Predictions of Mixed Hydrate Phase Equilibria and the Swapping of CH4 Hydrate with CO2 and CO2+N2 Mixtures

Natural gas hydrates are likely to contain more carbon than in all other fossil fuel reserves combined. Thus, if CH₄ stored in the natural gas hydrates can be recovered, the hydrates will become a potential clean energy resource for next 1000 years. Methods currently being employed for recovery of CH₄ from hydrate deposits include depressurization, thermal stimulation and inhibitor injection, which all involve the dissociation of hydrate and the release of significant volumes of water. These processes may cause geomechanical stress on the reservoir leading to subsidence. Another method of production of CH₄ gas from the gas hydrate is through the injection of CO₂. CO₂ sequestration in natural-gas-hydrate reservoirs by replacing the CH₄ is potentially attractive as it serves the dual purpose of long-term storage of a greenhouse gas (CO₂) and the production of natural gas (CH₄). Recent experimental studies (Park et al., PNAS, 103(34), 12690, 2006) have shown that the fraction of recovered methane can be improved by using a mixture of N₂ and CO₂ as opposed to pure CO₂.

Based on an analytical solution to the Lennard-Jones Devonshire approximation to the van der Waals-Platteeuw statistical mechanics model for hydrate equilibrium, the cell potential method is used to predict the phase equilibrium data of the mixed hydrate.  The reference parameters and cell potential parameters are obtained by fitting to the experimental simple hydrate data and are used to predict the mixed hydrate equilibria without fitting to any experimental data. Reference parameters vary with the guest molecules, depending on the size of the guest molecule. The method is validated by testing its predictive ability against available experimental data and has been shown to predict hydrate structural transitions accurately. It is verified by predicting structural transitions for cyclopropane to occur at 256.3 K and 274.5 K. The reliability of the method is demonstrated by its ability to predict cage occupancies accurately.

Natural gas hydrates can be formed either by bacterial activity at shallow depths or by thermal pyrolysis of fossil organic matter which contains methane and significant amounts of other higher hydrocarbons (C₂-C₅) and other non-hydrocarbon gases.  To understand the replacement of CH₄ from the hydrate it is important to know the phase equilibrium data of the mixed hydrates. In this work, the phase equilibria of many different binary mixed hydrates such as CH₄-C₂H₆, CH₄-CO₂, CH₄-N₂ and N₂-CO₂ have been predicted so as to assess the production of the CH₄ from natural gas hydrate reservoirs formed by thermogenic gases. Interestingly, both CH_4, and C₂H₆ form a structure I hydrate when the gas phase contains only one component but the CH₄-C₂H₆ mixed hydrate forms structure-II at various gas mixture compositions. Likewise, N₂ as a simple hydrate forms structure II, but along with other guest molecules it can form structure I. These structural transitions are predicted for these mixed hydrates. N₂-CH₄ and N₂-CO₂ predictions are confirmed using experimental results. Additionally, phase equilibria for ternary hydrates, including N₂-CO₂-CH₄ are calculated. Using this thermodynamic framework, the replacement of CH₄ in hydrate with N₂-CO₂ mixture is studied and is verified using the experimental data published by Park et al. (2006). In this system, 23% of CH₄ in the hydrate is replaced by N₂ whereas 62% of CH₄ is replaced by CO₂ i.e.; approximately 85% of CH₄ encaged in hydrate is recovered by using N₂-CO₂ (80:20) mixture.  An optimization of the N₂-CO₂ ratio was performed using our thermodynamic model, thus predicting the maximum recovery of CH₄ from a natural gas hydrate reservoir while simultaneously sequestering CO₂.