Microscopic Pore Structure and Improved Permeability Characterization of CO_x Argillite Based on SEM Images

Abstract

Permeability is one of the important parameters to evaluate the long-term sealing performance of Callovo-Oxfordian (COx) argillite for high-level radioactive waste repository. In this study, scanning electron microscope images were used to analyze the pore structure of COx argillite. The pore size distributions (PSD) obtained by the discrete algorithm and two continuous algorithms (Munch continuous algorithm and Song algorithm) are quite different, the range of pore size obtained by the discrete algorithm is larger than those obtained by continuous algorithms. The results of continuous algorithms are closer to that from mercury intrusion porosimetry. For the permeability prediction, the continuous methods show better results, but there is still a big gap. By further considering the contribution of pores with different sizes to permeability, the results obtained by continuous methods are very close to the measured result. The values of A2 obtained by the Munch continuous algorithm are 2.2 × 10−16, 1.8 × 10−16, and 0.94 × 10−16 m2 for magnifications ranging from 2,000 to 8,000 times; the corresponding values obtained by the Song algorithm are 2.42 × 10−16, 2.02 × 10−16, and 1.12 × 10−16 m2, respectively, while the laboratory result is 1.67 × 10−16 m2. More accurate permeability can be obtained by more representative PSD. Therefore, for soil materials with uniform pore distribution, more accurate permeability can be obtained by considering continuous algorithms and pore contribution rate.