Thermodynamics of hydrate systems using a uniform reference state

Abstract

AbstractFormation of natural gas hydrates during processing and transport of natural gas has historically been a significant motivator for hydrate research. The last three decades have also seen the focus increasingly shifting towards CH4 hydrates as a potential energy source. And in the context of climate changes, the impact of hydrate‐related processes is coming more to the forefront as well. This interest is not only limited to leakage fluxes of CH4 from natural gas hydrates but also flux from conventional hydrocarbon systems entering the seafloor at temperature and pressure allowing for hydrate formation. Alternative ways to treat formally overdetermined hydrate systems is an important focus in this work. The most common method used for assessment of hydrate phase transitions involves several fitted parameters to calculate the free energy difference between liquid water and empty hydrate. This technique calls for an empirical fitting of fundamental thermodynamic properties. Numerical codes based on this method limit the models to hydrate formation only from free gas and liquid water. This is at least true for all commercial and academic codes that were examined prior to this work. This work addresses the advantages in using residual thermodynamics for all phases, including hydrates. In addition to making it possible to handle many alternative hydrate routes leading to hydrate formation or dissociation, the presented method also opens a way to calculate a variety of needed thermodynamic properties (e.g., enthalpies of pure components and mixtures) in a simple and consistent way. This approach will be illustrated through calculations of various hydrate phase transitions, examples of free energy calculations for comparison of phase stability, and calculation of enthalpies of hydrate formation. Calculated enthalpies are compared with experimental data as well as results derived from applying the Clapeyron equation. Mechanisms for conversions of in situ CH4 hydrate to facilitate safe CO2 storage are also discussed. A very simple Clapeyron‐based scheme for calculation of enthalpies for hydrate phase transitions is also proposed.