Dissolution kinetics model calculations were performed for the interaction between three types of basaltic rocks in Japan (Fuji and Hachijyojima fresh basalts and Kitamatsuura altered basalt) and groundwater injected CO₂. Dissolution rates of the basalts experimentally determined by the authors (Shikazono et al., 2008) and database of dissolution rate constants of silicate minerals in the basaltic rocks in PATHARC (Talman et al., 2000) were used for the calculations. The results of calculations indicate that most of dissolved carbon in groundwater injected CO₂ can be fixed as carbonates in long-term period. The efficiency of carbon fixation is in an order, Hachijyojima>Fuji>Kitamatsuura. But the efficiency is not so different for three basaltic rocks in the fixation of carbon in underground sequestration of CO₂. It is inferred that mineral trapping of CO₂ by carbonates in basalt aquifer is useful for the long-term fixation of carbon in underground sequestration of CO₂.

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  To evaluate the influence of rock properties on the geological sequestration of carbon dioxide, laboratory flow-through experiments were performed under 80 °C for 65 days by a permeability test machine. This technique enables to estimate the dissolution rate and dissolution time of rocks by the interaction with CO₂ dissolved solution.
  This experiment uses two solutions, a pH 3.8-, CO₂ 0.107 mol/L dissolved-solution (CO₂ undersaturated solution) and another with pH 6.4 distilled water. These solutions are independently passed through rock samples, using samples of tuffaceous sandstone including 32% unweathered anorthite collected at Yoichi, Hokkaido. Si, Ca, Mg, K, and Na ions are progressively dissolved to keep the time for 20 hours after increasing temperature at 80 °C with CO₂ dissolved solution. After the initial reaction, ion concentrations decrease and became a constant dissolution state. In contrast, these elements, except for Si hardly dissolve under the freshwater flow-through conditions at 80 °C for 65 days.
  The surface area of rock increased for that of the starting material and many pits with diameters of about 0.5 μm were made and a 0.1 μm thick Si-rich layer was made only on the anorthite surface by reaction with the CO₂ dissolved solution. The hydraulic conductivity of the rock did not change during 65 days in contact with the rock-CO₂ dissolved solution and-distilled water.
  The Ca dissolution rate was calculated to be 7.4×10^-17 mol/cm²/sec under the CO₂ system, which is definitely greater than with the freshwater system: 3.7×10^-18 mol/cm²/sec. The Si dissolution rate of the tuffaceous sandstone under the freshwater system: 8.5×10^-17 mol/cm²/sec was almost the same under the CO₂ system: 1.9×10^-16 mol/cm²/sec. The dissolution time of Si from 1g of the tuffaceous sandstone mass containing 51 wt% of SiO₂ was calculated to be 2.4 years. The dissolution time of Ca would be 1.2 years under the CO₂ system and 322 years under the freshwater system from 1g of the rock containing 0.92 wt% of CaO. The interaction with the CO₂ dissolved dilute solution and rocks obviously forms many ions including solution to contact rock and minerals at early reaction.

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  Water qualities and occurrence of precipitates in 10 hot and mineral springs with large calcareous deposits in Japan were investigated in order to elucidate geochemical conditions of the precipitation of carbonate minerals and water-rock interactions in their reservoirs. Chemical analyses of water samples have revealed that the spring waters were rich in NaHCO₃ and CO₂ components. Out of 10 springs investigated, 2 springs contain Ca and CO₂-related soluble components with genetic relations to the dissolution of limestone underground; 4 springs also enriched in Ca were green-tuff type affected by gypsum dissolution. Other springs poor in SO₄ component have geochemical characteristics suggesting the contribution of deep-seated fluid. The pH of reservoir fluids is considered to be buffered by CO₂-HCO₃ speciation. This chemical condition probably promotes rock alteration including dissolution of calcite and plagioclase, resulting in formation of large calcareous deposits in the fields studied. The calcareous precipitates consist only of calcite in three fields with relatively low water temperatures; the other deposits contain aragonite with calcite possible due to their formation at high temperatures and high concentrations of Mg, Mn and SO₄ in spring waters. From a comparison of water chemistry at the points of precipitation, logarithmic critical saturation indices (SI) are empirically derived to be 0.7-0.9 for calcium carbonate and over 3.5, 0.5, 1.2 and 3.4 for dolomite, magnesite, siderite and dawsonite, respectively. Distribution coefficients of Sr and Mg between spring water and precipitates indicate that calcareous precipitates were formed near equilibrium conditions.
  The possible ware-rock interaction processes elucidated in this study should be taken into account for geochemical modeling of CO₂ geological reservoirs. The empirical values of SI, which are all positive for major carbonate minerals, can be the most important finding because geochemical simulation previously conducted commonly assumed precipitation at the point of saturation (SI=0).

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  Carbonate hot spring is a natural chemical reaction field for understanding CO₂ geological sequestration as a natural analogue. Natural analogue studies are particularly important to understand the kinetics of mineral precipitation which has potential difficulties in experimental investigations. Carbonate sinter is frequently formed in and around carbonate hot spring, which can be suitable to elucidate mechanisms of carbonate precipitation associated with flushing CO₂.
  Oku-Okuhachikuro hot spring, located in Kosaka town, Akita Prefecture, NE Japan, is an artificial hot spring after drilling of exploration for the Kuroko-deposits, and it is still active where carbonate sinter has still been forming continuously for more than thirty years after drilling. The temperature of spring water is 44 °C and water pH is 6.2, with discharge rate of 0.08 m³/min. The average chemical compositions of sinter correspond about 80 wt% CaCO₃, and 4 wt% Fe₂O₃, associated with minor (<1 wt% each) SiO₂, MnO, MgO, Na₂O and K₂O. Carbonate sinter is mainly composed of aragonite with a small amount of calcite; an intimate occurrence of these two forms of CaCO₃ is the most characteristic feature of this locality. However, mineral assemblage, texture and structure of carbonate sinter are different in relation to the distance from the blowout point. Near the blowout point, the sinter is well solidified and shows laminar structure having both of calcite and aragonite. Thickness of Ca-rich laminar ranges from 20 to 150 μm and Fe-rich one is from 10 to 80 μm. Calcite and aragonite assemblage is mainly observed in Ca-rich layer. The Fe-rich layer, however, is composed only of aragonite. The sinter along downstream becomes porous and is monomineralic having aragonite as CaCO₃. The observed relations on the special distribution of aragonite/calcite and the possible stability relations of these phases through EPMA and TG-DTA analyses suggest an importance of minor elements (Fe, etc.) in the precipitation of metastable carbonates: this possible effect of the precipitation of metastable phases should be taken into account in the consideration of geochemical processes of CO₂ mineral trapping.

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  Rock-water interaction in a Hot Dry Rock (HDR) system at Hijiori, Yamagata Prefecture, Northeast Japan, is studied from a view point of “georeactor” effect that is expected to fix CO₂ as Ca carbonates through mineral reactions in the HDR system. Mineralogy of HDR underground and precipitates in wells and pipelines are examined together with fluid geochemistry during a 3-month circulation test from June 1 to August 31, 2002. The study has revealed that, near the injection well, anhydrite in slightly altered granodiorite heat source (HDR) is dissolved in injected river water, which eventually precipitates as calcium carbonates. In HDR-2a well having lower temperatures, the carbonate precipitation using Ca in the produced hot water mainly took place in the pipeline system by degassing near the surface. In HDR-3 well with higher temperatures, Ca concentration in the fluid can be explained assuming calcite precipitation in the underground granodiorite at an amount up to 0.6 ton. Mass balance calculation on CO₂ in the whole system has further revealed that the amount of CO₂ injected into underground HDR was as large as 6 ton during the 3-month circulation test: this amount includes soluble CO₂ initially present in river water used for injection and atmospheric CO₂ absorbed in it. The calculation has shown different behaviors of CO₂ and related species (including calcium carbonate) between HDR-2a and HDR-3 wells. In the HDR-2a well, 67% of CO₂ reached the well has precipitated as calcium carbonate (aragonite and calcite) on pipeline system and 3% was discharged into atmosphere. In the HDR-3 well, the amount of CO₂ discharge is as high as 29% of the whole CO₂ reached there; the fixed CO₂ as calcite is estimated to be around 21%. The result of mass balance calculation indicates that the whole HDR system of the Hijiori site probably contributed mineralization of injected CO₂ associated with the circulation test. Equilibrium geochemical modeling adding extra- CO₂ in the Hijiori HDR system has been carried out to evaluate potential of CO₂ sequestration by the formation of carbonates. The modeling has shown that the potential of CO₂ mineral sequestration depends on the supply of Ca from the HDR, which probably has a strong linkage to the progress of geothermal alteration involving Ca-bearing minerals such as anhydrite and plagioclase.

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  Field experiments of CO₂ sequestration into the Ogachi hot dry rock (HDR; the temperature is 200 °C) site were performed to investigate mineralization of a part of CO₂ as carbonates through interaction with rocks (Georeactor; Ca extraction from rocks and carbonate fixation). In 2007, carbonated water (1 wt% CO₂; river water with dry ice) with several tracers was directly injected into well OGC-2, Test #1 (from September 2nd to 9th) and Test #2 (from September 11th to 16th). During the Test #2 experiment, additional river water was injected into well OGC-1, 100 m apart from well OGC-2, at 2 days after the injection of CO₂ water into well OGC-2 so as to push back carbonated water around this well. Water samples were collected at the depth of ~850 m by a sampler (500 ml in volume) and monitored for their chemical and isotopic compositions. The CO₂ concentrations in fluids collected decreased with duration time and were almost 2/3 of the expected concentration from tracer concentrations. This means that CO₂ injected into well OGC-2 can be removed from fluid by carbonate fixation.
  During the field experiments, dissolution or precipitation rates of calcite were determined by using a technique of “in situ observation of crystallization”. Calcite crystals covered with Au film were hold in a crystal cell and set in a crystal sonde. The crystal sonde was filled with He gas under a certain pressure and then put into well OGC-2. The fluid at depth of -850 m is introduced into the sonde and calcite can start to react with them. After 1 hour, the sonde was recovered to the surface, where the fluid in the sonde is pushed to outside by He gas. The recovered calcite crystal is analyzed for the dissolution or precipitation rates by a newly developed phase shift interferometer. The “in situ analyses” show that calcite precipitation was observed within 2 days after the injection. This probably indicates that the most of CO₂ injected has been fixed as carbonate, supporting our idea that HDR system can be useful for CO₂ sequestration from inland emission sources.

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