Geothermal Field

Advanced Research for CO₂ Fixation in Geothermal Field

<FY2002-FY2004>

Summary
This research was carried out conducted by Research Institute of Innovative Technology for the Earth (RITE), Central Research Institute of Electric Power Industry and Mitsubishi Materials Corporation, and was entrusted from New Energy and Industrial Technology Development Organization (NEDO).

1. Purpose

Reaction rate of CO₂ and some mineral increases and solubility of carbonate minerals decreases under high temperature condition. In the case of CO₂ storage into geothermal field, CO₂ fixation as carbonate minerals is expected. Once carbonate minerals precipitate forming a sealing layer, CO₂ storage will be possible under the sealing layer. Then this study is carried out to estimate the possibility of CO₂ storage technology into geothermal filed.

Concept of CO₂ fixation system

2. Results

Assessment of CO₂ storage capacity and cost in geothermal field

We discussed the possibility and application of the CO₂ fixation system in Japan. To assess the practical use, we selected the geothermal areas excluding the national park area and main CO₂ generate facilities (thermal power plants and cement factories) 20km or more away. Several geothermal areas for practical use exist in Hokuriku and Kyusyu area of Japan. Total capacity of CO₂ storage was estimated at 1.18 billion tons CO₂ on the basis of assumed rock porosities (average: 4%) and the solubility of CO₂ (3 wt%). The cost was evaluated at 5136 yen/ton- CO₂ in the case of storage rate of 1 million tons CO₂ per year and transport distance of 20km.

Laboratory experiment of CO₂-water-rock interaction under hydrothermal conditions

CO₂-water-rock interaction under hydrothermal conditions was investigated by the batch and column tests. The mineral and rock samples were plagioclase (labradorite) and granodiorite in Ogachi geothermal field, respectively. In the case of experimental with plagioclase, it was clarified that Ca and Si are released from the rock surface by CO₂-water-rock interaction. In the case of experimental with granodiorite, it was observed that anhydrite, mica and chlorite dissolved followed by precipitation of AlO (OH) and goethite on the rock surface.

Field experiment in Ogachi geothermal field

Field experiments were performed to investigate CO₂-water-rock interaction in Ogachi geothermal field. CO₂ dissolve water (4.3 wt%) was injected into the bored hole and granodiorite (Ogachi rock) was soaked. The temperature was 208 °C, and soaking time were 16.5 hours. After the soaking, the dissolution of anhydrite was observed as well as the laboratory experiment.

Simulations of CO₂-water-rock interaction under hydrothermal conditions

Laboratory and field experiments were simulated by geochemical code. The results indicated that plagioclase (anorthite) will dissolve and calcite, clay minerals (kaolinite) will precipitate subsequently. Then sealing layer will form as a result of CO₂ fixation.