An accurate model to predict the thermodynamic stability of methane hydrate and methane solubility in marine environments

An accurate thermodynamic model is proposed to predict the thermodynamic stability of methane hydrate in marine environments and methane concentration needed to form hydrate in the absence of gas phase. Taking into account the effect of capillary force and salinity on chemical potential of CH4 and H2O, this study extends the Van der Waals–Platteeuw model and our approach to calculate the Langmuir constant from angle-dependent ab initio intermolecular potentials to marine environments. The Gibbs–Thomson equation with correct parameters for hydrate–water interface is used to account for the capillary effect of porous sediments. The Pitzer model is used to calculate the activity coefficients of H2O and CH4 in methane–seawater system. Comparison with the experimental data shows that this model can predict the equilibrium P–T condition of CH4 hydrate in porous media and predict CH4 solubility at hydrate–water equilibrium with high accuracy. The prediction of this model shows that CH4 solubility in liquid phase at hydrate–liquid equilibrium will be reduced by increasing salinity, but it will be increased by reducing pore sizes of porous sediments. Online calculations of the P–T condition for the formation of methane hydrate and the methane solubility at a given salinity and pore sizes of sediments is made available on: www.geochem-model.org/models.htm.