Dissolution of precipitated salt under suppressed brine backflow in microfluidic porous media

In a wide range of natural and engineering systems, multiphase flow in porous media is accompanied by salt precipitation, which can significantly alter pore structure and reduce permeability. While salt deposition is commonly associated with persistent clogging, its evolution under conditions without bulk flow remains poorly understood. In this study, we use microfluidic experiments to investigate the fate of precipitated salt under quasi-static conditions. Salt deposits are first generated during a gas-brine displacement process, after which a small amount of brine is injected to one side of the chip while a constant gas back pressure is maintained to prevent capillary brine backflow. Under these conditions, time-resolved imaging reveals that previously formed salt deposits gradually dissolve. Surprisingly, water can return into regions containing dry carbon dioxide and precipitated salt crystals, disconnected from the continuous brine phase by a relatively large distance. It accumulates around crystals, dissolving them and forming separate brine “clusters”. The resulting phase distribution remains highly heterogeneous, with isolated brine regions that do not restore effective flow pathways. Possible mechanisms for water transport to the isolated salt deposits, including surface transport and water-vapor diffusion through the gas phase, are discussed and evaluated by comparing their characteristic timescales using simplified model analyses. These findings reveal a previously unrecognized post-drying dissolution pathway in porous media, demonstrating that precipitated salt can remain dynamically active even without observable bulk brine reconnection. The results demonstrate that drying does not necessarily terminate pore-scale evolution, providing a new perspective on the long-term evolution of salt precipitation in porous media.