An experimental study on quartz solubility in water under supercritical geothermal conditions

Abstract

Supercritical geothermal resource is a renewable and carbon-free energy with enormous potential. Quantitative assessment of the geochemical processes involved in the future operation of this resource requires a comprehensive understanding of quartz solubility in subcritical and supercritical water. However, quartz solubility behaviors from subcritical to supercritical conditions are unclear, and the quartz solubility model requires refinement by improving the accuracy of experimental measurements. In this study, a series of in-situ sampling and measurements are conducted to investigate quartz solubility at 300–500 °C and 25–50 MPa by exploiting a novel and specifically designed experimental apparatus. The results indicate that the use of in-situ sampling and resistance continuous monitoring procedures can bring an accurate and reliable estimation of quartz solubilities near the supercritical point and in the low-density region. It also emerged that the quartz solubility in water is significantly affected by temperature and pressure both under subcritical and supercritical conditions, showing a positive correlation with pressure at isothermal and a retrograde behavior near the supercritical point. These features are interpreted by the thermodynamic theory, quartz hydrolysis process, and mathematical model. By combining our experimental results with high-quality data from the literature, a five-parameter density model is provided for the calculation of the silica concentrations in aqueous fluids up to supercritical geothermal conditions. To overcome the current limitations in the modeling of supercritical geothermal fluids, future experimental work needs to collect additional data on silica in real geofluids (e.g., saline fluid) to develop an advanced model. The findings of this study can help for a better understanding of geochemical processes in supercritical geothermal systems, as well as ore deposits and vent structures.