Arjen Mascini , Marijn Boone, Stefanie Van Offenwert, Shan Wang, Veerle Cnudde, Tom Bultreys 

Multiphase flow is important for many natural and engineered processes in subsurface geoscience. Pore-scale multiphase flow dynamics are commonly characterized by an average balance of driving forces. However, significant local variability in this balance may exist inside natural, heterogeneous porous materials, such as rocks and soils. Here, we investigate multiphase flow in heterogeneous rocks with different wetting properties using fast laboratory-based 4D X-ray imaging. The mixed-wet dynamics were characterized by displacement rates that differed over orders of magnitude between 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 dynamics highlight the need for further studies on the fundamental controls on multiphase flow in geomaterials, which is crucial to design, e.g., groundwater remediation and subsurface CO2 storage operations.