Hydrothermal experiments and thermo-kinetic modelling of water-sandstone interactions

The aim of this work was to improve the understanding of the chemical behaviour of core samples of porous rock during percolation. Experiments were performed at 150°C on samples of Triassic sandstone from the Vosges massif (NE France) with daily sampling of the fluids after their transfer through the reactor containing the core. Fluids were found to describe dissolution-precipitation phenomena occurring within the rock during the percolation. A steady state of dissolution was rapidly reached after several hours. Although mass transfers to the fluid were small, up to 1% of the total rock was dissolved, which is significant for the corresponding duration of the experiments (11 days). The use of geochemical, thermo-kinetic computer models allowed a more precise interpretation of the water-rock chemical interactions responsible for the evolution of the concentration of the major aqueous elements. These reactions were included for both dissolution of primary minerals and precipitation of secondary phases. The simulations which best explained the concentration variations of the major elements in the experimental solutions were those where the sandstone alteration mass balance was essentially due to the dissolution of primary minerals. This supports the hypothesis of a strong kinetic control on the formation of secondary phases in relation to relatively more rapid irreversible dissolution of some primary minerals. The duration of the reactions established both experimentally and by computer simulations confirms that the chemical dissolution reactions occur rapidly and that the extracted solutions were saturated with respect to the primary minerals. Hematite, kaolinite and a magnesian smectite were predicted to precipitate from a computer-simulated solution chemically comparable to the experimental ones, but these phases were not observed as products of the experiments. Consequently, a kinetic model for the precipitation of secondary clay phases should be developed.