Capillarity-driven supersolubility in dual-porosity systems

Capillary phenomena are widespread in unsaturated media such as soils, building stones, deep depleted aquifers and still gas storage/sequestration reservoirs. Processes linked to capillarity depend on pore size and environmental conditions such as relative humidity. When capillary forces are high, water’s internal pressure can reach negative values representative of a tensile state. In those conditions, the capillary tensile water provokes compaction in moist granular materials. In geochemical terms, thermodynamics predict a significant effect on the solvent properties of tensile water. However, very little experimental work has validated thermodynamics models or shown the porosity arrangements that make capillarity a significant player to control the geochemical balance and poromechanics of porous media. In this study, we designed an experimental setup in glass microtubes (ø 200 µm) conducive to capillary tension. In the tubes, salts (halite and sodium sulfate) precipitated from an evaporating solution to build a strongly contrasted dual-porosity system. Large pore bodies (ø 200 µm) coexisted with nanometric pores that induced capillary tension in the whole volume. We observed mineral–liquid interactions (mass gain/loss, crystal shaping), especially how salt solubility changed as a function of capillary conditions. The water’s tensile state increased salt solubility, changing the reactions’ equilibrium constants at constant composition as predicted by thermodynamics. In addition, mineral grains showed clear evidence of poromechanical tensile stress, driving salts to move towards the interior of the tube or to crack. The results showed the potential significance of capillarity in heterogeneous porous media including nanopores. They should be considered in relation to how porous structures change, in the contexts of increasing droughts due to climate changes and increasing excavation work in deep sedimentary basins.