In situ characterization of the microstructure and porosity of Opalinus Clay (Mont Terri Rock Laboratory, Switzerland)

Clay rich formations are investigated thoroughly as candidate host rocks for the deep geological storage of radioactive waste. In the Mont Terri Rock Laboratory (Switzerland) Opalinus Clay is investigated with the aim to analyse its hydrological, geochemical and rock mechanical properties. Detailed investigation of the pore morphology and pore space contributes to the understanding of the sealing capacity, coupled flow, and associated deformation in clay. The Opalinus Clay formation is a fine-grained sedimentary rock deposited 180 Ma ago in a shallow sea compacted to a low porosity of 9–25% (depending on the measuring method used) and low permeability. On the regional scale the lateral variability of facies and lithology is low. Opalinus Clay is subdivided in six subfacies where the two end-members are the Shaly and the Sandy facies. To minimize damage of the clays microstructure the samples investigated, in this study, were carefully dried, as described in Chapter 2. After drying samples were imaged using a combination of Broad-Ion-Beam (BIB) milling and Scanning-Electron-Microscope (SEM) imaging. The method allows direct imaging of the clay fabric and porosity down to the nm scale. Visible porosity in the SE-micrographs was than segmented either manually or automatically (Chapter 3). A combination of BIB-milling and SEM imaging allows large representative area investigation of claystones, and a summary on porosity, pore morphology and pore size distribution of undamaged Opalinus Clay Shaly facies microstructures can be found in Chapter 4. This approach is able to provide a qualitative study of porosity as well as quantification of the pores directly from the micrographs at the nm - µm scale. Although the overall micro fabric differs per layer and per facies (Chapter 5), a low variability of the microstructure and porosity has been observed within each mineral phase. For example microstructurally the BIB polished cross-sections, produced in samples that have a high content of clay matrix (> 60%), display comparable microstructures for Sandy and Shaly facies samples of Opalinus Clay. The same is true for the pore size distribution and prominent pore shape per mineral phase, independent of the facies or polished cross-sections investigated; the pores within the clay matrix follow a similar pore size distribution and possess similar pore shapes. The large mineral grains, fossil shells and mineral aggregates are completely surrounded by the clay matrix, where the kind and amount of minerals depend on the facies of Opalinus Clay. The microstructure and porosity of the Shaly facies has also been investigated in 3D using a combination of µ-CT, BIB-SEM and FIB-SEM techniques (Chapter 6). The µ-CT measurements are used to get a 3D microstructural overview down to the µm scale. Afterwards a 2D BIB-polished cross-section was made through the µ-CT sample to examine the 2D microstructure down to the nm scale. FIB-SEM was used to study the 3D pore connectivity on two selected areas (Clay matrix and fossil shell) down to the nm scale. Next to the samples from the undisturbed Shaly and Sandy facies of Opalinus Clay, samples from the ‘Main fault’ (Mont Terri rock laboratory, Switzerland) were investigated, for the details see Chapter 7. Differences in microstructure with the undisturbed samples are the numerous micrometer wide calcite, celestine and pyrite filled veins present in the disturbed samples. Furthermore, damage zones of about 15 µm wide featuring less porosity are present. In order to investigate the pore connectivity Wood’s metal injection experiments were performed (Chapter 8). Preliminary results show that Wood’s metal although present in the bigger cracks and a few pores did not intruded a greater part of the pores present in the sample. This can indicate that that the connecting pore throats are smaller than 9 nm wide.