On the challenges of greyscale‐based quantifications using X‐ray computed microtomography

For X‐ray computed microtomography (μ‐CT) images of porous rocks where the grains and pores are not fully resolved, the greyscale values of each voxel can be used for quantitative calculations. This study addresses the challenges that arise with greyscale‐based quantifications by conducting experiments designed to investigate the sources of error/uncertainty. We conduct greyscale‐based calculations of porosity, concentration and diffusivity from various μ‐CT experiments using a Bentheimer sandstone sample. The dry sandstone is imaged overtime to test the variation of greyscale values over sequential scans due to instrumentation stability. The sandstone is then imaged in a dry and contrast‐agent saturated state at low resolution to determine a porosity map, which is compared to a porosity map derived from segmented high‐resolution data. Then the linearity of the relationship between the concentration of a contrast agent and its corresponding attenuation coefficient is tested by imaging various solutions of known concentration. Lastly, a diffusion experiment is imaged at low resolution under dynamic conditions to determine local diffusivity values for the sandstone, which is compared to values derived from direct pore‐scale simulations using high‐resolution data. Overall, we identify the main errors associated with greyscale‐based quantification and provide practical suggestions to alleviate these issues.

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