Development of a Thermodynamic Framework for the Simulation of Mixed Gas Hydrates: Formation, Dissociation, and Co2-ch4 Exchange

Abstract

Natural gas hydrate deposits contain CH4 along with other hydrocarbon and non-hydrocarbon gases like C2H6, C3H8 etc. 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 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 N2 headspace. Current reservoir simulators like HydrateResSim, TOUGH+HYDRATE, MH-21 HYDRES and STOMP-HYD can predict the production of CH4 from pure CH4 hydrate reservoirs. 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 has been 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. TOUGH+ and STOMPHYD has also been recently modified to account for CO2 hydrate formation.

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