Dissociation Heat of Methane-Carbon Dioxide Hydrate Mixtures

Replacement of methane with carbon dioxide in hydrate is proposed as a strategy for geologic sequestration of carbon dioxide (CO2) and/or production of methane (CH4) from natural hydrate deposits. This replacement requires a better understanding of the characteristics of gas hydrate mixtures as well as physical property changes during gas exchange. This study explores dissociation behavior and dissociation heat of CH4-CO2 hydrate mixtures along two pathways: depressurization and temperature change. CH4-CO2 hydrate mixture samples were prepared from two different well-defined gas mixtures. During hydrate dissociation, gas was periodically sampled and analyzed using gas chromatography to quantify its composition. The CO2 concentration in the vapor phase became richer during dissociation because the initial hydrate composition contained relatively more CO2 than the vapor phase. The composition change in the vapor phase during hydrate dissociation affects the dissociation pressure and temperature—the richer CO2 in vapor phase leads to lower dissociation pressure. The dissociation heat of a CH4-CO2 hydrate mixture was computed by fitting the Clausius-Clapeyron equation to the PT trace of a dissociation test. It is observed that the dissociation heat of a CH4-CO2 hydrate mixture lies between the limiting values of pure CH4 hydrate and CO2 hydrate, varying with the composition of hydrate.