Silica Nanoparticles Produced by Flash Vaporization from Supercritical Fluids

Abstract

The dissolution and precipitation of silica minerals plays critical roles on the formation and maintenance of the geothermal reservoirs in the crusts (Founier, 1999; Saishu et al., 2014). The solubility of silica minerals is a strong function of temperature and water density; therefore, a drop of fluid pressure from lithostatic to hydrostatic is thought to cause significant silica precipitation. Recently, a model has been produced that fluid pressure can drop below hydrostatic condition at fault jog induced by earthquake rapturing, resulting in the formation of gold-quartz veins (Weatherly and Hanely, 2013). However, this interesting "flash vaporization" model has not been verified experimentally. The significant change in quartz solubility occurs at conditions of near critical points of water. Therefore, the understanding of features of silica in superheated vapor is also important for the development of supercritical geothermal reservoirs. In this study, we conducted flash experiments from sub to supercritical conditions (250, 350, 400, 450 ˚C at 36MPa). The input high-silica solution was created by dissolution of granite in distilled water (Si = ~250 mg/kgH2O). We set the input solution to 110 ml autoclave, raised T-P conditions to the target values, then opened the valve to air. We found that near isothermal decompression from liquid to vapor, or from supercritical fluid to vapor within 5 seconds. The precipitates was caught by the alumina filter placed at the outlet of the autoclave. At all conditions, we found the stacking of spherical particles of amorphous silica in size of 0.1 to 6 μm (modal values are 500 to 900 nm). This size distribution could be produced by nucleation and aggregation of silica particles during the vaporization of the water droplet. Such nanoparticles of amorphous silica can change to quartz in solutions, and the small size and high mobility of amorphous silica particles produced from the decompression of supercritical fluids could play important effects on the fracture sealing and scaling of geothermal pipelines and turbines during the exploitation of supercritical reservoirs. [1] Founier, RO., 1999, Economic Geology, 94, 1193-1211. [2] Saishu, H., Okamoto, A., Tsuchiya, N., 2014. Terra Nova, 26, 253-259. [3] Weatherley, D.K., Henley, R.W., 2013. Nature Geosciences, 6, 294.