Grain-scale permeabilities of texturally equilibrated, monomineralic rocks

Permeability (k) was measured on synthetic, monomineralic aggregates of qua rtz and calcite, which serve as analogs for texturally equilibrated, fluid- and melt-bearings rocks such as those in the deep crust and mantle. Porous quartzites were synthesized in the presence of silica-saturated H2O and H2 O + NaCl, at 850 degrees C and 1.4 GPa. Permeabilities, measured at near-at mospheric conditions on the quenched samples and scaled to a common grain-d iameter (d), increase systematically by about four orders of magnitude as f luid fraction (phi) increases from 0.006 to 0.170. In this range, the k-phi relation can be approximated by the equation k = d(2)phi(3)/200. We were n ot able to determine permeabilities at fluid fractions lower than 0.006 (du e to microfractures that formed during depressurization), but we expect the same relation - possibly with a slightly lower exponent on phi - to apply. Permeabilities measured on porous marbles, synthesized at 800 degrees C an d 1.0 GPa in the presence of H2O, are lower than values measured on quartzi tes for a given fluid fraction. At phi > 0.027, an equation similar to that which applies to quartzites can be used to describe the k-phi relation, bu t using (phi - 0.015) in place of phi. The lower permeability of the calcit e aggregates reflects the fact that not all grain edges are 'wetted', and t hus some fraction of fluid is present in isolated pores. The 'critical' flu id fraction below which all pores are isolated and k is zero lies somewhere between 0.005 and 0.027. Our measured permeabilities closely approximate v alues predicted by numerical modeling of fluid how in texturally equilibrat ed rocks. This correspondence is somewhat surprising, considering that flui d 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 texturall y equilibrated rocks. Despite differences in fluid distribution, the simila rity of k-phi relations indicates that average length and cross-sectional a rea of grain-edge channels is the same in a texturally equilibrated rock as it is in a hypothetical rock with uniform pore geometry. (C) 1998 Elsevier Science B.V. All rights reserved.