THREE-DIMENSIONAL IMAGING OF POROSITY AND TRACER DISTRIBUTION IN A POROUS GEOLOGIC MEDIA USING X-RAY MICRO-CT

Porosity and solute concentration distribution are important parameters necessary for the assessment of diffusive solute transport. This work presents the measurement of these key parameters on a sub-mm scale. The spatial distribution of these parameters has been measured using micro-computed tomography (micro-CT) at a spatial resolution of approximately 20 µm in three-dimensions (3-D). Tracer concentration measurements were conducted in time series during diffusion experiments employing potassium iodide tracer (KI) in dolomite (Silurian Amabel formation in Guelph, Ontario Canada). The sample was prepared using a diamond core drill providing a core of 11 mm diameter and 18 mm length. The core sample was sealed around the perimeter surface at one end, and saturated with a background solution of potassium nitrate (KNO₃). Micro-computed tomography scans were then acquired to provide a reference dataset unaffected by KI tracers. The tracer was subsequently introduced to a reservoir at the lower boundary of the vertically oriented core and micro-CT scans were acquired in time series while tracer diffused upward through the pore space of the core sample. Following completion of the diffusion experiment, the sample was immersed in the KI tracer solution under vacuum in order to saturate all pores with tracer. A final micro-CT scan was acquired for the tracer-saturated sample. The acquired datasets were processed and analyzed using an in-house developed ManualSeed Registration Code and ImageJ, an open-source software package from the US National Institute of Health (NIH), to determine spatial distributions of porosity and relative tracer concentration. Results have provided direct visualization of macro-pore connectivity and evidence of preferred pathways in solute transport. Spatial distributions of diffusion-accessible and inaccessible porosity are attainable. The datasets for porosity and tracer concentration can be used as input for the validation of diffusive solute transport numerical models. Noise within the acquired datasets has been quantified to determine the limitations of the measurements.