Carbon dioxide capture and storage (CCS) in deep geological formations has recently emerged as an important option for reducing atmospheric greenhouse gas. CO₂ injected into deep underground is considered to be dissolved in formation water filling the reservoir formation and to reduce water pH by dissociation. The acidified water is expected to react with minerals forming the reservoir rocks to promote their dissolution and alteration toward a final goal of the precipitation of new phases that are stable in the presence of a large amount of CO₂. A series of rock-water interaction summarized above is called “geochemical CO₂ trapping” that is expected to stabilize CO₂ geological storage system in a long period. This and the next issues of the Japanese Magazine of Mineralogical and Petrological Sciences collect papers concerning to the rock-water interaction associated with CO₂ geological storage. The storage options as a background of individual study are largely similar to those reviewed in IPCC (2005), however, a unique option matching with geology of the Japanese Islands as a young volcanic arc is also included.

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  Two dimensional numerical simulation was performed using reactive geochemical transport code TOUGHREACT based on the geology of the Tokyo Bay area to understand long-term geochemical evolution of aqueous and mineral phases after CO₂ injection and to elucidate key issues affecting CO₂ mineral trapping. The injected CO₂ migrates upward in the reservoir (the Umegase Formation) towards the cap rock (the Kokumoto Formation) driven by buoyant force. In the CO₂ plume, water pH drops from 7.8 (initial) to 4.9 by dissolution and dissociation of CO₂. The acidified water containing CO₂ and related species induces mineral dissolution and promotes complexing of dissolved ions, which increase the water density and cause its downward movement to the bottom of the reservoir. This process gives rise to convective mixing which further promotes CO₂ solubility trapping. In the peripheral zone of the CO₂ plume, the acidified water comes in contact with intact reservoir water, giving rise to the precipitation of calcite (and siderite, in lesser amount). The precipitation of calcite and other carbonates might form a low permeability barrier surrounding the CO₂ plume, and is expected to increase the storage security with time.
  Among newly formed carbonates, calcite becomes the most dominant in a long period after injection cease. Dawsonite, which is occasionally considered to be important in trapping CO₂, is found to have limited stability that requires high CO₂ partial pressure at the conditions of saline aquifer storage. Our study also finds that the formation of dawsonite is in a close relation to the dissolution of plagioclase, and that its precipitation is strongly suppressed by the formation of kaolinite. After reviewing rates of dissolution of carbonates and feldspars, we conclude that the precipitation of dawsonite is controlled by plagioclase dissolution kinetics which determines the supply of Al as a major constituent of dawsonite. Plagioclase is a dominant clastic mineral in sandstones in geologically young formations of the Japanese Islands. Its dissolution could be a key issue in geochemical modeling of CO₂ aquifer storage.

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  The Gibbs free energy change, ΔG_r, dependence of the anorthite dissolution rate under a supercritical CO₂-water system was measured as part of a long-term assessment of CO₂ geological sequestration. The surface observation on a nanoscale described herein showed that the ΔG_r dependency follows to the rate function with three steps (i.e., the horizontal step retreats without the etch pit, the etch pit formation assisted by dislocations, and the spontaneous formation of the etch pit) instead of transient state theory and a traditional sigmoidal curve, where the rate increases abruptly more distant from equilibrium by spontaneous formation of the etch pit over the entire surface. Such a variation of the dissolution rate can affect largely not only the change in porosity or permeability of reservoir rocks and the resultant flow property of formation water, but also the precipitation rate of secondary minerals. Our preliminary numerical simulation revealed that the difference of the rate function form according to ΔG_r dependence produces the time gap more than two orders on both the dissolution of anorthite and the precipitation of secondary minerals. On the other hand, extremely slow rates were observed depending on surface conditions for the same ΔG_r condition. Therefore, understanding of transient processes before a steady state is also necessary for evaluation of the mineral dissolution rate under natural conditions including CO₂ geological sequestration.

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  Experiments on CO₂-water-rock interaction at hydrothermal conditions have been performed to investigate mineral dissolution and precipitation phenomena, mainly focusing on calcite deposition from the alteration of plagioclase and rocks during CO₂ sequestration into geothermal fields. Plagioclase from Shiga and granodiorite from the Ogachi hot/dry rock field (7 g; grain size is 0.5 to 2 mm) were independently enclosed with Kyoto tap water in a Teflon reaction container. The container is filled with CO₂ (10 MPa) or N₂ gas after evacuating and heated up to 150 °C in an electric furnace with rotation (1 rpm). After 1 to 15 days, the solutions were analyzed for their chemical compositions after filtration and the mineral surfaces were observed by using SEM-EDS.
  The concentration of Ca in the solutions reacted with CO₂ quickly increases within 1 day and is+∼50 mg/L higher than those without CO₂ (with N₂ gas). The saturation index shows that the solutions with CO₂ are saturated with respect to carbonate such as calcite and aragonite during the reaction. Newly formed calcium carbonate (possibly calcite) was detected by SEM-EDS observation on the plagioclase surfaces, but the other minerals such as kaolinite were not. These results indicate that Ca can be released from rocks (silicates) easily and might be removed as CaCO₃ during CO₂ sequestration into relatively high temperature (geothermal) fields. Also, Ca-rich plagioclase has a high potential of CO₂ fixation as carbonate.

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  This paper is a short review of a new technology, “Georeactor”, which sequesters and fixes CO₂ into geothermal fields by carbonate mineralization. In countries such as Japan, where have many active volcanic areas, one possibility is to sequester CO₂ into hydrothermal regions. The chemical reaction rates between CO₂-saturated water and rocks are usually faster than those at room temperature. For example, the following reaction (alteration reaction forming kaolinite from plagioclase):
CaAl₂Si₂O₈ (plagioclase)+H⁺+HCO₃^-+H₂O=CaCO₃ (calcite)+Al₂Si₂O₅(OH)₄ (kaolinite) is a good example that gives rise the carbonate precipitation. This reaction moves towards the right-hand side with increasing temperatures and CO₂ in aqueous phase (or HCO₃^- as a soluble species), suggesting a potential of CO₂ sequestration. The calcite- and kaolinite (clay)-rich rock produced through the reaction is expected to form a cap rock for the geothermal reservoir, with increasing storage safety. Furthermore, the high temperature of a geothermal field is favourable for immediate mineral carbonation, which would also contribute to the storage safety. In Japan, total rock volume in geothermal fields is estimated to be 4900 km³. The CO₂ storage capacity is calculated to be about 20 billion tons CO₂ (∼17 times Japan's total annual CO₂ emissions).

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