The use of Micro-CT for the Determination of Diffusion Parameters: Strengths and Limitations

Diffusive solute transport is a key issue in the assessment of rocks of low hydraulic conductivity for waste containment purposes. It is therefore pertinent to measure porosity as it is an important control on solute transport, and solute concentration distributions as they are the fundamental indicator of transport direction and rate. This work present the three-dimensional (3-D) measurement of these parameters using micro-computed tomography (micro-CT) in a laboratory controlled experiment. The spatial distributions of these parameters have been measured at approximately 20 µm in 3-D within Silurian dolomite from the Amabel formation in Guelph Ontario Canada. The non-destructive nature of the micro-CT technique enabled the measurement of tracer concentration in time series during a diffusion experiment using potassium iodide (KI) tracer. A reference scan of the sample was initially acquired without KI in the system and then KI tracer was subsequently allowed to diffuse into the system via an attached tracer reservoir. Micro-CT scans were acquired in time series as diffusion progressed, and a final micro-CT scan was acquired after totally immersing the sample in KI solution to saturate all the accessible pores. The micro-CT data were analyzed to determine the precision of measurement and the sensitivity to tracer concentration differences. Results of the data analysis showed an average precision of about 8 % in measured x-ray attenuation values from replicate datasets. Accounting for error propagation in data processing, porosity at each voxel has been measured with a relative error margin of ± 16 % and concentration distribution at different times of diffusion has been measured with a relative error ± 32 % at each voxel. The concentrations generally increase with increasing diffusion time. This micro-CT technique provides the visualization of porosity in 3-D and concentration distributions at different times.