Effects of grain size distribution on the permeability of compacting aggregates: Results from rock-analogue experiments and microphysical models

Abstract

We investigate the evolution of permeability with porosity and grain size in compacting aggregates of brine-saturated salt. The porosity of aggregates with different initial grain size distributions was reduced by a step-wise application of compactive strain, retaining the effective normal stress steady state flow with a pore pressure differential of 0.1 MPa across two granular layers in parallel. We found that permeability decreased with decreasing porosity according to a power law relation, consistent with predictions from capillary models (i.e. Kozeny-Carman Equation), but with an exponent that changes with porosity. The Kozeny-Carman model overestimates observed permeabilities in all experiments by at least one order-of-magnitude as a result of the assumed monodispersed distribution of grain diameters and therefore pore throat sizes. We extend the Kozeny-Carman model using an expression for the tortuosity that is dependent on the actual grain size. The broader grain size distributions, with abundant small grains, returns lower permeabilities than those predicted using the average or median grain size.