Secondary Mineralogy Associated with Supercritical Fluid Formation at Magmatic-Geothermal Contact Zone

Abstract

An increasing energy demand prompts exploration of high-temperature and supercritical resources associated with intrusion-based geothermal systems. Utilization of such fluids may lead to an increase in power production. However, the presence of supercritical fluids is associated with formation of mineral deposits that can potentially jeopardize the utilization of the supercritical reservoir. The formation of such deposits is yet poorly constrained. Here, we studied the effects of reservoir parameters such as boiling temperature or fluid source composition on secondary mineralogy upon supercritical fluid formation. For this purpose, four flow-through experiments with varying inlet fluid compositions were carried out at supercritical conditions. The experimental findings were subsequently compared with geochemical modeling results. The experimental and modeling results revealed that the types of mineral deposits formed upon supercritical fluid formation were dependent on fluid source composition, pH and boiling temperature. In low-NaCl high-temperature geothermal systems, conductive heating and boiling of subcritical fluids to supercritical conditions may result in the formation of alteration sequences consisting of Fe-Mg-Al silicates (clays or chlorites), Na-K feldspar, wollastonite, quartz and minor amounts of salt. At boiling temperature above 250 °C, the amount of precipitates may be twenty times larger than in geothermal systems with boiling temperature below 200 °C. In high-NaCl high-temperature geothermal systems, the amount of secondary minerals forming may be sixty times larger than in geothermal systems with boiling temperature below 200 °C, and three times larger than in low-NaCl high-temperature geothermal systems. Surprisingly, mineral amounts precipitating in high-temperature geothermal systems with elevated HCl concentrations may be as large as in low-NaCl geothermal systems. However, the highly acidic character in such systems favors precipitation of salts and silica into silica gel structures. This could potentially contribute to scaling and corrosion observed in some high-temperature geothermal wells, such as the IDDP-1 well in Krafla (Iceland).