Effects of spatially heterogeneous porosity on matrix diffusion as investigated by X-ray absorption imaging

High-resolution X-ray absorption imaging was used to investigate the effects of spatially heterogeneous porosity on matrix diffusion. Experiments were performed on four, centimeter-scale slabs of Culebra dolomite taken from the Waste Isolation Pilot Plant (WIPP) site. These tests involved the diffusion of potassium iodide into a single edge of each brine-saturated rock slab, while X-ray absorption imaging was used to measure the two-dimensional relative concentration distribution at different times during the experiment. X-ray imaging was also used to measure the heterogeneous, two-dimensional porosity distribution of each rock slab. The resulting high-resolution data provide unique insight into the spatially varying diffusion characteristics of each heterogeneous rock sample, which traditional methods such as through-diffusion experiments cannot. In these tests, significant variations in the diffusion coefficient were calculated over the relatively small length (centimeter) and time scales (months) investigated. Results also indicated that these variations were related to the heterogeneous porosity characteristics of each rock sample. Not only were the diffusion coefficients found to depend on the magnitude of the porosity but also on its spatial distribution. Specifically, the geometry, position, and orientation of the heterogeneous porosity features populating each rock slab appeared to influence the diffusion characteristics.