Reducing the amount of CO2 emitted by fossil fuel combustion is imperative to curb global warming. A promising method is to sequester CO2 into the earth's crust as carbonates. In geothermal fields, injected CO2 may react with surrounding minerals, e.g., Ca feldspar and anhydrite, and may precipitate CaCO3 at locations that are remote from the injection point, forming cap rocks.
We performed preliminary laboratory and field testing to investigate CO2/rock water reaction. The field test site is located at Ogachi, northeast Japan, where Hot Dry Rock (HDR) geothermal power exploitation tests had been conducted previously. Frozen CO2 was injected into an artificial reservoir with an initial temperature of 210 C using a 1100 m wellbore. The reservoir is mylonitized granodiorite with fractures mostly filled with anhydrite (CaSO4). Fluids at 1030 m depth were sampled in situ and then analyzed at the surface. It was found that dissolution of Ca was enhanced by CO2 injection. This result, together with our laboratory data, supports the feasibility of sequestering CO2 as CaCO3 in geothermal fields.

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