Evolution of crack permeability during fluid-rock interaction. Example of the Brezouard granite (Vosges, France)

Dissolution and precipitation of minerals during water-rock interaction inf luence the permeability of cracked rock (sealing or wide-opening of fractur es). The fracture parameters have consequently to be considered as non-cons tant variables that can evolve with time and during the alteration. The aim of this study is to quantify these feedback effects using the meteoritic w eathering of the Brezouard granite (Vosges, France). Chemical modifications of the rock and the resulting hydraulic evolution of the crack network are numerically quantified. A first permeability calculation is based on the g eometrical description of the crack networks (theory of percolation). Each crack is assumed to be a thick disc defined by its radius, thickness, and o rientation. The image analysis of oriented sections shows four crack famili es with different orientations, volumic densities, radii ad apertures. Thes e crack parameters are used in the calculation of the initial crack porosit y (4.5%), of the permeability tensor (0.180 mD for the maximum component) a nd of the surface area of the water rock interface (234 m(2)). The second a pproach is a geochemical modeling of the granite weathering at room tempera ture (25 degreesC). The rock alteration is modeled as a function of time us ing the computer code KINDIS and the quasi-stationary state approximation. A standard rain water enters and percolate the granite cracks (open water-r ock system). The spatial extent of the alteration is deduced using the Wash burn's equation assuming a capillary propagation of the fluid in the cracke d rock. 23 yr of water rock interactions are simulated. 8 successive parage nesis of alteration are observed through 13 in of rock before a steady stat e is reached. Dissolution of the granite primary minerals is the main pheno menon that leads to an increase of the connected porosity (from 4.5 to 4.75 %) as well as the permeability (from 0.180 to 0.215 mD). These effects decr ease with depth. (C) 2001 Elsevier Science B.V. All rights reserved.