Experimental simulation of greywacke–fluid interaction under geothermal conditions

► Experimental simulation of water–rock interaction. ► Geothermal reservoir conditions. ► Mineral dissolution/precipitation processes. ► Reservoir rock of the Taupo Volcanic Zone. A continuous-flow, high temperature–pressure apparatus has been used to simulate the reaction of greywacke with distilled water and re-injection brine. During the initial room temperature period of approximately one week, the effluents contained elevated Ca and Mg and had increased pH values due to calcite dissolution. After a temperature increase to 210°C at 35bars, the distilled water simulation reached quartz saturation (∼320mg/kg SiO2) after two days. In the re-injection brine experiment, Al dropped below detection limit and SiO2 concentration reached a steady state value of ∼460mg/kg after the temperature shift to 203°C. Except for amorphous silica, all SiO2 polymorphs remained supersaturated. Newly-formed secondary minerals observed in the distilled water experiment included a Ca–Na–K–Mg–Fe aluminosilicate (a clay mineral), a Ca–Na aluminosilicate (possibly clinoptilolite) and calcite. From the re-injection brine simulation, secondary minerals included a Ca–Na–K–Mg–Fe aluminosilicate (clay), a Ca–Na aluminosilicate (possibly clinoptilolite) and chlorite. These minerals are consistent with those expected during hydrothermal alteration under the P–T conditions of the experiment. Slow precipitation kinetics explains the lack of equilibration with respect to quartz in the re-injection experiment using Wairakei brine; however, other cation activities appear consistent with the thermodynamically predicted mineral assemblages. The experiments underline the utility of fluid-rock laboratory simulations in studying the effects of brine re-injection into aquifers with which the fluid is no longer in chemical equilibrium.