Vapor condensation in fractured porous media revealed by in-situ rapid neutron tomography and numerical modeling

The study of phase change in processes involving two-phase flow in porous media remains relatively under-explored due to the intricate nature arising from the strong coupling between heat and mass transfer and the heterogeneity of the medium. However, condensation in porous media plays a crucial role in various applications, including steam-based gas recovery, underground contamination removal, the integrity of geothermal, CO₂ storage reservoirs, durability of concrete structure, and porous fabric and insulation condensation. The objective of this study is to provide a deeper understanding of the subject by conducting rapid neutron tomographies during vapor injection experiments and introducing a novel numerical approach to model the process.

The identification and quantification of water is revealed using 3D rapid in-situ neutron imaging, acquired at 30-second intervals per tomography. Such temporal resolution is possible thanks to the high neutron flux of the Institute Laue Langevin Grenoble (ILL) using the imaging instrument NeXT (Neutron and X-ray Tomograph [1]). The experiments were preceded by a calibration and correction campaign where the quantification of water content was fitted to empirical correlation and the spurious deviations arising from the scattering of neutrons were accounted for using the black body (BB) grid method [2]. The in-situ experiment consists of the injection of a predefined mixture of air and water vapor at a constant flow rate into cylindrical samples of Fontainebleau sandstone with a splitting crack along their height. Successive rapid neutron tomographies are acquired during the injection of vapor to investigate the evolution of water content and condensation process inside the sample. Furthermore, X-ray tomography is performed prior to the vapor injection, and part of the sample is scanned by synchrotron microtomography with 6.5 micrometers pixel size. This allows for extracting the microstructure and morphology of the crack and porous matrix, and its impact on the spatio-temporal accumulation of liquid water, and understanding its migration within the crack and matrix. The results [3,4] show that water initially emerges near the inlet and spreads toward distant areas. Condensed water generally has the tendency to occupy tighter spaces within the sample. The condensed water diffuses into the porous matrix due to capillary effects and pressure buildup in the crack.

Preliminary results of a numerical model developed in OpenFoam are also discussed. The model solves heat transfer and two-phase flow equations with phase change mass transfer terms. It is capable of modeling water condensation and temperature fields within a domain of heterogeneous porous media contributing additional insights to the phenomena.

References:

1) Tengattini, A., et al., Nucl. Instrum. Methods Phys. Res., 968, 163939 (2020)

2) Boillat, P., et al., Optics Express, 26(12), 15769-15784 (2018)

3) Gupta, R. et al., Cem. Concr. Res., 162, p. 106987. (2022)

4) Nemati, A., et al., Transp. Porous Media, 150(2), 327-357 (2023)