Water vapor condensation in porous media: Effects of fracture, porosity, and flow rate revealed by rapid 4D neutron imaging

Abstract

This study investigates water vapor condensation processes in a fractured porous medium (sandstone), focusing on the effects of fracture conductivity, matrix porosity, and imposed flow rate of the vapor. Cylindrical samples of Fontainebleau sandstone were pre-fractured using the Brazilian splitting test and subjected to a constant vapor and air mixture flow rate. Rapid in situ neutron tomography, captured every 30 s, visualized time-resolved water migration through the porous media. Owing to the high neutron attenuation by hydrogen, the obtained contrast allowed quantitative 3D tracking of condensed water within the material bulk. The experimental data demonstrated the reliability of neutron imaging through consistent measurements of overall water content, as verified by imposed macroscopic boundary conditions. The results showed that cracks act as preferential pathways for vapor migration, leading to the formation of initial wetting fronts and subsequent capillary absorption into the matrix from inner fracture surfaces. The presence of a crack slowed the propagation of the water front in the porous matrix, allowing more water to accumulate within it. Low porosity samples exhibited lower water contents in the matrix, resulting in faster propagation of the water front due to the limited water absorption capacity of the matrix. In this case, water accumulates in the crack in the form of water patches, especially at lower flow rates, where the vapor’s exerted pressure is lower, allowing more water to remain in the porous matrix. At higher flow rates, cracks remained drier compared to the matrix due to increased injection pressure, which transferred water into the matrix more effectively. Rapid transitions of water patches were observed in the high flow rate low porosity case, highlighting dynamic water movement in the crack. The study underscores the importance of heterogeneity in the form of cracks in the absorption of condensed water and highlights the roles of capillary effects and pressure-driven flow in the transportation of water within fractured porous media. It provides novel full-field experimental datasets, which are invaluable for further numerical simulations.