The mining systems of methane hydrate reservoirs using CO2 hydrate are proposed, i.e. the artificial block system and the CH4-CO2 replacement system. In order to accomplish the concepts, it is necessary to understand the mechanism of formation kinetics of gas hydrates. There are few experimental data on the growth rate of CO2 hydrate, particularly in the temperature range below melting point of ice, and the mechanism has not been well understood.
In this study, two types of experiments were carried out on formation kinetics of CO2 hydrate below melting point of ice, using Raman spectrometry and gas consumption method.
Spectra of CO2 trapped in the hydrate cages were directly observed by Raman spectrometry. The result showed the amount of CO2 molecules increased with time, and increasing rates depend on the temperature and pressure conditions. From the experiment of gas consumption method, it was found that the growth rate of CO2 hydrate has two stages. It is indicated that the first stage of growth is rapid growth mode caused by quasi-liquid-layer or locally melted water by the latent heat, and then the latter stage is slower growth mode between gas and ice. In the temperature range of-5 ∼-1 °C, more than 40 % of total reaction gas was consumed in the first stage of growth. Considering that the reaction at the first stage is similar to that of H-LW-V system, an empirical equation was applied to the experimental results. Reaction coefficients in the first stage were estimated to determine the growth of CO2 gas hydrate. From these results, we address that proper use of the first stage growth of CO2 gas hydrate is important to develop the advanced mining systems for methane hydrate.
