Natural gas hydrates, a special type of non-stoichiometric crystalline inclusion compounds, are likely to contain more carbon than in all other fossil fuel reserves combined. Natural gas hydrate deposits contain CH4 along with other hydrocarbon and non-hydrocarbon gases like C2H6, C3H8 etc. In the prediction of CH4 production from hydrate reservoirs, the pressure and temperature of the I-H-V and Lw-H-V three-phase lines are of particular interest as they describe the limits of hydrate formation and dissociation conditions. 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 developed for variable reference chemical potential difference and enthalpy difference parameters is used to predict the phase equilibrium data of the mixed hydrates. Three-dimensional phase equilibria and structural transitions occurring in the mixed hydrates like CH4-C2H6, CH4-CO2 and CH4-N2-CO2, CH4-C2H6-C3H8 are predicted accurately without fitting to experimental data. These data obtained are validated by calculating the theoretical solubility of the gases in pore water during the dissociation experiments of pure CH4 hydrate and mixed hydrate (90%CH4+6%C2H6+4%C3H8) with a N2 headspace.
Field-scale methane hydrate production experiments are extremely complex and very expensive. Reservoir simulators can be used to predict production potentials of hydrate wells and to determine which technique best suits for that hydrate reservoir. Current reservoir simulators like HydrateResSim and Tough+Hydrate can predict the production of CH4 from pure CH4 hydrate. The regression equations developed by Kamath and Moridis are implemented into these simulators to obtain the equilibrium pressure and temperature data of CH4 hydrate. In order to predict production from natural gas hydrates it is essential to incorporate the phase equilibria of mixed hydrates into the reservoir simulators. The NETL-maintained gas hydrate reservoir simulator HydrateResSim is modified to predict the production of CO2 from CO2 hydrate and formation of CO2 hydrate from injected CO2. The phase equilibria of CH4-CO2 mixed hydrate obtained by the cell potential method is incorporated into the simulator to understand the replacement process of CH4 by CO2 in the CH4 hydrate as it can be potential method for recovery of CH4 from the hydrate deposits along with the sequestration of CO2.