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(2020) New Developments in Diffusion Measurements Using Laboratory-Based X-Ray Sources

Al T, Morfin S, Hafezian G, Cadieux C & Kumahor S


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08b: Room 3, View in program

Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.

Submitted by Laura Kennell-Morrison on
In the context of the methods described, and improving ability to resolve tracer migration at very low porosities, are there any thoughts as to how low a porosity sample can be investigated in terms of solute mobility. The presentation notes improvements to porosities of <1%, but is there a lower limit (e.g., 0.5%, 0.25%); presumably this will be impacted by the nature of the connected porosity within the rock?
Yes, the connected, or accessible porosity is the critical parameter, and as we know from sedimentary rocks, the accessible porosity can be tracer specific. The granitic rocks that were used for the diffusion experiments described in this presentation have water-accessible porosity ranging from 0.31% to 0.40%, with an average value of 0.35%. The pandemic cut short our research activities so we don't have information on the magnitude of anion exclusion in these rocks, but we can expect that the iodide-accessible porosity is lower than the water-accessible porosity. At this point we don't know what the lower porosity limit might be. The data presented here for granite, and in a related presentation in this session (Cadieux et al), represent our first attempt with the new method of rotating the sample during image acquisition, so there is still room for improvement by tweaking the method. However, the lower threshold for porosity that corresponds to a detection limit for the diffusion coefficient, is also dependent on the mineralogy of the rock (effective atomic number) and the behaviour of the tracer. It is actually easier to image transport of non-conservative tracers because the sorption causes accumulation of tracer mass on the pore walls, leading to an increase in the signal-to-noise ratio. This is evident by a comparison of the iodide versus cesium diffusion profiles presented by Cadieux et al.. The signal (delta mu) for cesium is almost double that for iodide, while the cesium concentration at the influx boundary is half of that for iodide.

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