Clathrate hydrates are crystalline inclusion compounds resulting from the hydrogen bonding of water (host) molecules enclosing relatively small (guest) molecules, such as hydrogen, noble gases, carbon dioxide, hydrogen sulfide, methane and other low-molecular-weight hydrocarbons. They form and remain stable under low temperatures – often well below the ambient – and high pressures – ranging from a few bar to hundreds of bar, depending on the guest molecule. Long considered as either an academic curiosity or a nuisance for the oil and gas producer facing pipeline blockage, they are now being investigated for applications as diverse as hydrogen or methane storage, gas separation, cold storage and transport, water treatment, etc. The ubiquitous presence of natural gas hydrates not only in the permafrost, but also in deep marine sediments, has been identified, and their role in past and present environmental changes and geohazards, as well as their potential as an energy source, are under intense scrutiny.

These perspectives are motivating an ever-increasing research effort in the area of gas hydrates, which addresses both fundamental issues and applications. Gas hydrates exhibit fascinating yet poorly understood phenomena. Perhaps the most fascinating feature exhibited by gas hydrates is self-preservation, or the existence of long-lived metastable states in some conditions far from stable thermodynamic equilibrium. Strong departures from equilibrium are also noted in gas hydrate compositions, depending on their formation and kinetic pathways. A proper understanding of these two effects could serve in developing gas storage and selective molecular-capture processes. The memory effect, or the ability of gas hydrates to reform rapidly in an aqueous solution where gas hydrates have been freshly melted, is another puzzling phenomenon. Gas hydrates are likely to be soon exploited for storing gas (guest) molecules or for separating or capturing some of them selectively; yet, the occupancy rates of the different hydrate crystal cavities by the various guest molecules are not fully understood. Very little is known as well on hydrate formation and stability in the extreme conditions (e.g.

This volume addresses geoscience issues and potential industrial applications. The first part is devoted to field study and laboratory experiments of hydrate-bearing sediments. Marine gas-hydrate deposits are very complex geological structures, which often host rich and diverse ecosystems. They can be studied via multiple approaches, which all entail three major steps: an exploratory step to locate the deposit, a sampling and in situ measurement step and further onshore analyses. Thus, this part is meant to provide the reader with a general overview of the tools and techniques commonly used during the three aforementioned steps. It ends with a detailed description of the physicochemical properties of hydrate-bearing sediments with new results obtained from high-pressure flow-through experiments to investigate hydrate dynamics. The second part presents modeling approaches of the geochemical and geomechanical behavior of hydrate-bearing sediments, with applications to the Nankai gas production test and other settings. Finally, the last part presents a field case study for a giant hydrate-bearing pockmark and potential industrial applications: the volume ends with state-of-the-art reviews on the promises and challenges of using clathrate hydrates in technologically important areas - geological storage of CO₂ in sub-marine sediments, the capture of CO₂ from gaseous methane-rich streams, and cold storage and distribution.

Livio RUFFINE
IFREMER

Daniel BROSETA
University of Pau and Pays de l’Adour

Arnaud DESMEDT
CNRS – University of Bordeaux

February 2018