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.

Image‐based pore‐scale modeling is an important method to study multiphase flow in permeable rocks. However, in many rocks, the pore size distribution is so wide that it cannot be resolved in a single pore‐space image, typically acquired using micro‐computed tomography (micro‐CT). Recent multi‐scale models therefore incorporate sub‐voxel porosity maps, created by differential micro‐CT imaging of a contrast fluid in the pores. These maps delineate different microporous flow zones in the model, which must be assigned petrophysical properties as input. The uncertainty on the pore scale physics in these models is therefore heightened by uncertainties on the representation of unresolved pores, also called sub‐rock typing. Here, we address this by validating a multi‐scale pore network model using a drainage experiment imaged with differential micro‐CT on an Estaillades limestone sample. We find that porosity map‐based sub‐rock typing was unable to match the micrometer‐scale experimental fluid distributions. To investigate why, we introduce a novel baseline sub‐rock typing method, based on a 3D map of the experimental capillary pressure function. By incorporating this data, we successfully remove most of the sub‐rock typing uncertainty from the model, obtaining a close fit to the experimental fluid distributions. Comparison between the two methods shows that in this sample, the porosity map is poorly correlated to the multiphase flow behavior of microporosity. The method introduced in this paper can help to constrain the sources of uncertainties in multi‐scale models in reference cases, facilitating the development of simulations in complex reservoir rocks important for for example, geological storage of CO2.

The simultaneous flow of multiple fluid phases through a porous material is an important process encountered in many natural and manmade systems. In earth sciences, it is critically important for the injection and safe storage of CO 2 in deep saline aquifers (Bui et al., 2018), geological energy storage (Mouli-Castillo et al., 2019) and the study of subsurface contaminant transport (Mercer & Cohen, 1990).The pore-scale dynamics of multiphase flow in porous media are known to be governed by a competition between the driving forces on the fluids: capillary, viscous

Fluid flow through intricate confining geometries often exhibits complex behaviors, certainly in porous materials, e.g., in groundwater flows or the operation of filtration devices and porous catalysts. However, it has remained extremely challenging to measure 3D flow fields in such micrometer-scale geometries. Here, we introduce a new 3D velocimetry approach for optically opaque porous materials, based on time-resolved x-ray micro-computed tomography (CT). We imaged the movement of x-ray tracing micro-particles in creeping flows through the pores of a sandpack and a porous filter, using laboratory-based CT at frame rates of tens of seconds and voxel sizes of 12 μm. For both experiments, fully three-dimensional velocity fields were determined based on thousands of individual particle trajectories, showing a good match to computational fluid dynamics simulations. Error analysis was performed by investigating a realistic simulation of the experiments. The method has the potential to measure complex, unsteady 3D flows in porous media and other intricate microscopic geometries. This could cause a breakthrough in the study of fluid dynamics in a range of scientific and industrial application fields.

Fluid invasion into porous materials is very common in natural and industrial processes. The fluid invasion dynamics in simple pore networks are governed by a global balance of capillary, viscous and inertial forces. However, significant local variability in this balance may exist inside natural, heterogeneous porous materials. Here, we imaged slow fluid intrusion in two sister samples of a heterogeneous sandstone, one water-wet and one mixed-wet, using high-resolution 4D X-ray imaging. The pore-by-pore fluid invasion dynamics were quantified, revealing a new type of mixed-wet dynamics where 19% of the fluid invasions were orders of magnitude slower than in directly neighboring pores. While conventional understanding predicted strongly capillary-dominated conditions, our analysis suggests that viscous forces played a key role in these dynamics, facilitated by a complex interplay between the mixed-wettability and the pore structure. These previously unknown dynamics highlight the need for further studies on the fundamental controls on multiphase flow in complex natural porous materials, which are abundant in e.g. groundwater remediation and subsurface CO2 storage operations.