Pore-Scale Visualization and Quantification of Transient Solute Transport Using Fast Microcomputed Tomography

Solute transport is important in a variety of applications regarding flow in porous media, such as contaminant groundwater remediation. Most recent experimental studies on this process focus on field-scale or centimeter-scale data. However, solute spreading and mixing are strongly influenced by pore-scale heterogeneity. To study this, we developed a novel methodology to quantify transient solute concentration fields at the pore scale using fast laboratory-based microcomputed tomography. Tracer injection experiments in samples with different degrees of pore-scale heterogeneity (porous sintered glass and Bentheimer sandstone) were imaged in 3D by continuous scanning at a time resolution of 15 s and a spatial resolution of 13.4 μm. While our calibration experiments indicated a high uncertainty (1σ) on the concentration in single voxels due to imaging noise (± 27% of the total concentration range), we show that coarse gridding these values per individual pore significantly lowers the uncertainty (± 1.2%). The resulting pore-based tracer concentrations were used to characterize the transport by calculating the solute's arrival time and transient (filling) time in each pore. The average velocities estimated from the arrival times correspond well to the interstitial velocities calculated from the flow rate. This suggests that the temporal resolution of the experiment was sufficient. Finally, the pore-based transient filling times, the global concentration moment and the global scalar dissipation rate calculated from our experiments, indicated more dispersion in the sandstone sample than in the more homogeneous sintered glass. The developed method can thus provide more insight in the influence of pore-scale heterogeneity on solute transport