Fractured hard rocks

Rutqvist, J M. Chijimatsu, L. Jing, A. Millard. T.S. Nguyen, A. Rejeb, Y.Sugita and C.F. Tsang, Evaluation of the impact of thermal-hydrological-mechanical couplings in bentonite and near-field rock barriers of a nuclear waste repository in sparsely fractured hard rock, this volume, 2004. 

In terms of numerical methods, the dominance by FEM with equivalent continuum approach might not be most suitable for sparsely or moderately fractured hard rocks and more advanced methods and codes using discrete approach are needed. The issue of applicability of the equivalent media approach, the associated scale effects, and uncertainty evaluations need to be fully explored. The processes are dominated by coupled stress-flow problems and effects of thermo-chemical effects need more attention. More works for soils, clays, sands and other similar media, which are equally, if not more, important in the fields of geo-engineering and environments, seem also needed. 

EVALUATION OF THE IMPACT OF THERMAL-HYDROLOGICAL-MECHANICAL COUPLINGS IN BENTONITE AND NEAR-FIELD ROCK BARRIERS OF A NUCLEAR WASTE REPOSITORY IN SPARSELY FRACTURED HARD ROCK... 

Stress induced permeability change is of crucial importance in various kinds of applications such as nuclear waste disposal in deep geological formations, geothermal energy utilization and underground excavations. In particular, coupling between the stress and permeability is a key element in understanding the nature of flow in the fractured rock (Rutqvist and Stephansson, 2003). This is because fractures, which are the main pathways of fluid flow in fractured hard rocks, are heavily dependent on the stress conditions for their deformations. 

Fig. 1. Variation of pH with Cl in groundwaters from fractured hard rocks from the Canadian (Bottomley et al. 1990, 1994) and Fennoscandian (Nurmi et al. 1988 Nordstrom et al. 1989a Smellie Laaksoharju, 1992) Shields, the northern Swiss basement (Pearson et al. 1989) and the Carnmenellis granite, UK (Edmunds et al. 1984), together with those generated using EQ3/6 (version 7.2a, Wolery 1992) and thermodynamic data (from Johnson et al. 1992) for an assemblage of low temperature minerals in equilibrium with water and different concentrations of Cl . The mineral assemblage (with controlled element in parenthesis) employed was chalcedony (Si) albite (Na) K-feldspar (K) laumontite (Ca) chlorite (Mg) kaolinite (Al) calcite (HCO3-).

Fig. 3. Variation of Pqo with temperature for pore fluids from sedimentary basins (open symbols) and groundwaters in fractured hard rocks (closed symbols). Open diamonds - Paris Basin (Michard Bastide 1988), open squares - US Gulf Coast (Smith Ehrenberg 1989), open circles - North Sea (Smith Ehrenberg 1989). Closed symbols and other sources of data are as for Fig. 1.

Pco2 in groundwaters from fractured hard rocks in Shield regions tends to decrease with increasing depth (Fig. 4). Although carbon... 

Fig. 4. Variation of PcOi with depth for groundwaters in fractured hard rocks. Sources of data are as for Fig. 1.

Mass action relationships for aluminosilicate-carbonate-water reactions will be the most important constraint in buffering pH if their reaction rates are fast enough to accommodate changing environmental conditions (kinetic constraint), and there are sufficient amounts of these minerals in the rock (mass balance constraint). To attempt to address the kinetic constraint first, we can consider how fast a mineral typical of low temperature alteration in both sediments and fractured hard rocks (albite) behaves. Albite (Na-plagioclase) has a congruent dissolution... 

Aluminosilicates abound in most rocks considered suitable for the disposal of radioactive wastes (acidic-basic igneous and metamorphic rock types, and clays), so that the abundance of these minerals as potential buffers of pH should not be a limiting factor. Calcite may be less abundant in fractured hard rock systems than sedimentary rocks such as clays. 

Rcoi low (< 10 bars) closed system behaviour fractured hard rocks away from tectonic/magmatic activity, particularly in shield regions... 

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