Mass transport and reaction in water-saturated and unsaturated rocks

Abstract

Water-rock interaction involves the mass transport driven by fluid flow and the chemical reaction. This paper presents investigations of fluid flow and reaction under water-saturated and unsaturated conditions from the pore scale to the field scale. The results of flow-through dissolution experiments using a saturated and unsaturated sandstone show that the pore surfaces are covered by the water film with a thickness of 7-18 nm, which allows the dissolution and diffusion even if the pores are filled with air. With respect to mass transport at the core scale, which pore structure (porosity and pore size) determines the ease of fluid flow (permeability) is a long-standing problem. The maximum pore-throat radius was determined by measuring the pressure at which gas initially penetrates a water-saturated medium. The maximum pore-throat radius is highly correlated with the permeability of various natural and artificial porous media over a wide range of permeability values. To study fluid flow on a larger scale, I focused on quartz-calcite veins in metapelite, which is considered as a fossil fluid conduit in the plate boundary at 10-15 km depth. Noble gas isotope analyses of fluid inclusions trapped in the veins show that long-distance fluid migration from the slab mantle or mantle wedge toward the shallower plate boundary.