OS1C-1：THE IMPACT OF WATER FILM THICKNESS ON THE RATE OF MIXED HYDRATE FORMATION

Khuram BAIG, Bjørn KVAMME, Tatiana KUZNETSOVA
Department of Physics and Technology, University of Bergen, Norway,

    Natural gas hydrates are crystalline structures in which methane and other small hydrocarbons are enclathrated in water. Hydrates abound around the world both in permafrost and submarine regions. They are of substantial importance for the environment and global climate because of the threat of dissociation by contact with under-saturated water coming through fractures and faults. But they also represent a huge energy resource. Even by more conservative estimates, the volume of captured gas reserves in gas hydrates exceeds that of conventional fossil energy sources. Most hydrates forming or dissociating in natural or industrial processes exist in constant non-equilibrium due to the boundary conditions and the Gibbs’ phase rule. Adsorption on solid surfaces will increase the number of phases by two in addition to the hydrate and the fluid phases. With only two components in the simplest case of water and methane, and pressure and temperature defined by local conditions, hydrate formation will never be able to achieve thermodynamic equilibrium. As a consequence, the kinetics of hydrate formation and dissociation will be dominated by competing growth and dissociation transitions and requires kinetic theories that include minimization of free energy under constraints of mass and energy transport.

　　Conversion of reservoir methane hydrate into carbon dioxide hydrate is an interesting option offering a win-win combination of energy production with safe long-term storage of carbon dioxide to minimize the carbon dioxide footprint. As described theoretically and verified experimentally, carbon dioxide 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 carbon dioxide will form new hydrate from free water trapped in pores. Heat released by this process will contribute to dissociation of in situ methane 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 carbon dioxide hydrate will depend on the contact area between injected carbon dioxide and liquid water. We have therefore investigated three methane 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 phase.