(389d) Gas Hydrates Modeling: Spanning Multiple Scales

Gas hydrates are nonstoichiometric crystalline inclusion compounds formed by the physically-stable interactions between water and relatively small guest molecules. They are stable at relatively high pressures and low temperatures and are found naturally below permafrost on shore or under the seafloor, within ocean sediments or even in deep lake sediments. The gas hydrate stability zone (GHSZ) is the range of depths of subsea or subsurface at which natural gas hydrates can form and remain stable at in situ temperature, pressure and local gas composition. The stable regimes are governed by the hydrate thermodynamic phase equilibria.  The phase equilibria of hydrates containing single or multiple gas components can be calculated using our cell potential method. This method is based on an analytical solution to the Lennard-Jones Devonshire approximation to the van der Waals-Platteeuw statistical mechanics model for hydrate equilibrium, along with variable reference parameters to account for lattice distortion. Bazant and Trout proposed temperature-dependent Langmuir constants that define the volumetric interaction of the potential energy between the guest and host molecules. The Langmuir constants of various guest molecules like CH₄ and CO₂ have been calculated by integrating the full, six-dimensional configurational integral for the guest gases in a hydrate lattice using our calculated ab initio potentials. Three-dimensional (P-T-y) phase equilibria surfaces and hydrate phase compositions of various mixed hydrates including CH₄-N₂, CH₄-CO₂ and CH₄-N₂-CO₂ obtained using the cell potential method have been incorporated into macro scale reservoir simulation tools such as HydrateResSim for the prediction of production of CH₄ by injecting either pure CO₂ or N₂+CO₂ gas mixtures. The CH₄ hydrate reservoir behavior is studied during injection of pure CO₂ and N₂ and found that in both cases there is a possibility of blockage near the injection well due to secondary hydrate and ice formation respectively. A solution may be to inject  a mixture of N₂ +CO₂ therefore  HydrateResSim has been modified for the ternary gas system CH₄-N₂-CO₂.