Regeneration of capillary water in unsaturated zones

Claudie Hulin (1, 2) , Lionel Mercury (1, 2)

1 ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327
2 Milieux Poreux - UMR7327

In porous media subject to drying conditions such as arid regions and excavation zones (deep gas injection or nuclear waste disposal), capillarity is involved in weathering processes because it modifies the geochemical and poromechanical balances within the porous network. Heterogeneous porous media like sedimentary rocks can host significant volumes of tensile capillary water in large pore bodies, and the negative pressure within is controlled by capillary forces exerted at nanometric pore throats. We have developed experiments using synthetic bimodal pore systems conducive to capillary tension. In microtubes, salts precipitated in an evaporating solution to build a dual-porosity system. A large volume (ø 200 µm) became trapped behind nanometric pores, where high capillary tension was applied. We investigated the gas-water interactions there, especially how gas nucleated in the trapped liquid and how it subsequently changed size. After gas nucleation, the decreasing of gas volume that we observed has been attributed to two complementary geochemical effects. On the one hand, the water’s tensile state increases gas solubility, as predicted by thermodynamics: capillarity is a “gas-in” process. On the other hand, while the total volume of the gas-water assemblage remains constant, the water’s molar volume increases by capillary forces. Consequently, capillary forces exerted at the nano-throats can (re)induce a superheated monophasic liquid state from a biphasic liquid-gas assemblage even after gas nucleation. Tensions required for gas shrinkage have been estimated at 7 ± 3 MPa and 53 ± 15 MPa. This regeneration process offers opportunities for water to regularly return to a capillary state, making the capillary lifetime less limited than expected. This shows that pore heterogeneity in rocks submitted to drying processes results in tension for water in pores that is long-lived. As a consequence, capillarity may significantly impact the long-term geochemical budget through its effects on gas and solid solubility and/or poromechanics (compaction, tensile stress, fracturing, etc.), so that it may play an important role in the weathering of drying porous materials.

Mots clés: Thermodynamics; Water retention; Metastability; Phase transitions

Domaines: Hydrologie; Planète et Univers [physics]

Claudie Hulin, Lionel Mercury. Regeneration of capillary water in unsaturated zones. Geochimica et Cosmochimica Acta, 2019, 265, pp.279-291. ⟨10.1016/j.gca.2019.07.058⟩. ⟨insu-02267563⟩