Phase Transitions in a Dual Porous Media Connecting Nano-scale Throats to Millimeter-scale Pores
Résumé
Principal ambiguities exist in the hydro-chemical coupling within unsaturated hydrosystems when the surface area of liquid-vapor interfaces increases dramatically through capillary effects, while modifying the chemical potential of aqueous solutions. Such archetypical situations are created along with the containment barriers of underground storage made of thinly porous formations of very low permeability, either the impervious caprock (CO2 storage) or host rock (nuclear waste). Due to the coexistence of a dry reservoir (CO2 bubble along the cap rock, ventilated tunnels inside argillaceous formation) with the containment barriers, capillary-driven processes propagate behind the liquid-vapor capillary bridges in the whole hydraulically connected water reservoir. To study the capillary-driven mass balance and its associated geochemical features, capillary water bodies were experimentally investigated within a synthetic dual-pore model with perfectly controlled inner geometries. These Lab-on-a-Chip dual-porous media were designed based on two macro channels, which are connected through a series of nanochannels (Fig. 1). The access holes located on the macro reservoirs make it possible to control/change the capillary states tuning the relative humidity (RH) in one reservoir, while flowing an aqueous solution of known composition in the opposite reservoir therefore infilling the nanochannels. Tuning the evaporative demand allows us to provoke evaporation, therefore, to change salinity. The optical observations of the evaporation kinetics were recorded with respect to the depth of nanochannels (ranging from 5 to 100 nm). Evaporative demand can control the phase transition (crystallites precipitation, water cavitation) in the nanochannels or the macro reservoirs. The primary outcome corresponds to the coupling between pore-size controlled and capillary-controlled geochemistry in porous media. Also, a modified Kelvin-Laplace equation and the classical nucleation theory have been used to interpret the observations.