Many methods for the production of natural gas from hydrate reservoirs have been proposed during the latest 3 decades. Reducing pressure, thermal stimulation, or injection of thermodynamic hydrate inhibitors are three examples. A typical problem is, however, that different methods for producing hydrates are not evaluated thermodynamically prior to planning expensive experiments or even more expensive pilot tests. This can be due to the lack of a thermodynamic toolbox for the purpose. On a macroscopic level, there are two criteria that need to be met: the Gibbs free energy change has to be favorable and sufficient heat must be supplied in order to fulfill the first law of thermodynamics. Another challenge is the lack of focus on the limitations of the hydrate phase transition itself. The interface between the hydrate and liquid water is a kinetic bottleneck that requires efficient breaking of water hydrogen bonds. Reducing pressure does not address this problem. Injection of CO2, however, will lead to the formation of a new CO2 hydrate. This released heat from this hydrate formation is an efficient heat source for dissociating the in situ CH4 hydrate. Adding limited amounts of N-2 increases the permeability for the injection gas. Addition of a surfactant increases the gas/water interface dynamics and promotes heterogeneous hydrate formation. In this work, we demonstrate a residual thermodynamic scheme that opens up for detailed thermodynamic analysis of different routes to hydrate formation and dissociation. It is demonstrated that addition of 20 mol % N-2 to CO2 is thermodynamically feasible for generating a new hydrate in the pores. The available hydrate formation enthalpy when adding N-2 is reduced as compared to pure CO2 but still considered as sufficient. Up to 3 mol % ethanol in the free pore water is also thermodynamically feasible. The addition of alcohol will not significantly disturb the ability to form a new hydrate from the injection gas. The released enthalpy from the formation of the new hydrate is also considered as sufficient compared to what is needed for dissociation of the in situ CH4 hydrate. Homogeneous hydrate formation from dissolved CH4 and/or CO2 is limited in amount and is not important. However, the hydrate stability limits related to the concentration of the hydrate former in the surrounding water are important. Mineral surfaces can act as hydrate promoters through direct adsorption, or adsorption in water, which is structured by the mineral surface charges. Examples from theoretical studies are discussed.
