Serpentinite

Development of a Technology for CO₂ Fixation and Sequestration Using Serpentinite rock mass

<FY2004-FY2006>

Summary

The aim of this study is to develop a technology for CO₂ fixation and sequestration using serpentinite rock mass.

1) In-situ Test
The in-situ CO₂ injection tests were carried out at the Iwanaidake, Hidaka-cho, Hokkaido, Japan. The result of water analysis of flow through tests with various flow rates and concentrations of CO₂ dissolved water showed that clear peaks of Mg and C concentration were seen under the condition of the high CO₂ concentration. On the other hand, Mg carbonate was precipitated in groundwater under the condition of low CO₂ concentration and high flow rate. These results indicated that groundwater with low pH value was produced when CO₂ concentration was high, and dissolved Mg ion was supplied from cracks when the flow rate was increased. Moreover, the dilution rate in 5 m between two wells in the in-situ tests was about 16%, and a fixed rate of CO₂ was estimated to be about 5% from the change in the concentration of C in produced water.

2) Laboratory Experiments
The experiments that circulated the CO₂ dissolved water to the rock samples with cracks were carried out. In the experiment that used dunite-origin serpentinite, the Mg carbonate (giorgiosite) of about 0.008 cm in thickness was generated along cracks. In the experiments that used harzburgite-origin serpentinite, the Fe carbonate (siderite) was observed. According to these results, CO₂ fixation rate of dunite-origin serpentinite and harzburgite-origin serpentinite were estimated to be 4.6e-10 mol/cm²/s and 0.56 to 2.4e-10 mol/cm²/s, respectively. Moreover, it is showed that the precipitation of the reaction product determined the rate of reactions between CO₂ and serpentinite.

3) Reaction and Transportation Model
Considering the values and the findings obtained by the in-situ tests and the laboratory experiments, the reaction and transportation model was improved to be applicable to various serpentinite masses. The potential CO₂ injection amount was estimated at 7,000 - 138,000 t/year/well in one injection depth using this model. It is likely that a massive serpentinite has more advantage in CO₂ injection amount than a foliated serpentinite, and hydraulic fracturing is effective.

4) Efficient CO₂ fixation system
Large 43 rock masses were selected from domestic serpentinite rock masses, and a potential amount of CO₂ fixation by serpentinite was estimated using the reaction and transportation model. The CO₂ injection amount was estimated at 1.11~1.47 billion tons. Furthermore, the injection cost excluding separation and transportation cost was estimated at 1,400 - 1,500 yen/t-CO₂ for a massive serpentinite rock mass.