Multi-scale characterization of porosity in Boom Clay (HADES-level, Mol, Belgium) using a combination of X-ray μ-CT, 2D BIB-SEM and FIB-SEM tomography

Abstract

The Oligocene age Boom Clay is a potential host material for radioactive waste disposal in Belgium. To better understand the physical basis of transport mechanisms of radionuclides, we aim to characterize the pore space and its connectivity at nm-scale in 3D. In the present study, X-ray μ-CT and FIB-SEM (focused ion beam scanning electron microscopy) tomography were combined, to investigate the 3D pore space of a Boom Clay sample from the Mol-1 borehole (depth corresponding to the level of the HADES-URF – ‘high activity disposal experimental site underground research facility’) at the Mol–Dessel research site for radioactive waste disposal (Belgium). BIB-SEM (broad ion beam scanning electron microscopy) was used to bridge the gap in resolutions between X-ray μ-CT and FIB-SEM and to optimize the selection of a relevant spot for FIB-SEM. Pore network extraction (PNE) modeling (Dong and Blunt, 2009 [1]) was used to simplify the results into a set of pore bodies and pore throats, which are suitable for a statistical description. Resulting pore-size distributions are interpreted to be power-law distributed over ∼6 orders of magnitude, showing the scale-invariance of the pore space. We present a conceptual model of the 3D pore network in Boom Clay. The extracted 3D pore network model can be used to estimate transport properties – in digital rock models.