GAS HYDRATES

The entrapment of natural gas molecules in ice at very low temperatures forms an ice-like solid which is a metastable complex called a gas hydrate. Gas hydrates are clathrates. A clathrate is a chemical complex that is formed when one type of molecule completely encloses another type of molecule in a lattice. In the case of gas hydrates, hydrogen-bonded water molecules form a cage-like structure in which mobile molecules of gas are absorbed or bound.
The presence of gas hydrates can complicate field operations. For example, the existence of hydrates on the ocean floor can affect drilling operations in deep water. The simultaneous flow of natural gas and water in tubing and pipelines can result in the formation of gas hydrates that can impede or completely block the flow of fluids through pipeline networks. Heating the gas or treating the gas-water system with chemical inhibitors can prevent the formation of hydrates, but increases operating costs.

Gas hydrates are generally considered a problem for oil and gas field operations, but their potential commercial value as a clean energy resource is changing industry perception. The potential as a gas resource is due to the relatively large volume of gas contained in the gas hydrate complex. In particular, Makogon, et al. [1997] have reported that one cubic meter of gas hydrate contains 164.6 m3 of methane. This is equivalent to one barrel of gas hydrate containing 924 ft3 of methane, and is approximately six times as much gas as the gas contained in an unimpeded gas-filled pore system [Selley, 1998, pg. 25]. The gas in gas hydrates occupies approximately 20% of the volume of the gas hydrate complex. Water occupies the remaining 80% of the gas hydrate complex volume.

Gas hydrates can be found throughout the world [Selley, 1998;Makogon, et al., 1997]. They exist on land in sub-Arctic sediments and on seabeds where the water is near freezing at depths of at least 600 to 1500 feet. For instance, favorable conditions for gas hydrate formation exist at sea floor temperatures as low as 39°F in the Gulf of Mexico and as low as 30°F in some sections of the North Sea. According to Makogon, et al. [1997], over 700 trillion m3 in explored reserves of methane in the hydrate state exist. Difficulties in cost effective production have hampered development of the resource.

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