Saturated solute transport micro-CT dataset in Savonnières limestone

• 10.1029/2019WR025880

Abstract — 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.

• https://doi.org/10.1016/j.advwatres.2015.05.012

Abstract — Over the past decade, the wide-spread implementation of laboratory-based X-ray micro-computed tomography (micro-CT) scanners has revolutionized both the experimental and numerical research on pore-scale transport in geological materials. The availability of these scanners has opened up the possibility to image a rock’s pore space in 3D almost routinely to many researchers. While challenges do persist in this field, we treat the next frontier in laboratory-based micro-CT scanning: in-situ, time-resolved imaging of dynamic processes. Extremely fast (even sub-second) micro-CT imaging has become possible at synchrotron facilities over the last few years, however, the restricted accessibility of synchrotrons limits the amount of experi- ments which can be performed. The much smaller X-ray flux in laboratory-based systems bounds the time resolution which can be attained at these facilities. Nevertheless, progress is being made to improve the quality of measurements performed on the sub-minute time scale. We illustrate this by presenting cutting-edge pore scale experiments visualizing two-phase flow and solute transport in real-time with a lab-based environmental micro-CT set-up. To outline the current state of this young field and its relevance to pore-scale transport research, we critically examine its current bottlenecks and their possible solutions, both on the hardware and the software level. Further developments in laboratory-based, time-resolved imaging could prove greatly beneficial to our understanding of transport behavior in geological materials and to the improvement of pore-scale modeling by providing valuable validation.