Andesite and CO2-Saturated Water Interaction at Different Temperatures and Flow Rates Using a Flow-Through Reactor

This study aims to elucidate the geochemical reactions between CO2-saturated water and rocks in CO2-enhanced geothermal system (CO2-EGS) reservoirs by focusing on andesite found in island arc regions, such as Japan. Laboratory flow tests of CO2-saturated water (3 wt.% CO2) and rocks (particle size: 0.14–1 mm) were conducted under varying temperature (150–250 °C) and flow rate (0.3 and 1.0 mL/min) conditions using a flow-through reactor. Elevated temperatures enhanced the dissolution of silicate minerals, reflected by increased Na+, K+, Ca2+, and Si concentrations, whereas those of Fe2+ and Al3+ remained low, suggesting secondary mineral precipitation. The dissolution process was dominant at 150 °C. Al-bearing minerals, such as gibbsite and boehmite, as well as clay minerals, including beidellite and kaolinite, were predominant at higher temperatures (200–250 °C). Carbonate minerals were not observed, attributable to low pH and limited availability of divalent cations. Flow rate substantially influenced Si dissolution rates, with lower flow rates promoting longer residence times and higher Si dissolution rates. These results indicate that the test conditions simulate the environment around the injection well, where the fluid is acidic and dissolution is the main reaction in the rock. Although a small amount of secondary minerals precipitated and the Si dissolution rates were of the same order of magnitude as those for labradorite, it may be considered that andesite has less impact on permeability variations than basalt near the injection well in CO2-EGS reservoirs.