Thermodynamics of the Sequestration of Carbon Dioxide in Methane Hydrates in Porous Media

Concerns about 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 hydrates. 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 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. In addition, naturally occurring hydrates are normally made up of mixtures of natural gases, often predominately methane. The thermodynamics of any potential conversion of hydrates involving methane to carbon dioxide hydrate would therefore be affected by the size of the pores in which the conversion would take place, as well as the composition of the gas. We have developed a model that can be used to interpret or predict hydrate formation in porous media for any pore size distribution, allowing for the calculation of the heats of formation of these hydrates in porous media as a function of pore size and temperature. These results are applied to mixtures of methane and carbon dioxide. This allows for a preliminary assessment of the thermodynamic feasibility of converting hydrates formed from methane to carbon dioxide hydrate in porous media involving various size pores.