Study of Mix CO2/CH4 Hydrate Phase Transitions vs the Thickness of Surrounding Water Film

Conversion of reservoir CH4 hydrate into CO2 hydrate is an interesting option offering a win-win combination of energy production with safe long-term storage of CO2 to minimize the CO2 footprint. As described theoretically and verified experimentally, CO2 is capable of inducing and maintaining a solid state exchange process of conversion. This mechanism will be slow since it is kinetically controlled by solid state mass transport through the hydrate. In parallel to this, the injected CO2 will form new hydrate from free water trapped in pores. Heat released by this process will contribute to dissociation of in situ CH4 hydrate and thus provide a second conversion mechanism with its rate controlled by liquid state transport processes. Understanding the kinetics of gas hydrate formation and dissociation is crucial for the development of theoretical models describing gas exchange processes and providing a basis for efficient design of production schemes.

In this work, we combine a non-equilibrium description of hydrate and fluid thermodynamics with the phase field theory (PFT) for simulation of phase transition kinetics. The phase field theory approach allows one to minimize the free energy while taking into account the implicit couplings to mass and heat transport as well as hydrodynamics. The hydrodynamic treatment is important to distinguish between situations when gas released in the course of dissociation will dissolve into surrounding water (slow dissociation), and more rapid dissociation creating dispersed gas bubbles that will affect the available dissociation interface and influence heat transport. We studied the conversion of CH4 hydrate into either CO2 hydrate or mixed CO2-CH4 hydrate to investigate the relative impact of the two mechanisms. The efficiency of mechanism based on formation of new CO2 hydrate will depend on the contact area between injected CO2 and liquid water. We have therefore investigated three CH4 hydrate systems surrounded by varying amounts of initial liquid water. It was found that the kinetic rate of conversion increased with the thickness of initial free water phases.