Enthalpies of Hydrate Formation from Hydrate Formers Dissolved in Water
Authors: Kvamme Bjørn 
Published: March 2019

The international interest in the energy potential related to the huge amounts of methane trapped in the form of hydrates is rapidly increasing. Unlike conventional hydrocarbon sources these natural gas hydrate deposits are widely spread around the world. This includes countries which have limited or no conventional hydrocarbon sources, like for instance Japan. A variety of possible production methods have been proposed during the latest four decades. The pressure reduction method has been dominant in terms of research efforts and associated investments in large scale pilot test studies. Common to any feasible method for producing methane from hydrates is the need for transfer of heat. In the pressure reduction method necessary heat is normally expected to be supplied from the surrounding formation. It still remain, however, unverified whether the capacity, and heat transport capabilities of surrounding formation, will be sufficient to supply enough heat for a commercial production based on reduction in pressure. Adding heat is very costly. Addition of limited heat in critical areas (regions of potential freezing down) might be economically feasible. This requires knowledge about enthalpies of hydrate dissociation under various conditions of temperature and pressure. When hydrate is present in the pores then it is the most stable phase for water. Hydrate can then grow in the concentration range in between liquid controlled solubility concentrations, and the minimum concentration of hydrate in water needed to keep the hydrate stable. Every concentration in that range off concentrations results unique free energy and enthalpy of the formed hydrate. Similarly for hydrate dissociation towards water containing less hydrate former than the stability limit. Every outside liquid water concentration results in unique enthalpy changes for hydrate dissociation. There are presently no other available calculation approaches for enthalpy changes related to these hydrate phase transitions. The interest of using CO2 for safe storage in the form of hydrate, and associated CH4 release, is also increasing. The only feasible mechanism in this method involves the formation of new CO2 hydrate, and associated release of heat which assist in dissociating the in situ CH4 hydrate. Very limited experimental data is available for heats of formation (and dissociation), even for CH4. And most experimental data are incomplete in the sense that associated water/hydrate former rate are often missing or guessed. Thermodynamic conditions are frequently not precisely defined. Although measured hydrate equilibrium pressure versus temperature curves can be used there is still a need for additional models for volume changes, and ways to find other information needed. In this work we propose a simple and fairly direct scheme of calculating enthalpies of formation and dissociation using residual thermodynamics. This is feasible since also hydrate can be described by residual thermodynamics though molecular dynamics simulations. The concept is derived and explained in detail and also compared to experimental data. For enthalpy changes related to hydrate formation from water and dissolved hydrate formers we have not found experimental data to compare with. To our knowledge there are no other alternative methods available for calculating enthalpy changes for these types of hydrate phase transitions. And there are no limits in the theory for which hydrate phase transitions that can be described as long as chemical potentials for water and hydrate formers in the relevant phases are available from theoretical modeling and/or experimental information.