On the determination of pore size distribution by nitrogen Adsorption and Mercury intrusion Porosimetry for claystone

¹ Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), DREE/SPDR/LETIS, Fontenay-aux-Roses, 92260, France
² Ecole des Ponts ParisTech, Laboratoire Navier/CERMES, Marne La Vallée, France.

^* Corresponding author: mehana.allache@asnr.fr

Abstract

Understanding the evolution of Callovo-Oxfordian (COx) claystone porosity under alkaline conditions is crucial for assessing its role as a geological barrier for nuclear waste deep geological disposal facility. This study evaluates the pore size distribution of COx claystone using two experimental techniques: nitrogen adsorption and mercury intrusion porosimetry (MIP). The nitrogen adsorption data were analysed using the Barrett-Joyner-Halenda (BJH) model and the non-local density functional theory (NLDFT). These were further combined in a Total Pore Volume (TPV) approach to better describe the full pore size spectrum. Results show significant discrepancies between the methods. BJH overestimates mesopore volume (~3.8 nm) due to the cavitation and pore blocking artifacts, while MIP underestimate total porosity due to the compression effects and its inability to detect micropores. DFT provides a more reliable micropore and mesopore characterization, yet its detection range (<50 nm) remains limited, requiring high- resolution data at low relative pressures for accurate analysis. To overcome these limitations, the TPV method combine DFT and MIP data, offering a more continuous and accurate pore size distribution. The results indicate that alkaline exposure increases macroporosity while reducing microporosity, potentially affecting the permeability and self-sealing capacity of COx claystone.