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HypothesisCapillary-dominated multiphase flow in porous materials is strongly affected by the pore walls’ wettability. Recent micro-computed tomography (mCT) studies found unexpectedly wide contact angle distributions measured on static fluid distributions inside the pores. We hypothesize that analysis on time-resolved mCT data of fluid invasion events may be more directly relevant to the fluid dynamics.ExperimentWe approximated receding contact angles locally in time and space on time-resolved mCT datasets of drainage in a glass bead pack and a limestone. Whenever a meniscus suddenly entered one or more pores, geometric and thermodynamically consistent contact angles in the surrounding pores were measured in the time step just prior to the displacement event. We introduced a new force-based contact angle, defined to recover the measured capillary pressure in the invaded pore throat prior to interface movement.FindingsUnlike the classical method, the new geometric and force-based contact angles followed plausible, narrower distributions and were mutually consistent. We were unable to obtain credible results with the thermodynamically consistent method, likely because of sensitivity to common imaging artifacts and neglecting dissipation. Time-resolved mCT analysis can yield a more appropriate wettability characterization for pore scale models, despite the need to further reduce image analysis uncertainties.

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

The mathematical models for the capillary-driven flow of fluids in tubes typically assume a static contact angle at the fluid–air contact line on the tube walls. However, the dynamic evolution of the fluid–air interface is an important feature during capillary rise. Furthermore, inertial effects are relevant at early times and may lead to oscillations. To incorporate and quantify the different effects, a fundamental description of the physical processes within the tube is used to derive an upscaled model of capillary-driven flow in circular cylindrical tubes. The upscaled model extends the classical Lucas–Washburn model by incorporating a dynamic contact angle and slip. It is then further extended to account for inertial effects. Finally, the solutions of the different models are compared to experimental data. In contrast to the Lucas–Washburn model, the models with dynamic contact angle match well the experimental data, both the rise height and the contact angle, even at early times. The models have a free parameter through the dynamic contact angle description, which is fitted using the experimental data. The findings here suggest that this parameter depends only on the properties of the fluid but is independent of geometrical features, such as the tube radius. Therefore, the presented models can predict the capillary-driven flow in tubular systems upon knowledge of the underlying dynamic contact-angle relation.

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.