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Hydrates are a possibility in oil/gas exploration, production, transportation, or processing, which involves water and molecules smaller than n-pentane. When small (< 9 Å) nonpolar molecules contact water at ambient temperatures (typically < 100°F) and moderate pressures (typically > 180 psia), a water crystal form may appear—a clathrate hydrate.
In the petroleum industry, there are four clathrate hydrate technological areas:
- Safety and flow assurance in oil/gas drilling, production, and transmission lines
- Stranded-gas transmission to market in a hydrated state
- Seafloor stability, affecting subsea-equipment foundations and climate
- Energy recovery from hydrates in permafrost and in deep-sea locations
At the appropriate combinations of temperature, pressure, and low-molecular-weight gases, water molecules arrange themselves into coplanar 5- or 6-membered rings, which then form three-dimensional (3D) polyhedra around the gases:
These individual polyhedra then combine to form specific crystalline lattices. In these solids, one volume of water in the hydrate state may “enclathrate” 70 to 300 volumes of gas. Such solids can be formed with N2, H2S, CO2, C1, C2, C3, and iso-butane. Larger molecules like n-butane and cyclopentane require the presence of some smaller molecules. Natural gas hydrates are to be distinguished from the common inorganic-salt hydrates such as CuSO4•5H2O.
More information on hydrate structures and formation conditions (pressures, temperatures, and compositions) are specified in Equilibrium of water and hydrocarbon systems with hydrates
A general phase diagram for water, hydrocarbon, and solid hydrate is shown in Fig. 1. There are essentially five regions:
- Hydrate + gaseous hydrocarbon (+ excess liquid water)
- Hydrate + liquid hydrocarbon (+ excess liquid water)
- Ice + gaseous hydrocarbon
- Liquid water + gaseous hydrocarbon
- Liquid water + liquid hydrocarbon
The temperatures at which gas hydrates form are significantly higher than the temperatures at which water ice will form. The exact pressure/temperature (PT) values for this equilibrium vary with hydrocarbon-gas composition and with the dissolved salt content in the liquid water phase. (This salt will not enter the gas-hydrate crystal structure, but it will control the chemical activity of the water from which the hydrate forms.) Hydrates can form more readily (i.e., at higher temperatures) from oil than in pure methane.
Additional information on hydrates
Other pages in PetroWiki provide additional information on aspects of hydrates and the challenges they can cause in production and transportation.
- Phase behavior of water and hydrocarbon systems
- Equilibrium of water and hydrocarbon systems with hydrates
- Predicting hydrate formation
- Hydrate problems in production
- Preventing formation of hydrate plugs
- Hydrate plug removal
- Transporting stranded gas as hydrates
- Gas hydrates in nature
Future hydrate research directions
Wherever small molecules contact water, the potential for a hydrate phase should be considered. As a minimum, hydrates should be included as one extreme in models of fluid phase behavior. In many cases, solid hydrates may control the system phase behavior.
Industrial problems with pipeline flow assurance will continue to provide the major incentive for development of hydrate technologies. The new, high-pressure challenges of deepwater gas stability and production require extensions of the existing thermodynamic database, along with better understanding of hydrate inhibition. Accurate means of determining in-situ hydrate detection, distribution, sediment properties, and controls are needed for a comprehensive picture to draw together the two communities interested in hydrates inside and outside pipelines. Hydrate time-dependent behavior (involving mass and heat transfer, and kinetics) is problematic, but the best work emanates from a three-decade effort at U. Calgary under Prof. Raj Bishnoi.
Transporting stranded gas as hydrate is currently being investigated. It is likely that useful production information will be obtained by studying ocean hydrate effects on sediment slumping, related to subsea-structure foundations and the climate, which are currently largely unknown.
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 an engineering breakthrough is required for energy recovery from hydrates to be economically feasible. Yet, the potential to tap the Earth’s largest hydrocarbon energy resource cannot be ignored.
Makogon, Y.F. 1997. Hydrates of Hydrocarbons, 412. Tulsa, Oklahoma: PennWell Books.
Sloan, E.D. 1998. Clathrate Hydrates of Natural Gases, Ch. 4-6, second edition. New York City: Marcel Dekker.
Max, M.E. ed. Natural Gas Hydrate in Oceanic and Permafrost Environments, 414. Dordrecht, The Netherlands: Kluwer Academic Publishers.
Noteworthy papers in OnePetro
Sloan, E.D. 2000. Hydrate Engineering, Vol. 21, Ch. 2, 44, 46-47. Richardson, Texas: Monograph Series, SPE. http://store.spe.org/SPE_Product.aspx?ProductId=839&CategoryId=6
Paull, C.K. and Dillon, W.P. ed. 2001. Natural Gas Hydrates: Occurrence, Distribution, and Detection, Vol. 124, 315. Washington, DC: Monograph Series, AGU. http://www.agu.org/books/gm/v124/