Dear Colleagues,

Natural gas hydrates and their associated free gas accumulations represent a significant source of natural gas. Hydrates have the potential to help underpin the transition to cleaner energy, either as a source of energy by itself, or as a target for CO₂ storage combined with the release of natural gas. The combination of the latter option with known technology for steam cracking of hydrocarbons allows for H₂ to be delivered energy. Natural hydrate dissociation also plays a role in climate change due to complex feedback, and has implications for submarine geo-hazards.  

Natural gas hydrates are found in shallow marine sediments and permafrost around the world. Hydrates form in specific thermodynamic conditions and when enough natural gas and water is available. Hydrates dissociate due to changes in independent variables that control hydrate stability, such as increases in ocean temperature. Hydrates also dissociate continuously around the world due to fracture systems that bring seawater (containing almost no CH₄) into hydrate-bearing sediments.

The implications of hydrate dissociation are that it contributes to the carbon content in the oceans, and in many cases, also causes direct leaks through the water column and potentially into the atmosphere. Occasional sub-sea landslides that cause tsunamis (tidal waves) are another effect of hydrate destabilisation.

There are still several knowledge gaps in our understanding of gas hydrate systems. Typically, hydrate formation in the sediments affects the geophysical, hydro-mechanical and geochemical properties of the host sediments. Generally, remote geophysical methods are used to quantify gas hydrates and determine changes in these geophysical properties. The extent of changes in geophysical properties due to hydrates depends, however, on the amount (saturation) and distribution (morphology) of the hydrates. Moreover, there are complex thermodynamic interactions between hydrates (solid phase), water (liquid phase), gas (free or in solution) and mineral surfaces. Molecular interactions and phase transitions are, by nature, nanoscale processes on a volumetric scale. Dynamic multiscale couplings from the pore scale to macroscopic scale still lead to many challenges and several effects that cannot be directly interpreted from field data with the current level of methods used in geophysics and geochemistry. A fundamental challenge is the lack of proper modelling of the dynamic couplings between a flowing multiphase system, including hydrates, and the inversion of geophysical responses. As an example, seismic responses are essentially responses due to changes in density and elasticity of the medium, which again is a function of all the coexisting phases in the sediments and flow on a different level, ranging from molecular flow (diffusion) to various levels of hydrodynamics. Even the coupling to flow dynamics in individual pores is complex, and this complexity increases with additional flow couplings between pores, and with macroscopic effects on a reservoir scale and field scale such as geological heterogeneities, local dynamic effects of fractures, and many other effects. Further progress will require developments in specific sub-topics on different scales, including the nanoscale and upwards. It is, however, difficult to make substantial further progress without a higher degree of interdisciplinary scientific work than what has been achieved up to now.  

There is also a need for a deeper understanding on how the technical feasibility of the various hydrate dissociation methods are controlled by thermodynamic laws, and on couplings to various levels of flow. The primary goal is the safe and sustainable production of energy from hydrates at commercially feasible costs.

Prof. Dr. BjØrn Kvamme
Dr. Sourav Sahoo
Guest Editors

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• gas hydrate
• multiscale and multidisciplinary
• seismic
• laboratory elastic and electrical drilling systems
• swapping or exchange of CO₂ and CH₄ hydrates
• field-validated coupled models
• high-resolution imaging of hydrates
• integrated petrophysical, geological and geophysical data analyses