An Integrated Imaging Study of the Pore Structure of the Cobourg Limestone-A Potential Nuclear Waste Host Rock in Canada

With the development of imaging technology, tools to quantitatively describe pore structure, morphology, and connectivity have been widely applied on low permeable rocks; however, it is still questionable to what extent this information can be used to predict permeability. Applicability and comparability of different techniques are discussed here for the Middle Ordovician Cobourg limestone (Canada), a rock dominated by calcite grains of variable sizes (mu m-cm) and heterogeneously distributed quartz, dolomite, pyrite, and meshy clay minerals. Absolute porosities determined by helium pycnometry (HP) in literature are approximately 1.6% (+/- 0.9%), and gas permeabilities range from 10(-20) to 10(-19) m(2). Porosities obtained from BIB-SEM are much smaller compared to those from HP (16-69% of HP). Pores found in clays are smaller, slit-shaped, and more densely spaced when compared to those in calcite minerals. Connectivity between pores could not be resolved with 3D micro-CT or FIB-SEM reconstructions, which have a resolution limit of 8 mu m and 10 nm, respectively. However, assuming the pores to be connected, laboratory-derived permeability data could be fitted using a simple capillary bundle model, including information about the visible pore size distributions obtained from BIB-SEM images and a tortuosity range of 8 to 15.

Automated summary

With the development of imaging technology, tools to quantitatively describe pore structure, morphology, and connectivity have been widely applied on low permeable rocks; however, the predictability of permeability remains questionable. Different techniques for the Middle Ordovician Cobourg limestone in Canada are discussed, showing differences in porosities determined by helium pycnometry and BIB-SEM. Connectivity between pores could not be resolved with 3D micro-CT or FIB-SEM reconstructions.