Hydro-mechanical response of Opalinus Clay shale: dependency on composition and burial depth

Shales are sedimentary formation currently studied for their involvement in many geo-energy related applications, such as radioactive waste disposal, CO2 sequestration, oil and gas extraction. In Switzerland the Opalinus Clay shale has been selected has host formation for the construction of geological disposal for radioactive waste. Opalinus Clay is a Jurassic shale widespread in the northern part of the country, characterised by sedimentation planes (bedding). In the shale formation, several lithostratigraphic units have been identified. Also, the current and maximum depths that the formation has experienced are not homogeneous among the locations. Most of the research in the last 20 years have been concentrated on one of the lithofacies (characterised by high clay content), the shaly facies, in the rather shallow location of the Mont Terri Underground Rock Laboratory, and on a few other deeper sites, in the north-east of the country. This thesis aims to study the impact of the composition and burial depth on the hydro-mechanical response of the Opalinus Clay. The results serve to estimate the geomaterial response for compositions and locations (e.g., where the disposals are planned) where the possibility to directly test the shale may be limited because of cost and time efforts. An experimental study was carried out, to test the hydro-mechanical response, in saturated and unsaturated conditions, of samples from various locations and compositions. Samples from sub-superficial locations (<100m depth) showed similar responses, at the laboratory scale, to those from deeper locations, in terms of permeability and compressibility, although clear signs of weathering were detected at the field scale for the first few tens of meters. The hydro-mechanical response of the Opalinus Clay shale was interpreted as a layered geomaterial, composed as an alternation of two-layer types. The first (shaly) is composed of a clay matrix in which grains of quartz, calcite or other minerals are embedded. The latter (sandy) is composed by a compact quartz structure, whose pores are filled with clay. The properties of each layer type were estimated via the gathered experimental data. For each tested sample the volumetric fraction of shaly layer and the corresponding hydro-mechanical response were estimated. The results were verified against the experimental data, finding good agreement. The volumetric response observed during hydraulic loading performed in free stress conditions was characterized by significant anisotropy and dependence on the sample composition. Irreversibility of the strains was investigated by performing X-ray tomographies of a specimen subjected to a hydraulic load. Cracks opening was detected both during swelling than during drying, but in different locations in the samples. It was highlighted that samples with lower clay content, such as those belonging to the sandy facies, showed higher stiffness and shear strength, that may be considered more desirable properties for underground constructions. On the other hand, samples with higher clay content (such as those belonging to the shaly facies) were found to have a higher capacity to swell upon saturation, and therefore to potentially self-seal cracks. Further, they have lower permeability and pore throat diameter. From this perspective, the high-clay-content part of the formation has more desirable properties to behave as a barrier to fluid flow in the radioactive waste disposal context.

Laloui, Lyesse
Ferrari, Alessio
Lausanne, EPFL

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 Record created 2019-10-29, last modified 2020-02-06

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