Mineralogical alteration and associated permeability changes induced by a high-pH plume: Modeling of a granite core infiltration experiment

摘要

Within the framework of the HPF project (Hyperalkaline Plume in Fractured Rock) at the Grimsel Test Site (Switzerland), a small scale core infiltration experiment was performed at the University of Bern. A high-pH solution was continuously injected, under a constant pressure gradient, into a cylindrical core of granite containing a fracture. This high-pH solution was a synthetic version of solutions characteristic of early stages in the degradation of cement. The interaction between the rock and the solutions was reflected by significant changes in the composition of the injected solution and a decrease in the permeability of the rock. Changes in the mineralogy and porosity of the fault gouge filling the fracture were only minor. One-dimensional reactive transport modeling, using a modified version of the GIMRT code, was used to interpret the results of the experiment. Dispersive and advective solute transport, mineral reaction kinetics and a coupling between porosity and permeability changes were taken into account. In order to obtain a reasonable agreement between models and experimental results, reactive surface areas of the order of 105 m(2)/m(3) rock had to be used. These values are much smaller than the values measured for the fault gouge filling the fracture, which are in the order of 10(6)-10(7) m(2)/m(3) rock. However, the results could be improved by adding a small fraction of fine grained mineral, which could explain the high initial peaks in At and Si concentration. With the inclusion of this fine grained fraction, the initial surface areas in the model were within the range of the measured specific surface areas of the fault gouge. The fact that the decrease in permeability was significant despite the minor changes in mineralogy, suggests that permeability may be controlled by changes in the structure of the rock (pore geometries) rather than by only the bulk volumetric (porosity) changes. (c) 2006 Elsevier Ltd. All rights reserved.