About Natural Gas, Shale & Gas Hydrates

No Comments » November 2nd, 2013 posted by // Categories: Science & Technology



 

 

Natural Gas

Natural gas is a naturally occurring hydrocarbon gas mixture consisting primarily of methane (CH4), but commonly includes varying amounts of other higher alkanes (CnH2n+2) and even a lesser percentage of carbon dioxide (CO2), nitrogen N2), and hydrogen sulfide (H2S).

 

Sources of Natural Gas

Schematic cross-section of the subsurface illustrating types of natural gas deposits

Source:  US Energy Information Administration:

http://www.eia.doe.gov/oil_gas/natural_gas/special/ngresources/ngresources.html

File:GasDepositDiagram.jpg

 

Shale Gas

Shale gas is natural gas that is found trapped within shale formations.  Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility.  Shale gas has become an increasingly important source of natural gas in the United States since the start of this century, and interest has spread to potential gas shales in the rest of the world. In 2000 shale gas provided only 1% of U.S. natural gas production; by 2010 it was over 20% and the U.S. government’s Energy Information Administration predicts that by 2035, 46% of the United States’ natural gas supply will come from shale gas.

Map of major shale gas basis all over the world from the EIA report World Shale Gas Resources: An Initial Assessment of 14 Regions Outside the United States .

Source:  http://www.eia.gov/analysis/studies/worldshalegas/

File:EIA World Shale Gas Map.png

 

Gas Hydrates

Methane clathrate (CH4•5.75H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Originally thought to occur only in the outer regions of the Solar System where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of Earth.

The nominal methane clathrate hydrate composition is (CH4)4(H2O)23, or 1 mole of methane for every 5.75 moles of water, corresponding to 13.4% methane by weight, although the actual composition is dependent on how many methane molecules fit into the various cage structures of the water lattice. The observed density is around 0.9 g/cm3, which means that methane hydrate will float to the surface of the sea or of a lake unless it is bound in place by being formed in or anchored to sediment. One litre of fully saturated methane clathrate solid would therefore contain about 120 grams of methane (or around 169 litres of methane gas at 0°C and 1 atm) [Molecular weight of CH4 is 16.  22.4 liters of methane weigh 16 grams at STP. Observed density of 0.9 g/cm3 is 900 grams per liter of hydrate, of which 13.4% is 120 grams of methane, which yields about 169 liters at STP.)

 

Worldwide distribution of confirmed or inferred offshore gas hydrate-bearing sediments.

Source: USGS Open-File Report 96-272. http://pubs.usgs.gov/of/1996/of96-272/fig2.html

File:Gas hydrates 1996.svg

 

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