Assessing the Thermodynamic Feasibility of the Conversion of Methane Hydrate into Carbon Dioxide Hydrate in Porous Media

Concerns about the potential effects of rising carbon dioxide levels in the atmosphere have stimulated interest in a number of carbon dioxide sequestration studies. One suggestion is the sequestration of carbon dioxide as clathrate hydrates by injection of carbon dioxide into methane hydrate. Energy-supply research estimates indicate that natural gas hydrates in arctic and sub-seafloor formations contain more energy than all other fossil fuel deposits combined. The simultaneous sequestration of carbon dioxide and the production of methane by injection of carbon dioxide into deposits of natural gas hydrates, if possible, represents a potentially efficient and cost effective option for the sequestration of carbon dioxide. Data in the literature show that the conversion of bulk methane hydrate into carbon dioxide hydrate is thermodynamically favored. These results are not directly applicable to naturally occurring hydrates, because the hydrates in these locations are embedded in sediments. The thermodynamics of any potential conversion of CH4 hydrate to CO2 hydrate will therefore be affected by the size of the pores in which the conversion of CH4 hydrate to CO2 hydrate would take place. We have developed a model that is able to explain and predict equilibria in porous media for any pore size distribution. This model can be used to calculate the heats of dissociation for these hydrates in porous media as a function of pore size and temperature. These results allow for an assessment of the thermodynamic feasibility of converting CH4 hydrate to CO2 hydrate in porous media involving various size pores. We have used this model to derive a simple, explicit relation for the hydrate formation conditions in porous media, as well as the enthalpy of dissociation for these hydrates.