Phase separation and convection in heterogeneous porous media: Implications for seafloor hydrothermal systems

We examine experimentally the effect of phase separation of a binary fluid on convection and heat transfer in porous domains with heterogeneous permeability fields. In mid-ocean ridge hydrothermal systems, a dense brine phase and a coexisting vapor phase form during the phase separation of seawater at supercritical conditions; this process strongly influences the chemistry of vents and the convective transfer of heat. Furthermore, convection and phase separation are likely to occur in oceanic crust that is highly heterogeneous in nature. Because of high temperatures and pressures required for phase separation in saline systems, a binary fluid [H₂o-2-butoxyethanol], with a lower consolute point of 48.5°C, was used as a proxy for seawater. A saturated pseudo-two-dimensional porous domain containing high permeability [k] zones embedded in a lower permeability matrix was heated from below and cooled from above. As with homogeneous systems, chemical stratification develops in heterogeneous domains, with dense fluid accumulating in low-velocity regions at the bottom of the porous matrix. Furthermore, in both types of porous media, the efficiency of thermal transport is reduced relative to single-phase systems. Heterogeneities in the permeability field, however, can act to amplify these effects; in systems with vertical high-k zones, the thickness of the dense-phase layer increases in the low-k regions, and thermal transport is suppressed even more. Also, the experiments show that regions of both high and low permeabilities can act as a store for brines and facilitate the formation of much thicker brine layers than are otherwise predicted by models based on homogeneous oceanic crust.