Environmental Engineering Reference
In-Depth Information
Natural gas also contains small amounts of noncombustible components, including
nitrogen, carbon dioxide, and hydrogen sulfide.
Commercial natural gas is generally derived from both land-based plants and
marine organic matter, usually together with oil. Over geologic time, almost all nat-
ural gas migrates through the Earth's crust and eventually leaks to the atmosphere.
Frequently such migration is blocked by rock formations, giving rise to reservoirs
where large quantities of gas are trapped.
Gas that cannot be extracted with conventional production technology is termed
“unconventional gas.” The main types of unconventional gas are shale and gas hy-
drates.
Gas hydrate is a solid crystalline substance composed of water and natural gas
(primarily methane) in which water molecules form a cage-like structure around
the gas molecules. The cage structure of the hydrate molecule concentrates the
component gas so that a single cubic meter of gas hydrate will yield approximately
160 cubic meters of gas. Gas hydrate forms under conditions of moderately high
pressure and moderately low temperature and is widespread in marine sediments
of outer continental margins and in sediments in polar regions.
Conventional natural gas reserves, as assessed by different organizations
between 2007 and 2009, converge around 187.5 trillion cubic meters (Tcm) (or a
thousand billion cubic meters). Production in 2009 was 3.0 Tcm. At this rate of
production the reserves should last 62.5 years. The main reserves are in the Middle
East (40.6%), Europe and Eurasia (40.6%), Asia Pacific (8.7%), Africa (7.9%),
North America (4.9%), and South and Central America (4.3%).
Shale is a sedimentary rock consisting of clay, quartz, and other materials. It is
one of the most common rock formations and can be found everywhere. However,
most shale has insufficient permeability to allow significant fluid flow and, there-
fore, is not a suitable source of natural gas. The recent shale gas boom in North
America is the result of technological advances in creating extensive artificial frac-
tures around horizontal (rather than vertical) well bores. Within just over 10 years,
the share of shale gas in US supplies rose from 0.3 Tcm in 1996 to 2 Tcm in 2008.
The technology used in this case involves hydraulic fracturing of the shale reser-
voir in order to allow gas trapped in the rock to escape.
What is the expected life of fossil fuel reserves and resources?
Table 4.1 summarizes the situation in 2006 concerning the consumption of proven
reserves of fossil fuels defined as concentration of naturally occurring solid, liquid,
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