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Experiments on CO 2 -water-rock interaction at hydrothermal temperatures have been performed to investigate dissolution and precipitation phenomena, including Ca extraction from rocks that might occur during CO 2 sequestration into geothermal fields. Distilled water samples were exposed to a CO 2 atmosphere at a temperature of 25°C and pressures up to 6 MPa. The resulting solutions were then reacted with granodiorite samples from the Ogachi hot/dry rock field and labradorite, at 200°C and 120°C respectively. The calcium concentrations in the solutions that had reacted with CO 2 were twice those with N 2 instead of CO 2 . Combined with the results of thermodynamic calculations, these observations indicate that calcium can be released from rocks (silicates) easily and might be removed as CaCO 3 and/or CaSO 4 during CO 2 sequestration into geothermal fields.