Pore Space and Fluid Phase Characterization in Round and Angular Partially Saturated Sands Using Radiation-Based Tomography and Persistent Homology

Abstract

The multiphase flow transport properties find ubiquitous applications in partially saturated granular media, particularly in the vadose zone due to alternate drying and wetting cycles. The spatial heterogeneity in the degree of saturation at the pore scale has a significant impact on the transport properties of the porous materials. In the present work, three-dimensional (3D) computed tomography (CT) images of the two-phase flow experiments are used to visualize and investigate the pore space and fluid phase microstructure of sands with rounded and angular grain morphology. The dual image modality utilizing X-rays and neutrons is used to obtain 3D information on the arrangement of solids and water in a sand sample. Advanced analysis of the experimental data is carried out using a mathematical framework known as persistent homology. This approach allows quantification of geometry as well as connectivity of pore space and fluid phase directly from the CT images with powerful implications in exploring pore-scale physics of multiphase granular materials. Persistent homology predicted percolation length of 52.5 µm and 41.86 µm consistent with the hydraulic conductivity measurements for rounded sand and angular sand, whereas the value of global void ratio equal to 0.512 and 0.69, respectively, suggests the contrasting response. The fluid distributions obtained from numerical predictions using the pore morphology method (PMM) are compared with results from experiments utilizing persistent homology. These results demonstrate the need for incorporating pore-scale physics in boundary value problems related to multiphase flow in granular materials.