Nondestructive Visualization and Quantification of 3-D Microstructure of Granular Materials and Direct Numerical Simulations

Abstract

This paper summarizes the key concepts from the recent published work of the authors on using both neutron and X-ray imaging techniques to study partially saturated sand and water flow through compacted sand. The goal of the manuscript is to serve as a review paper, building on discrete contributions from cited publications for geomechanics community as the topic is rather new and concepts are connected. For this study, neutron and micro-CT-based X-ray imaging was performed at Helmholtz-Zentrum-Berlin (HZB) in Germany. Due to different attenuation characteristics of neutrons and X-rays to three phases (silica, air, and water) of partially saturated sand, radiation-based images provide unique but complementary information in a nondestructive fashion. Water phase is very precisely identified with neutron radiation-based images, and sand (silica) phase is well identified with X-ray images. An automatic image registration technique is implemented to combine neutron and X-ray images in the same coordinates for a detailed quantitative evaluation of microstructural features in three dimensions. In situ imaging experiment of flow through compacted sand was performed based on the dual modality imaging concept. The initial 3-D pore geometry was obtained from dry compacted sand specimen by using x-ray. The water flow pattern was monitored by using time-lapsed neutron radiography and tomography after a target water injection step. The initial microstructure obtained with X-ray tomography is also used to perform direct numerical simulations. Experiments based on using neutron and X-ray imaging technique thus providing a unique opportunity to characterize partially saturated sand and investigate multiphase flow behavior through porous media. Direct numerical simulation based on realistic geometry can account for complex pore geometry including heterogeneity of the pore structure.