Natural gas hydrates in sediments can never reach thermodynamic equilibrium. Every section of any hydrate-filled reservoir is unique and resides in a stationary balance that depends on many factors. Fluxes of hydrocarbons from below support formation of new hydrate, and inflow of water through fracture systems leads to hydrate dissociation. Mineral/fluid/hydrate interaction and geochemistry are some of the many other factors that determine local hydrate saturation in the pores. Even when using real sediments from coring it is impossible to reproduce in the laboratory a natural gas hydrate reservoir which has developed over geological time-scales. In this work we discuss the various stages of hydrate formation, with a focus on dynamic rate limiting processes which can lead to trapped pockets of gas and trapped liquid water inside hydrate. Heterogeneous hydrate nucleation on the interface between liquid water and the phase containing the hydrate former rapidly leads to mass transport limiting films of hydrate. These hydrate films can delay the onset of massive, and visible, hydrate growth by several hours. Heat transport in systems of liquid water and hydrate is orders of magnitude faster than mass transport. We demonstrate that a simple mass transport model is able to predict induction times for selective available experimental data for CO₂ hydrate formation and CH₄ hydrate formation. Another route to hydrate nucleation is towards mineral surfaces. CH₄ cannot adsorb directly but can get trapped in water structures as a secondary adsorption. H₂S has a significant dipole moment and can adsorb directly on mineral surfaces. The quadropole-moment in CO₂ also plays a significant role in adsorption on minerals. Hydrate that nucleates toward minerals cannot stick to the mineral surfaces so the role of these nucleation sites is to produce hydrate cores for further growth elsewhere in the system. Various ways to overcome these obstacles and create realistic hydrate saturation in laboratory sediment are also discussed.

Hydrate-based CO₂ sequestration is a novel approach that can not only realize permanent CO₂ sequestration but can also form an artificial cap to prevent its upward migration. In this work, a self-developed large-scale 3D apparatus was employed to investigate hydrate formation characteristics in hydrate-based CO₂ sequestration at a constant liquid CO₂ injection rate through a vertical well for the first time. Temperature and pressure evolutions in the sediment were analyzed in detail. Key indicators, including cumulative sequestered CO₂, CO₂ in hydrate and liquid phases, the instantaneous hydrate conversion, and liquid CO₂ retention rates, were calculated. The results show that hydrate continuously forms with increased CO₂ injection and exhibits strong heterogeneity due to the variation in hydrate formation rate and quantity. Severe liquid CO₂ heterogeneous figuring phenomena occur since hydrate deteriorates the effective pore structure and topology, resulting in relatively small cumulative sequestered CO₂ when a large amount of CO₂ is released from the outlet. Meanwhile, the instantaneous hydrate conversion and liquid CO₂ retention rates have large fluctuations owing to water consumption and variation in the effective contact area between liquid CO₂ and water. However, hydrate formation does not cause blockage of wellbore and formation nearby under given experimental conditions, which is beneficial for hydrate formation in deeper sediment. This study provides insights into hydrate formation and liquid CO₂ immigration regularity during hydrate-based CO₂ sequestration and demonstrates its feasibility at a field scale.

The formation of natural gas hydrates during processing and transport of natural has historically been one of the motivations for research on hydrates. In recent years, there has been much focus on the use of hydrate as a phase for compact transport of natural gas, as well as many other applications such as desalination of seawater and the use of hydrate phase in heat pumps. The huge amounts of energy in the form of hydrates distributed in various ways in sediments is a hot topic many places around the world. Common to all these situations of hydrates in nature or industry is that temperature and pressure are both defined. Mathematically, this does not balance the number of independent variables minus conservation of mass and minus equilibrium conditions. There is a need for thermodynamic models for hydrates that can be used for non-equilibrium systems and hydrate formation from different phase, as well as different routes for hydrate dissociation. In this work we first discuss a residual thermodynamic model scheme with the more commonly used reference method for pressure temperature stability limits. However, the residual thermodynamic method stretches far beyond that to other routes for hydrate formation, such as hydrate formation from dissolved hydrate formers. More important, the residual thermodynamic method can be utilized for many thermodynamic properties involved in real hydrate systems. Consistent free energies and enthalpies are only two of these properties. In non-equilibrium systems, a consistent thermodynamic reference system (ideal gas) makes it easier to evaluate most likely distribution of phases and compositions.

Ocean temperature difference energy, low-temperature seawater resources, liquefied natural gas cold energy, comprehensive utilization; temperature difference power generation