高温岩体システムでの炭酸カルシウム沈積からみたジオリアクターによるCO_2地中貯留の可能性
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概要
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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.
- 2009-09-30
著者
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