Variations in the morphology of porosity in the Boom Clay Formation: insights from 2D high resolution BIB-SEM imaging and Mercury injection Porosimetry

The direct characterisation of the pore space of Boom Clay, using Broad Ion Beam (BIB) cross-sectioning combined with SEM imaging and Mercury Injection Porosimetry (MIP) enable to characterise the pore space variability in Boom Clay at the nm- μm scale within representative 2D areas, and to relate microstructural observations to fluid flow properties. Results show total SEM-resolved porosities of 10-20 %, different characteristic mineral phase internal pore morphologies, and intra-phase porosities. Most of the nano-porosity resides in the clay matrix, while in the silt-rich samples, larger inter-aggregate pores contribute to a major part of the resolved porosity. MIP with access to pore-throat diameters down to 3.6 nm, shows total interconnected porosities between 27-35 Vol%. The difference between BIB-SEM visible and MIP measured porosities is explained by the resolution limit of the BIB-SEM, and the limited size of the BIB-polished cross-section areas analysed. Compilation of the results provides a conceptual model of the pore network in fine- and coarse-grained samples of Boom Clay, where different mineral phases show characteristic internal porosities and pore morphologies and the overall pore space can be modelled based on the distribution of these mineral phases, as well as the grain-size distribution of the samples investigated.