An Integrated Imaging Study of the Pore Structure of the Cobourg Limestone—A Potential Nuclear Waste Host Rock in Canada
3. Imaging Experiments and Simulation
3.1. X-ray Micro-CT
3.4. Image Analysis
3.5. Permeability Simulation
4.1. X-ray Micro-CT Image Analysis
4.2. BIB-SEM Image Analysis
4.2.1. Mineralogy and Pore Morphology
4.2.2. Porosity and Pore Size Distribution
4.3. FIB-SEM Image Analysis
4.4. Permeability Simulation
5.1. Methodological Comparison of Porosity
5.2. Pore Morphology and Pore Size Distribution
5.3. Implications on the Dominant Flow Pathways
- Identified pores in the calcite phase are bound to individual grains and therefore could potentially be isolated (Figure 6a). Although calcite is the dominant mineral phase, the clay mineral phase is finely dispersed throughout the whole sample, often enveloping the carbonate grains to a large extent (Figure 5).
- Shortest distance analyses of the FIB-SEM reconstruction indicate that pores in the clay mineral phase are closer to each other than those in the calcite phase, which indicates a higher possibility that these pores are connected.
- The dominant slit-shaped pores in the clay mineral phase are likely to form a connected pore network, considering the high elongation of these pores and highest visible relative porosity (1.06–1.46%).
- HP-derived grain densities on intact (plugs) and crushed samples (<63 µm particle size) showed no significant differences. Disruption or destruction of the rock fabric by grinding/milling should make an increasing percentage of pore volume accessible by physically opening isolated pores if existent. It can therefore be concluded that pores in calcite are connected for helium and contribute to transport.
- Extrapolated porosities from mineralogical representative map 1, including calcite and clay mineral phases, are similar to the HP-derived porosities. As the HP-derived porosity measured on plugs represents an effective porosity, this is another piece of supporting evidence that the pores in the calcite phase are not isolated.
- BIB-SEM-derived permeability coefficients for the clay mineral phase can only be matched to laboratory-derived permeability coefficients with a tortuosity value of 1. Such tortuosity appears unrealistic considering that pore throats could not be visualized (e.g., all pores are isolated based on FIB-SEM), and clays are finely dispersed throughout the rock, making up only 6 vol.% of the bulk rock.
- Slit-shaped pores and microfractures are generally more susceptible to deformation than cylindrical pores. Under realistic conditions (burial depths of 250–1000 m for underground mining repositories), the proportion of slit-shaped pores and microcracks would likely be significantly lower than in this study. Additionally, the calcite phase is much more rigid than the clay mineral phase, leading to better preservation potential of pores.
- Based on the mineral composition analysis of BIB-SEM images, the Cobourg limestone can be characterized as a tight rock dominated by calcite grains of variable sizes (µm –cm) surrounded by idiomorphic or subhedral minerals (quartz, dolomite, and pyrite) and meshy clay minerals.
- Qualitative and quantitative pore analyses indicate that calcite and clay mineral phases together contribute over 90% of the total pore areas in two BIB-SEM maps. The clay pores are relatively small and close to each other, dominated by slit-shaped pores, and are interpreted as interparticle pores. The pores associated with calcite have a larger pore size range and are distant from each other, equidimensional, or elongated in shape, and can mostly be considered as intraparticle pores.
- The calcite and clay mineral phases show different pore size distribution, which can be characterized by a power law relationship between pore area and normalized pore frequency. The power law exponent D values measured in the clay mineral phase (2.50–2.59) are higher than those measured in the calcite phase (1.93–2.18).
- The discrepancy in porosity values measured by HP and BIB-SEM can be attributed to the sample heterogeneity and the relatively lower resolution of BIB-SEM. The similarity between extrapolated porosities (down to 1 nm) and bulk porosities measured by HP indicates that valid porosity models can be reached by BIB-SEM imaging and interpretation towards smaller-sized pores, if a representative mineralogy is present in the studied map.
- Based on detailed characterizations of the pore size distribution and pore morphology, and the uncertainty with respect to the connectivity of pores in the calcite and clay mineral phases, it cannot be concluded that the clay mineral phase exclusively provides pathways for flow; rather, it is possible that microfractures in the interface between calcite and clay phases enhance the flow, as well as interconnected pore networks in the calcite phase. However, this has to be investigated in more detail in the future.
Data Availability Statement
Conflicts of Interest
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|Sample||Calcite (%)||Dolomite/Ankerite (%)||Clay Minerals (%)||Quartz (%)||Pyrite (%)||Others (%)||Carbonate (%)||Silicate (%)|
|BIB-SEM map 1||89.5||1.2||8.4||0.8||0.1||0.0||90.7||9.2|
|BIB-SEM map 2||61.2||7.6||20.9||8.4||2.0||0.0||68.7||29.3|
|BIB-SEM overview map||84.1||4.0||6.5||4.6||0.4||0.5||88.1||11.1|
|XRD analysis *||80.0||8.4||6.4||5.0||0.3||0.0||88.4||11.4|
|BIB-SEM map 1||Absolute||0.58||0.00||0.09||0.00||0.00||0.68|
|BIB-SEM map 2||Absolute||0.15||0.01||0.22||0.01||0.02||0.41|
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Hu, Z.; Lu, S.; Klaver, J.; Dewanckele, J.; Amann-Hildenbrand, A.; Gaus, G.; Littke, R. An Integrated Imaging Study of the Pore Structure of the Cobourg Limestone—A Potential Nuclear Waste Host Rock in Canada. Minerals 2021, 11, 1042. https://doi.org/10.3390/min11101042
Hu Z, Lu S, Klaver J, Dewanckele J, Amann-Hildenbrand A, Gaus G, Littke R. An Integrated Imaging Study of the Pore Structure of the Cobourg Limestone—A Potential Nuclear Waste Host Rock in Canada. Minerals. 2021; 11(10):1042. https://doi.org/10.3390/min11101042Chicago/Turabian Style
Hu, Zhazha, Shuangfang Lu, Jop Klaver, Jan Dewanckele, Alexandra Amann-Hildenbrand, Garri Gaus, and Ralf Littke. 2021. "An Integrated Imaging Study of the Pore Structure of the Cobourg Limestone—A Potential Nuclear Waste Host Rock in Canada" Minerals 11, no. 10: 1042. https://doi.org/10.3390/min11101042