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

Abstract

Dissolution and precipitation of minerals during water–rock interaction influence the permeability of cracked rock (sealing or wide-opening of fractures). The fracture parameters have consequently to be considered as non-constant variables that can evolve with time and during the alteration. The aim of this study is to quantify these feedback effects using the meteoritic weathering of the Brézouard 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 geometrical description of the crack networks (theory of percolation). Each crack is assumed to be a thick disc defined by its radius, thickness, and orientation. The image analysis of oriented sections shows four crack families with different orientations, volumic densities, radii ad apertures. These crack parameters are used in the calculation of the initial crack porosity (4.5%), of the permeability tensor (0.180mD for the maximum component) and of the surface area of the water rock interface (234m2). The second approach is a geochemical modeling of the granite weathering at room temperature (25°C). The rock alteration is modeled as a function of time using the computer code kindis and the quasi-stationary state approximation. A standard rain water enters and percolate the granite cracks (open water–rock system). The spatial extent of the alteration is deduced using the Washburn's equation assuming a capillary propagation of the fluid in the cracked rock. 23yr of water rock interactions are simulated. 8 successive paragenesis of alteration are observed through 13m of rock before a steady state is reached. Dissolution of the granite primary minerals is the main phenomenon that leads to an increase of the connected porosity (from 4.5 to 4.75%) as well as the permeability (from 0.180 to 0.215mD). These effects decrease with depth.