Experimental study and modelling of granite-distilled water interactions at 180°C and 14 bars

Abstract

Distilled water was percolated for 38 days through Soultz granite (France) from the candidate site of the future Hot Dry Rock (HDR) geothermal exchanger at 180°C and 14 bars in a dynamic experimental system (plug-flow system with Ti reaction cell: I.D. 50 mm and L. 120 mm). The amplitude of dissolution and precipitation reactions was estimated after fluid analysis, and scanning electron microscope observations of the minerals were done following the percolation experiment. There were well developed etch pits on the surface of the feldspars. These were, therefore, the most reactive minerals. Dissolution and precipitation are distributed in zones along the flow path of the reacting fluid. Newly precipitated calcite is observed along the fluid flow path. Ferromagnesian saponite precipitates only at the outlet part of the system. The following mobility order was observed: SiO 2 > HCO 3 > F ∼ Na > Ca > Al > Li > Cs > Mg > Ba > Rb > B > Sr. By combining dissolution reaction kinetics, according to the law of the transition state theory, for the granite minerals, and the local chemical equilibrium approach for the precipitation of the secondary minerals and for the reactions of aqueous complexation, we were able to simulate the first reaction pathway of this experiment using the geochemical code EQ3/6 with the ‘fluid-centred flow-through' calculation mode. The experimental results (observations of minerals and chemial composition of theoutlet fluid) provided model constraints. Particular attention was paid to the effectively reactive surface of the minerals. The correction of the reactive surface in relation to the total measured surface was necessary so that calculated results would be in agreement, at least qualitatively, with observations. A good agreement is obtained only if the effective reactive surface of the minerals is taken to be from 0.03 to 3.2% of the total surface measured by the BET-N 2 method.