Mineralogical heterogeneity in fractured, porous media and its representation in reactive transport models

Abstract

Reactive transport models that simulate processes in porous media have, generally, required abstracted representation of porosity, permeability, and mineralogy. This study compares abstracted, homogeneous representations of porosity and permeability, mineral surface areas and distributions, to discrete distribution representation of these same properties. Discretization was accomplished by high-resolution (ca. 1 μm 2) characterization of fractured tuffaceous rock from Yucca Mountain, Nevada, using optical microscopy and X-ray fluorescence spectroscopy. A sample area of 10 6 μm 2 was mapped in detail, and the resulting element and porosity maps were digitized. The domain was decomposed into 12,208 cells that were 8.77 × 10 −6 m on a side. Simulations were conducted in which a dilute fluid enters the discretized porous medium at modest flow rates. Simulation results using a discrete mineral distribution point to the conclusion that slow flow rates, in which fluid residence times are on the order of days, provide fluid composition results that are very similar to those obtained from the homogeneous mineral distribution representation. At higher flow rates, where fluid residence times are on the order of hours, contrasts in fluid composition persist throughout the flow domain. The results demonstrate that the fluid composition characteristics in the homogeneous and discrete mineral representations will be similar only when the bulk average contact times for the individual mineral phases along the flow paths are approximately equivalent (within a few percent) for the two cases.