To evaluate the long-term behavior of a CO2 storage site, it is necessary to understand the geochemical reactions induced by CO2 injection into the water in an aquifer. We observed changes in the chemical composition of the formation water at the Nagaoka CO2 storage site, Japan, where a total of 10,400 tonnes of CO2 was injected into a 12-m-thick sandstone section of the Haizume Formation located at a depth of 1,100 m. The formation water was collected from the CO2 injection well during a pumping-up test carried out before the injection. Before the injection, the total dissolved solid in the sampled water was 8,000 ppm, indicating that 50,000 ppm of CO2 can be dissolved maximally in the formation water due to its solubility. After the CO2 injection, the Cased Hole Dynamics Tester (CHDT) tools sampled formation fluids by penetrating casing, and plugging the test hole in a single trip at the depths determined by the well logging. The fluid sample collected at a depth of 1114.0 m was almost free CO2. The other two fluid samples from depths 1108.6 m and 1118.0 m were mainly the formation water. At the depth of 1118.0 m, we observed an increase in the concentration of HCO3 due to the dissolution of injected CO2. Concentrations of Ca, Fe, Si, Mg and Mn also increased. These elements were potentially provided by the dissolution of hornblende, serpentine, pyroxene, chlorite, and gypsum. In particular, Ca, Mg, and Fe are important to neutralize acidified water and to fix CO2 as a carbonate.