Grain-scale permeabilities of texturally equilibrated, monomineralic rocks

Permeability (k) was measured on synthetic, monomineralic aggregates of quartz and calcite, which serve as analogs for texturally equilibrated, fluid- and melt-bearing rocks such as those in the deep crust and mantle. Porous quartzites were synthesized in the presence of silica-saturated H2O and H2O + NaCl, at 850°C and 1.4 GPa. Permeabilities, measured at near-atmospheric conditions on the quenched samples and scaled to a common grain-diameter (d), increase systematically by about four orders of magnitude as fluid fraction (φ) increases from 0.006 to 0.170. In this range, the k–φ relation can be approximated by the equation k=d2φ3/200. We were not able to determine permeabilities at fluid fractions lower than 0.006 (due to microfractures that formed during depressurization), but we expect the same relation — possibly with a slightly lower exponent on φ — to apply. Permeabilities measured on porous marbles, synthesized at 800°C and 1.0 GPa in the presence of H2O, are lower than values measured on quartzites for a given fluid fraction. At φ>0.027, an equation similar to that which applies to quartzites can be used to describe the k–φ relation, but using (φ−0.015) in place of φ. The lower permeability of the calcite aggregates reflects the fact that not all grain edges are `wettedʹ, and thus some fraction of fluid is present in isolated pores. The `criticalʹ fluid fraction below which all pores are isolated and k is zero lies somewhere between 0.005 and 0.027. Our measured permeabilities closely approximate values predicted by numerical modeling of fluid flow in texturally equilibrated rocks. This correspondence is somewhat surprising, considering that fluid distribution in our samples, and in real rocks, is much more complex than assumed in the numerical models. Assumptions implicit in numerical models — simple grain shape, a single grain size, non-rational crystal faces, and isotropic surface energies — do not apply to natural or synthetic texturally equilibrated rocks. Despite differences in fluid distribution, the similarity of k–φ relations indicates that average length and cross-sectional area of grain-edge channels is the same in a texturally equilibrated rock as it is in a hypothetical rock with uniform pore geometry.