Novel geothermal power generation systems are being developed that use supercritical CO₂ as the heat transfer medium. In this technology, some CO₂ injected into the underground reacts with surrounding water and rocks to form secondary minerals, such as carbonate minerals and clay minerals; however, the reaction mechanism in the vicinity of the injection well, the subject of this study, has not been clarified. As the first laboratory test, Rishiri Island basalt was reacted with distilled water at 250 °C for 15 days at four different CO₂ concentrations to investigate the difference in reaction depending on the CO₂ concentration. Na, K and Ca increased rapidly until 5 days of the reaction, with higher values at higher CO₂ concentrations; Mg showed characteristic behavior with higher values in the test without CO₂ (using Ar gas). The saturation index of each secondary mineral was calculated, and it was found that carbonate minerals were unsaturated and clay minerals, such as smectite, were supersaturated under all test conditions, which was in agreement with the experimental results. It is concluded that a small amount of clay minerals was formed in this test due to the low pH of the reaction solution caused by the high CO₂ concentration, indicating that dissolution was the main reaction for the rocks in the vicinity of the injection well.