Environmental Engineering Reference
In-Depth Information
South and
Central America
3.6
6.4 Others
West Europe 5.2
North and
West Africa
24.3
Middle East
5.4
Caspian Basin 6.8
23 Russia
Asia and Pacific 7.9
North America 17.4
Figure 2.15
World's natural gas reserves, percent. (
Source:
Same as in Figure 2.14.)
On the average, the heating value of NG is 23,500 Btu/lb (55 MJ/kg), or 1032 Btu per cubic foot
(38.5 MJ/m
3
).
Natural gas is a very desirable fuel for several reasons. First, it is easy to combust because,
being a gas, it readily mixes with air. Thus, the combustion is rapidly completed, and the boiler
or furnace volume is smaller than that required for oil or coal combustion. Second, the combusted
gas can directly drive a gas turbine with applications in power generation. Third, gas combustion
does not produce particulate and sulfurous pollutants. Fourth, NG produces one-half the amount
of CO
2
per unit heating value as does coal, and it produces three-quarters as much as oil.
The USGS estimate of the world's natural gas reserves is 6.75 E(15) cubic feet. Taking the
heating value of NG as 1032 Btu/ft
3
, the world's NG reserves amount to 6966 Q, comparable to
those of oil. The world's rate of consumption of NG in 1995 amounted to 77.5 Q. If that rate were
to continue in the future, the world's proven gas reserves would be depleted in about 85-90 years.
If gas consumption keeps increasing at a rate of 2.45%/y, the lifetime of the world's gas reserves
would be only about 50 years.
The gas reserves are distributed among the major reservoirs as follows (in percent): Middle
East (24.3), Russia, including Siberia (23), North America, including the United States, Canada,
and Mexico (17.4), Asia and Pacific (7.9), Caspian Basin (6.8), North and West Africa (5.4), West
Europe, including the North Sea (5.2), South and Central America (3.6), and others (6.4) (see
Figure 2.15).
2.7.5
Unconventional Gas Resources
Methane is known to exist also in unconventional reservoirs. These are (a) gas trapped in sand-
stone, (b) gas trapped in shale rock, (c) gas trapped in coal seams, (d) gas trapped in pressur-
ized underground water reservoirs (“geopressurized methane”), and (e) methane clathrates, also
called hydrates, found at some locations at the bottom of oceans and ice caps. Methane hy-
drates are basically ice crystals in which a methane “guest” molecule is trapped. Hydrates are
formed under the high pressures and low temperatures prevailing at the ocean bottom or ice
caps. The first four reservoirs in the United States alone may yield 600-700 Q, which is about
70% of the proven gas reserves in the United States and about 10% of the world's proven gas
reserves.
