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 effect s of spatially heterogeneous porosity on matrix diffusion. Experiments were performed on four, centimeter-scale slabs of Culebra dolomite taken from t he Waste Isolation Pilot Plant (WIPP) site. These tests involved the diffus ion of potassium iodide into a single edge of each brine-saturated rock sla b, while X-ray absorption imaging was used to measure the two-dimensional r elative 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 dat a provide unique insight into the spatially varying diffusion characteristi cs of each heterogeneous rock sample, which traditional methods such as thr ough-diffusion experiments cannot. In these tests, significant variations i n the diffusion coefficient were calculated over the relatively small lengt h (centimeter) and time scales (months) investigated. Results also indicate d that these variations were related to the heterogeneous porosity characte ristics of each rock sample. Not only were the diffusion coefficients found to depend on the magnitude of the porosity but also on its spatial distrib ution. Specifically, the geometry, position, and orientation of the heterog eneous porosity features populating each rock slab appeared to influence th e diffusion characteristics. (C) 2000 Elsevier Science B.V. All rights rese rved.