Experimental study on initial behavior of water-rock interaction in CO ₂-added hydrothermal system

In order to reduce the amount of CO₂ emitted to the atmosphere, injection of CO₂ into underground is considered one of the useful approaches. Existing data of CO₂ solubility in water suggests that hotter and deeper rock masses may also be candidates for CO₂ underground sequestration due to enhanced CO₂ solubility under high pressures even at high temperatures. To understand the initial reaction of several rocks in a CO₂-saturated hydrothermal system, experiments were conducted using a batch-type over a range of 100-350°C up to 250 bars and numerical computations of phase equilibria based on the experimental results were carried out. The rocks used in our study were granite, sandstone and calc-silicate rock. Solution chemistry and examination of the rocks' surface texture showed that, in the CO₂-saturated system, the release of alkali metals and alkaline-earth metals and the deposition of secondary minerals were promoted. According to EDX analysis and numerical phase equilibrium calculations, the secondary minerals formed might be diaspore, kaolinite, clay minerals, Ca-alminosilicates and calcite. However, there was no clear evidence for the calcite formation. It is implied that the precipitation rate of calcite is relatively slow and the formation of Ca-alminosilicates occur much easily than calcite even if solution is supersaturated with respect to calcite.