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The first intersection of clathrate hydrates and human endeavor took place in the late 1700s. A number of researchers (natural philosophers) working on the solubility of newly discovered airs (gases) observed unexpected ice‐like solids formed above the freezing point of ice when certain gases were passed into cold water or when such a solution was frozen. Davy identified these solids as two‐component water–gas compounds and named them “gas hydrates.” After some 140 years and much research, these solids were shown to be clathrates, materials where small molecules (guests) are trapped in an ice‐like lattice (host) consisting of hydrogen‐bonded water cages. During the time between initial discovery and final identification, gas hydrates confounded researchers by having a number of properties that countered concepts derived from mainstream chemistry. For instance, the hydrates were non‐stoichiometric, the water‐to‐gas ratios were not small whole numbers, and they decomposed upon heating or depressurization to give back the unchanged starting materials. The lack of chemical bonds between the water and the gas in the hydrates suggested that these were not real chemical compounds and in fact were the first examples of “chemistry beyond the molecule” – supramolecular compounds.

From phase equilibrium studies, we now know that when many gases and water are in contact under appropriate pressure (P) and temperature (T) conditions, a solid hydrate will form. The gas hydrates store gases, including natural gas, very efficiently with one volume of solid hydrate storing some 160 volumes of gas at standard temperature and pressure (STP). Since a number of gas hydrates are found naturally, this class of materials can be taken to be an unusual type of mineral. There are many sites in the geosphere where natural gas and water are in contact under the conditions required to form gas hydrate. Locations where this occurs are in sediments offshore of continental margins, under permafrost, and in some deep freshwater lakes.

Well before the discovery of natural gas hydrate in the geosphere, oil and gas engineers encountered blocked natural gas pipelines during cold weather operation which was initially attributed to ice formation from moisture in wet gas. Knowledge of earlier work on solid methane hydrate led Hammerschmidt in the 1930s to the correct explanation for the pipeline blocks – they were made of solid methane hydrate rather than ice. Since then, during the exploration, production and transport phases of hydrocarbon resources, blockage by natural gas hydrate formation has become a well‐known hazard, resulting in possible serious damage and loss of life, for example in the Deepwater Horizon oil spill of 2010. Much research has been carried out to prevent or manage hydrate formation in pipelines. Other problems related to gas hydrates have been identified, including marine geohazards, such as submarine landslides, and sudden gas releases from hydrate formations.

Because of the vast amounts of trapped natural gas in hydrate form globally, gas hydrates have been evaluated to be a significant unconventional natural gas resource. Hydrate deposits have been mapped in many locations around the world, usually using geotechnical methods that depend on the location of unexpected solid hydrate–liquid water interfaces which act as seismic reflectors. Test wells for gas production have been drilled in the Mackenzie Delta, Canada (Mallik 2L38), Alaska, the Nankai trough offshore Japan, and offshore China. Many problems have been encountered in producing gas from hydrate reservoirs, including the development of the best techniques for destabilizing the solid hydrate and capturing the resulting gas. Some hydrate deposits, often associated with hydrocarbon seeps or vents, exist as outcrops on the seafloor. Whereas most of the methane in hydrate reservoirs is of biogenic origin, hydrates associated with seeps or hot vents are formed from thermally altered hydrocarbons originally residing in deeper reservoirs. Some hydrate outcrops are home to specialized biological ecosystems where microbes feed on hydrocarbons and these in turn become a food source for “ice‐worms.”

Besides the marine and terrestrial natural gas hydrates, there are gas hydrates of air (mainly N₂ and O₂) deep inside glaciers. The hydrate zone starts at pressures where gas bubbles in the ice disappear. There has been much speculation regarding the existence of hydrates in extra‐terrestrial space, that is, on Mars, Titan, Enceladus, and the heads of comets. One of the best candidates for finding such a hydrate would appear to be that of CO₂. Although lots of spectroscopic data exist for free solid CO₂ and ice in extra‐terrestrial space, no sign of CO₂ hydrate has been found, see Chapters 3 and 13 for possible reasons.

The large gas capacity and water as cheap working fluid make gas hydrates interesting materials for industrial applications. Gas hydrates are generally selective for guest molecule adsorption which allows the separation of gas mixtures. Much as salt is excluded when brine is frozen in desalination processes, gas hydrates have the same ability, but now the properties of the solid phase can be adjusted. For example, depending on the choice of guest, the hydrate freezing point can be well above the ice point, so saving on refrigeration costs. The same principle applies to cool energy storage where the freezing point of the hydrate can be tuned to minimize operational costs. Further applications include dewatering of fruit juices, sewage sludge and wood pulp, and the storage of unstable molecules such as ozone and chlorine dioxide. A more exotic application was the separation of radioactive radon gas from a gas mixture.

The gentler conditions required for the formation and storage of methane in solid hydrate form as compared to the low temperature required for liquid nitrogen storage of liquefied methane has resulted in the evaluation of transport by these two means. Indeed, some cost advantages become apparent for solid hydrate transport if methane has to be transported from stray fields where construction of a liquid natural gas (LNG) plant is not cost effective. Other applications for storage have been explored, e.g. for hydrogen as fuel gas and CO₂ for greenhouse gas separation and storage.

Gas hydrates, because of their unique properties, have demonstrated some entertainment value. The “burning snowball” results when methane from decomposing methane hydrate is ignited and this phenomenon has been admired by many, both live and in print. In the early 1980s, it was proposed that sudden, massive decomposition of marine methane hydrates could be the cause of disappearances of ships and planes in mythically mysterious areas such as the Bermuda triangle. It appears the mystery novel “Death by gas hydrate” still needs to be written: hydrates have great potential as difficult to track murder weapons. The media also have frequently given news coverage of “burning ice” which is rediscovered every 10 years or so.

From the earlier examples, it is clear that gas hydrates are interesting and unusual materials partly because nature makes them and partly because there are many potential uses which unfortunately remain largely prospective. This book will emphasize the molecular chemical, physical, and material aspects of clathrate hydrates, that is, the many details needed to understand the macroscopic properties and processes mentioned earlier. Engineering and geological aspects of the gas hydrates have been covered admirably in a number of previous books mentioned later.

In Section 1.2 of this chapter, we highlight milestones of clathrate hydrate science up to the present. The history and context of some of these earlier developments are discussed in greater detail in Chapter 2 and in chapters that follow and the relevant references can be found there. From their beginning and in the decades that followed, many centers of clathrate hydrate research emerged in different parts of the world. From the early 1960s with the work of Don Davidson and coworkers, the National Research Council (NRC) of Canada in Ottawa emerged as one of the active centers of research in this field. In Section 1.3, we give a summary of the contributions to clathrate hydrate science made in the NRC of Canada during this time period. Contributors to the clathrate hydrate research at the NRC are acknowledged in Section 1.4. Some influential books and review articles on clathrate hydrates that appeared during this period and up to the current time are introduced in Section 1.5. International conferences focusing on clathrate hydrate science are listed in Section 1.6.