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Sloan 11/11/03 10:06 am Page 37 insight review articles Fundamental principles and applications of natural gas hydrates E Dendy Sloan Jr Center for Hydrate Research, Colorado School of Mines, Golden, Colorado 80401, USA (e-mail: esloan@.mines.edu) Natural gas hydrates are solid, non-stoichiometric compounds of small gas molecules and water They form when the constituents come into contact at low temperature and high pressure The physical properties of these compounds, most notably that they are non-flowing crystalline solids that are denser than typical fluid hydrocarbons and that the gas molecules they contain are effectively compressed, give rise to numerous applications in the broad areas of energy and climate effects In particular, they have an important bearing on flow assurance and safety issues in oil and gas pipelines, they offer a largely unexploited means of energy recovery and transportation, and they could play a significant role in past and future climate change U ntil mankind learns how to economically generate hydrogen for fuel cells, natural gas will be the premium fuel for this century for two reasons First, gas burns cleanly, causes few pollution problems and, relative to oil or coal, produces less carbon dioxide And second, liquid fuels are better used as feedstocks (raw material) for generation of petrochemicals Two examples herald this coming change: many power plants are being converted from coal to natural gas, and fleets of cars have been converted from petrol to natural gas fuel As we deplete the readily accessible reserves, we will need to obtain natural gas from conditions that are both more severe and more remote We will need to explore deep ocean environments with higher pressures, and permafrost environments with lower temperatures than we presently And gases that were previously thought to be uneconomical, such as those containing non-combustible components of nitrogen, carbon dioxide and hydrogen sulphide, will also be explored Such unusual conditions also promote the formation of a solid compound of gas and water — namely clathrates of natural gas — commonly called gas hydrates Here, I indicate the motivation for hydrate science and engineering; that is, the applications where technical workers find a use for the physics, chemistry and biology that are associated with science This is not to indicate that hydrate engineering is simply an applied science As indicated recently in a defining book1 on differences in technical philosophy, engineering frequently cannot afford the luxury of a thorough scientific foundation, and must proceed at risk As only one example, the past decade’s development of the new ‘low-dosage’ pipeline hydrate inhibitors proceeded in an Edisonian research mode (a process of intelligent guesswork, intuitive reasoning and testing), and scientific progress is currently being made to refine trial and error research gaps in inhibitor developments Below, I describe five major applications of hydrate research: flow assurance, safety, energy recovery, gas storage/transportation and climate change Before the applications are addressed, an introduction to hydrate structures and their overall properties is presented I conclude this review with an outline of future challenges For readers who want a more detailed understanding, several hydrate reviews2–8 are available Hydrate structures Clathrate hydrates typically form when small (1,000 bar), in unusual fluids such as black oils, hydrate–sediment mixtures, and the methanol-partitioning challenge indicated earlier As an example of one such challenge, hydrate– sediment mixtures have an unexplained thermal diffusivity maximum when plotted against sediment concentration As we begin to examine hydrates in nature, such challenges for time-independent properties will require decades to resolve However, the largest challenge is to describe the kinetics of hydrates42 The fact that hydrates are solid compounds makes their slow, solid-phase kinetics particularly challenging to researchers An additional challenge arises from the fact that hydrate solids form interfacial barriers between the liquid and vapour phases that typically compose them Hydrate research is most accurate when studying a time-independent target Typically, time-dependent (kinetic) research is much more difficult and at least an order of magnitude of accuracy is lost, relative to time-independent (thermodynamic) research The use of kinetic-model results to predict data from other laboratories is problematic Molecular dynamic simulations of hydrate kinetics have been hindered by stochastic nucleation and the large number of molecules and time required for growth processes More hydrate phase measurements are required to provide a needed breakthrough — a unified hydrate kinetics model Conclusions and outlook Wherever small molecules contact water, the potential for a hydrate phase should be considered The size ratio (guest to cavity) determines hydrate structural stability to a first-order approximation Other simple hydrate properties such as solid behaviour, density and concentration of guest molecules affect the major applications of hydrate safety, flow assurance, energy production and storage and climate change During the next decade, gas production will begin from permafrost hydrates associated with conventional gas reservoirs However, efficient production of ocean hydrates is problematic and requires an engineering breakthrough to be economically feasible Yet, the potential to tap the Earth’s largest hydrocarbon energy resource cannot be ignored Although hydrate thermodynamics are understood to an acceptable degree for most engineering applications, the kinetics arena will represent the largest challenge for advancing the information on hydrates Although we know quite a lot about what hydrates are, the question of how hydrates form is still very much unanswered FindNATURE | VOL 426 | 20 NOVEMBER 2003 | 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