(70c) Phase Equilibrium Predictions of Mixed Hydrates by the Cell Potential Method: Validation Using Experimental Data and Its Implementation Into a Reservoir Simulator

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 L_w-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-C₂H_6, 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.