The effect of microstructural heterogeneity on pore size distribution and permeability in Opalinus Clay (Mont Terri, CH): insights from an integrated study of laboratory fluid flow and pore morphology from BIB-SEM images

Opalinus Clay (OPA) is considered as potential host rock for the deep-geological disposal of radioactive waste. One key-parameter in long-term storage prediction is permeability. In this study we investigated microstructural controls on permeability for the different facies of OPA. Permeability and porosity were determined under controlled pressure conditions. In addition, the pore space was investigated by SEM, using high quality surfaces prepared by Broad Ion Beam milling (BIB). Water permeability coefficients range from 1.6×10-21 to 5.6×10 20 m². Helium pycnometer porosities range between ~21% and ~12%. The sample with the highest helium porosity (shaly facies) is characterized by the lowest permeability, and vice versa (carbonate-rich sandy facies). This inverse behavior deviates from the generally reported trend of increasing permeability with increasing porosity, indicating that parameters other than porosity affect permeability. Visible porosities from SEM images revealed that 67-95% of the total porosity resides within pores smaller than the SEM detection limit. Pore sizes follow a power law distribution with characteristic power law exponents (D), strongly differing among the facies. The carbonate-rich sandy facies contains a network of much larger pores (D(shaly)~2.4, D(carbonate-rich)~2.0), because of the presence of load-supporting sand grains that locally prevent clay compaction, being responsible for a higher permeability.