WHY DOES CO2 HYDRATE DISPOSED OF IN THE OCEAN IN THE HYDRATE-FORMATION REGION DISSOLVE IN SEAWATER

The fate of CO2 hydrate disposed of in the ocean is analyzed based on thermodynamic theory. It is found that CO2 hydrate and seawater form a stable system only when (a) the system pressure p and temperature T f all within the hydrate-formation region in the ocean (i.e. p > 4.5 MPa and T < 283 K) and (b) seawater is saturated or supersaturated with r espect to CO2. At ocean depths below 440 m, the pressure and temperatu re required for system stability are satisfied. However, since seawate r is highly unsaturated with respect to CO2, the requirement for full thermodynamic stability cannot be met because the hydrate and seawater are not in chemical equilibrium and the chemical potential for CO2 in the hydrate is larger than that in seawater. Therefore, the hydrate i s unstable and dissolves in seawater. Thus, CO2 hydrate disposed of in the ocean may not be a long-lived entity in the ocean as was predicte d previously by many investigators. The results of our study have been confirmed by laboratory simulations. (C) 1997 Elsevier Science Ltd.