Interactions between basalt and CO₂-saturated water at varying temperatures (100°C–300 °C) in a duplex reactor

This study experimentally investigated basalt–CO₂–water interactions at temperatures between 100 °C and 300 °C through flow-through experiments, with a focus on changes in solution composition and the formation of secondary minerals under varying temperature conditions relevant to enhanced geothermal systems (EGSs). A duplex reactor system was developed for the first time to simulate temperature variations in the reservoir, particularly in the vicinity of caprock. The Mg²⁺ and Fe²⁺ concentrations were higher at lower temperatures (100 °C and 150 °C), whereas the other major cations exhibited an increasing trend with temperature. In the duplex reactor test (reactor 1 = 100 °C; reactor 2 = 250 °C; Fr6), the Mg²⁺ and Fe²⁺ concentrations were lower, suggesting potential Mg²⁺ and Fe²⁺ precipitation in high-temperature conditions. The SEM-EDS results primarily identified aluminium oxyhydroxide and clay minerals at temperatures above 250 °C, which were mainly oversaturated in beidellite, montmorillonite, illite, kaolinite, and boehmite. Conversely, at temperatures below 150 °C, primarily aluminium oxyhydroxide minerals were identified, which were oversaturated in boehmite. However, among the clay minerals, kaolinite mainly exhibited oversaturation, while the other minerals remained undersaturated. The results of the duplex reactor test suggest that Mg²⁺ and Fe²⁺, which dissolved under lower-temperature conditions, may lead to the precipitation of clay minerals upon migration to higher-temperature regions within the EGS reservoir. This behavior is attributed to the decreasing solubility constants of these minerals with increasing temperature.