Production of natural gas from hydrate using carbon dioxide allows for a win-win situation in which carbon dioxide can be safely stored in hydrate form while releasing natural gas from in situ hydrate. This concept has been verified experimentally and theoretically in different laboratories worldwide, and lately also in a pilot plant in Alaska. The use of carbon dioxide mixed with nitrogen has the advantage of higher gas permeability. Blocking of flow channels due to formation of new hydrate from injected gas will also be less compared to injection of pure carbon dioxide. The fastest mechanism for conversion involves the formation of a new hydrate from free pore water and the injected gas. As a consequence of the first and second laws of thermodynamics, the most stable hydrate will form first in a dynamic situation, in which carbon dioxide will dominate the first hydrates formed from water and carbon dioxide / nitrogen mixtures. This selective formation process is further enhanced by favorable selective adsorption of carbon dioxide onto mineral surfaces as well as onto liquid water surfaces, which facilitates efficient heterogeneous hydrate nucleation. In this work we examine limitations of hydrate stability as function of gradually decreasing content of carbon dioxide. It is argued that if the flux of gas through the reservoir is high enough to prevent the gas from being depleted for carbon dioxide prior to subsequent supply of new gas, then the combined carbon dioxide storage and natural gas production is still feasible. Otherwise the residual gas dominated by nitrogen will still dissociate the methane hydrate, if the released in situ CH4 from hydrate does not mix in with the gas but escapes through separate flow channels by buoyancy. The ratio of nitrogen to carbon dioxide in such mixtures is therefore a sensitive balance between flow rates and formation rates of new carbon dioxide dominated hydrate. Hydrate instability due to undersaturations of hydrate formers have not been discussed in this work but might add additional instability aspects.
New insights into the dissociation of mixed CH4/CO2 hydrates for CH4 production and CO2 storage
