200 days. In spite of the depletion of this organic perturbation in the circuit water, sulfate reduction and methanogenesis continued to occur at a constant rate leading to near-to-constant concentrations of sulfate and bicarbonate as well as pH/pCO₂ conditions until the end of the experiment. The main sink for sulphur was iron sulfide, which precipitated as FeS (am) and FeS₂.The chemical and isotopic composition was affected by the complex interplay of diffusion, carbon degradation rates, mineral equilibria and dissolution rates, iron sulfide precipitation rates, and clay exchange reactions. The ¹³C signals measured for different carbon species showed significant variations which could only be partly explained. The main cations, such as Na, Ca and Mg remained remarkably constant during the experiment, thus indicating the strong buffering of the formation via cation and proton exchange as well as carbonate dissolution/precipitation reactions.     

Biomarker transformations as constraints for the depositional environment and for maximum temperatures during burial of Opalinus Clay and Posidonia Shale in northern Switzerland

Samples from two argillaceous formations (Opalinus Clay and Posidonia Shale) of near-identical maturity from northern Switzerland were subjected to a geochemical characterisation of organic matter and to confined-system pyrolysis experiments. Throughout the study area, the characteristics of organic matter are similar, indicating a spatially homogeneous sedimentary facies. Posidonia Shale contains marine organic matter deposited in a reducing environment, while a predominantly terrigenous source and a more oxidising environment of deposition was identified for Opalinus Clay. In the western and central parts of the study area, organic maturity is close to the onset of oil generation. In the easternmost part, a higher maturity has been reached due to a deeper burial below thick Tertiary Molasse deposits.Isothermal pyrolysis experiments were conducted at temperatures between 250 and 390 °C over 24 h. Bitumen yields increase along similar pathways for both Opalinus Clay and Posidonia Shale, but the maximum values are displaced by 10–20 °C. Data pertaining to maturity were determined from GC–MS analyses of saturated hydrocarbons, and specific attention was given to C₂₉-sterane and C₃₂-hopane isomerisation ratios. The evolution of these parameters with rising temperature is slightly different in the two formations, which is attributed to the contrasting organic facies. The pyrolysis data, together with literature data from natural basins, were used to calculate kinetic parameters for C₂₉-sterane and C₃₂-hopane, assuming a single-step isomerisation scheme according to the Arrhenius law. The resulting values based on pyrolysis data alone are very similar to those based on the combination of pyrolysis and natural data. Activation energies are similar in both formations, while the frequency factors are up to one order of magnitude higher for Posidonia Shale when compared to Opalinus Clay. For the Benken site, maximum temperature during Cretaceous burial was calculated on the basis of the kinetic data, using the TTI approach. The resulting temperatures of 75–80 °C are 5–10 °C below those derived in the literature from apatite fission-track analysis, vitrinite reflectance and basin modelling.     

Mechanical validation of balanced cross-sections: The case of the Mont Terri anticline at the Jura front (NW Switzerland)

Close to north-west front of the Jura thrust belt in Switzerland, the Mont Terri anticline is located at a disruption of the Muschelkalk decollement, due to a former normal fault. Numerous geological data are available thanks to a motorway tunnel cross-cutting the fold and an underground laboratory. Original field data, two geological maps, a borehole, and a highway tunnel constrain the inner structure. The anticline is characterized by an overturned frontal limb, a steep back limb, and a ramp cross-cutting older normal faults. Three geological cross-sections and three 2D kinematic models are proposed as various combinations of fault-bend folding, fault-propagation folding and detachment folding. The main motivation of this study is to illustrate how geometrical constructions can be constrained by the kinematic approach of limit analysis, which determines the optimal deformation, including faults, that verifies mechanical equilibrium and the Coulomb criterion. Matching some of the important kinematic steps assumed in each three interpretations to the optimal deformation leads us to define the compatible ranges of values of the frictional parameters. These ranges allow us to assess the likeliness of the important kinematic steps of each kinematic model. The present analysis provides a quantitative link between the assumed evolution of the internal structure, and the concomitant evolutions of the topography and of the frictional parameters.     

Rates of microbial hydrogen oxidation and sulfate reduction in Opalinus Clay rock

Hydrogen gas (H₂) may be produced by the anoxic corrosion of steel components in underground structures, such as geological repositories for radioactive waste. In such environments, hydrogen was shown to serve as an electron donor for autotrophic bacteria. High gas overpressures are to be avoided in radioactive waste repositories and, thus, microbial consumption of H₂ is generally viewed as beneficial. However, to fully consider this biological process in models of repository evolution over time, it is crucial to determine the in situ rates of microbial hydrogen oxidation and sulfate reduction. These rates were estimated through two distinct in situ experiments, using several measurement and calculation methods. Volumetric consumption rates were calculated to be between 1.13 and 1.93 μmol cm⁻³ day⁻¹ for H₂, and 0.14 and 0.20 μmol cm⁻³ day⁻¹ for sulfate. Based on the stoichiometry of the reaction, there is an excess of H₂ consumed, suggesting that it serves as an electron donor to reduce electron acceptors other than sulfate, and/or that some H₂ is lost via diffusion. These rate estimates are critical to evaluate whether biological H₂ consumption can negate H₂ production in repositories, and to determine whether sulfate reduction can consume sulfate faster than it is replenished by diffusion, which could lead to methanogenic conditions.     

Constitutive analysis of the mechanical anisotropy of Opalinus Clay

This paper aims to analyse the anisotropic features of behaviour of Opalinus Clay using the theory of plastic multi-mechanisms. The results of triaxial tests conducted with different load levels and directions showed that the mechanical behaviour of this shale is cross-anisotropic. The stiffer samples are those in which the loading direction is parallel to the bedding plane. This indicates that the preconsolidation stress depends on the orientation of the load with respect to the fabric of Opalinus Clay. It is proposed to interpret the observed cross-anisotropy with an elastoplastic model based on four plastic strain mechanisms that may be successively mobilised depending on the loading direction. The predicted stress–strain responses vary according to the directions of the space as a result of the hardening process, depending on the number of plastic strain mechanisms that have been mobilised. The numerical predictions show overall good agreement with the experimental data in terms of deviatoric stress versus axial strain, demonstrating that multi-mechanism plasticity is a suitable constitutive tool for the interpretation of the mechanical anisotropy of this shale.     

Activation energies of the self-diffusion of HTO, 22Na+ and 36Cl− in a highly compacted argillaceous rock (Opalinus Clay)

The temperature dependence of the self-diffusion of HTO, ²²Na⁺ and ³⁶Cl⁻ in Opalinus Clay (OPA) was studied using a through-diffusion technique, in which the temperature was gradually increased in the steady state phase of the diffusion. The measurements were done on samples from two different geological locations. The dependence of the effective diffusion coefficient on temperature was found to be of an Arrhenius type in the temperature range between 0 and 70 °C. A slight difference between the two locations could be observed. The average value of the activation energy of the self-diffusion of HTO in OPA was 21.1 ± 1.6 kJ mol⁻¹, and 21.0 ± 3.5 and 19.4 ± 1.5 kJ mol⁻¹ for ²²Na⁺ and ³⁶Cl⁻, respectively. The measured values for HTO are slightly higher than the values found for the bulk liquid water (HTO: 18.8 ± 0.4 kJ mol⁻¹). This indicates that the structure of the confined water in OPA might be slightly different from that of bulk liquid water. Also for Na⁺ and Cl⁻, slightly higher values than in bulk liquid water (Na⁺: 18.4 kJ mol⁻¹; Cl⁻: 17.4 kJ mol⁻¹) were observed.The Stokes–Einstein relationship, based on the temperature dependency of the viscosity of bulk water, could not be used to describe the temperature dependence of the diffusion of HTO in OPA. This additionally indicates the slightly different structure of the pore water in OPA.     

Characterization of dissolved organic matter in anoxic rock extracts and in situ pore water of the Opalinus Clay

Dissolved organic matter (DOM) from the Opalinus Clay, a potential host rock for the disposal of radioactive waste, was isolated under strictly anoxic conditions from ground rock material and compared with DOM of in situ pore water samples. For the extractions, deionized water, synthetic pore water (SPW, water containing all major ions at pore water concentrations but no organic matter) and 0.1 M NaOH were used. The influence of the solid-to-liquid ratio, extraction time, acid-pretreatment and O₂ exposure of the rock material on the isolated DOM were investigated. Liquid chromatography coupled with a total organic C detector (LC-OCD) and reverse-phase ion chromatography were used to characterize the DOM size distributions and to determine the low molecular weight organic acid (LMWOA) contents in the pore water samples and the rock extracts.The results revealed that only a small portion of the total organic C of the rock material (<0.38%) was extractable, even after removal of carbonates by acid-pretreatment. The concentrations of dissolved organic C (DOC) were found to range from 3.9 ± 0.4 to 8.0 ± 0.8 mg/L in the anoxic extracts. The pore waters exhibited similar DOC concentrations ranging from 1.2 to 15.8 ± 0.5 mg/L. The analysis by LC-OCD showed that the DOM extracted under anoxic conditions and the pore water DOM mainly consisted of hydrophilic compounds of less than 500 Da. The DOM extracted with SPW was most similar in size to the pore water DOM. Grinding the rock under oxic conditions increased the DOC yields and shifted the size distribution toward higher molecular weight compounds compared to the strictly anoxic treatment. Acetate, lactate and formate were identified in all extracts and in the pore water. In total, LMWOA accounted for 36% of the total DOC in both pore water and SPW extracts. The results imply that controlled anoxic conditions and the use of SPW as an extractant are required to isolate DOM from Opalinus Clay rocks which most resembles the in situ pore water DOM with respect to its size distribution and the LMWOA contents.     

Temperature and Damage Impact on the Permeability of Opalinus Clay

The effects of temperature on the water transport properties of intact and damaged Opalinus (OPA) clay are investigated by using a recently developed hollow cylinder triaxial cell [Monfared (Int J Rock Mech Min Sci 48:637–649, 2011b)] that allows full saturation and drainage conditions in low-permeability clays and shales. The volumetric response of saturated OPA clay sample during a drained heating test shows an irreversible contraction after a temperature threshold. The permeability tests which are performed before and after the heating test show that the induced irreversible sample contraction by thermal loading reduces the permeability of OPA clay sample. In order to study the effect of temperature on the permeability of a damaged sample of OPA clay, the permeability tests are performed on a saturated sample previously sheared by a standard drained triaxial loading. The test results show no significant effect of shear-type damage on the permeability of the sample at 25 °C and 80 °C. The experimental results presented in this paper show the crucial role of the thermally induced strains on sample permeability. Thermo-elastic dilation leads to a slight increase of the permeability, whereas thermoplastic contraction leads to a reduction.     

Sedimentary facies and depositional history of the Swan Islands,Honduras

Swan Island is a Honduran possession in the western Caribbean, located on the southeastern side of the Cayman Trench. Two sedimentary assemblages are found on the island: an older bedded sequence of mid-Tertiary age (Aquitanian or Burdigalian) and a younger sedimentary sequence of Late Pleistocene age. The older sequence is composed of a series of calcarenites, calcilutites, and siliciclastic mudstones; capping these are cliff-forming reefal carbonates of the younger sequence.The rocks of the older bedded sequence accumulated in deep water. Sedimentation consisted of a constant rain of pyroclastic debris interrupted by the episodic introduction of upslope carbonate material by turbidity currents. Uplift and deformation of this sequence was initiated sometime after the Early Miocene. By the Late Pleistocene, uplift had brought the rocks into water depths conducive to coral growth. Pleistocene sedimentation on the island was controlled by the interaction between tectonic uplift and eustatic sea-level changes. The primary controlling force on the tectonic history of the island is its proximity to the boundary between the North American and Caribbean plates.