Spontaneous Imbibition and Evaporation in Rocks at the Nanometer Scale.

Understanding multiphase fluid displacement dynamics in porous media is of great importance in efficiently designing hydrogen storage projects in porous reservoirs. During gas injection and extraction, cyclic evaporation and spontaneous imbibition processes have an impact on storage efficiency. In both imbibition and evaporation, capillary films on the surface of grains play a role in the transport of water through the pore space. In this study, we use atomic force microscopy to study the formation of these films in carbonate rock during imbibition and their dynamic behavior during evaporation. The imbibition dynamics are related to pore-scale processes determined by micro-CT experiments. We find that imbibition through the mesoporous structure of the grains is slower compared to imbibition in macropores. The formation of the water film on the outer grains is also slower, indicating that a film is evolving due to water flow through intragranular mesopores rather than film flow around the grains. Evaporation experiments reveal that the film shows both local swelling and shrinkage behavior, which we relate to pore-scale processes causing disconnection of the water film. Our results show the close relationship between pore-scale processes and water film dynamics during both spontaneous imbibition and evaporation. This work forms a basis for a more quantitative study of the impact of pore structure on wetting and drying dynamics and can be extended to reactive flow processes.