Measuring Diffusion Coefficients in Low-Porosity Rocks by X-Ray Radiography

Abstract

Deep geological repositories (DGR) are considered an effective long-term solution for radioactive waste disposal. Sedimentary (argillaceous formations) and crystalline rocks are currently under investigation worldwide as potential host formations for DGR. Their low porosity (<1-2 %) and very low hydraulic conductivity result in diffusion-dominated solute transport. There is a need to investigate their diffusion properties in detail, the long-established diffusion methods do not allow an evaluation of the spatial relationship between tracers and the characteristics of the geological medium. The aim of this project was to measure diffusion coefficients in low-porosity rocks (< 2%) using X-ray radiography and iodide tracer. The method is a non-destructive technique based on the principle of X-ray attenuation; it provides temporal- and spatially-resolved information of a highly attenuating tracer diffusing in a sample. Samples from the Cobourg Formation, an Ordovician argillaceous limestone from the Michigan Basin, and from the Lac du Bonnet batholith, an Archean granitic pluton were used in this study. X-ray radiography data from the Cobourg Formation indicate tracer accumulation occurs on dark argillaceous layers in the rock characterized by clay minerals and organic matter. It is proposed that the I– tracer solution underwent photo-chemical oxidation, leading to the formation of I2, a highly reactive volatile iodine species and I3–, which readily reacted with humic substances contained in the clay- and organic rich zones in the limestone samples. In the case of the granitic samples, attempts at measuring diffusion coefficients encountered several challenges. The results indicate that tracer signal can be detected, however diffusion signal is masked by imaging errors and noise.