Energy - Environmental Science - page 295

used. Indeed, the natural gas found above oil reser-
voirs in deep-sea and remote land areas is often
viewed as an unwanted by-product and is burned off.
This wastes a valuable energy resource and releases
carbon dioxide into the atmosphere.
Unconventional natural gas is found by itself in
other underground sources. So far, it costs too much to
get natural gas from such unconventional sources, but
the extraction technology is evolving rapidly.
When a natural gas field is tapped, propane and
butane gases are liquefied and removed as liquefied
petroleum gas (LPG). LPG is stored in pressurized
tanks for use mostly in rural areas not served by nat-
ural gas pipelines. The rest of the gas (mostly
methane) is dried to remove water vapor, cleansed of
poisonous hydrogen sulfide and other impurities, and
pumped into pressurized pipelines for distribution. At
a very low temperature, natural gas can be converted
to liquefied natural gas (LNG). This highly flamma-
ble liquid can then be shipped to other countries in re-
frigerated tanker ships. Some predict greatly increased
use of imported LNG in the United States if enough
shipping terminals can be built.
Russia has about 31% of the world's proven nat-
ural gas reserves, followed by Iran (15%) and Qatar
(9%). The United States has only 3% of the world's
proven natural gas reserves.
The long-term global outlook for natural gas sup-
plies is better than that for conventional oil. At the cur-
rent consumption rate, known reserves and undiscov-
ered, potential reserves of conventional natural gas
should last the world for 62-125 years and the United
States for 55-80 years, depending on how rapidly they
are used.
In total, conventional and unconventional supplies
of natural gas (the latter available at higher prices)
should last at least 200 years at the current consump-
tion rate and 80 years if usage rates rise 2% per year.

T rade-Offs

Heavy Oils from Oil Shale
and Oil Sand

Advantages Disadvantages

Moderate cost
(oil sand)

High cost (oil shale)

Low net energy
yield

Large potential
supplies, especially
oil sands in Canada Large amount of

water needed for
processing

Easily transported
within and
between
countries

Severe land
disruption from
surface mining

Water pollution
from mining
residues

Efficient
distribution
system in place

Air pollution
when burned

Technology is well
developed

CO 2 emissions
when burned

Figure 13-12 Trade-offs: advantages and disadvantages
of using heavy oils from oil sand and oil shale as energy re-
sources. Critical thinking: pick the single advantage and dis-
advantage that you think are the most important.

Figure 13-12 lists the advantages and disadvan-
tages of using heavy oil from oil sand and oil shales as
energy resources.

Trade-Offs: Advantages and Disadvantages of
Natural Gas
Natural gas is a versatile and clean-burning fuel, but it
releases the greenhouse gases carbon dioxide (when
burned) and methane (from leaks) into the
atmosphere.
Figure 13-13 (p. 296) lists the advantages and disad-
vantages of using conventional natural gas as an en-
ergy resource. Because of its advantages over oil, coal,
and nuclear energy, some analysts see natural gas as
the best fuel to help make the transition to improved
energy efficiency and greater use of solar energy and
hydrogen over the next 50 years.
Natural gas heats about two-thirds of American
homes, and it generates about 15% of the country's

Science: Natural Gas
Natural gas, consisting primarily of methane, is often
found above reservoirs of crude oil.
In its underground gaseous state, natural gas is a mix-
ture of 50-90% by volume of methane (CH 4 ), the sim-
plest hydrocarbon. It also contains smaller amounts of
heavier gaseous hydrocarbons such as ethane (C 2 H 6 ),
propane (C 3 H 8 ), and butane (C 4 H 10 ), and small amounts
of highly toxic hydrogen sulfide (H 2 S).
Conventional natural gas lies above most reservoirs
of crude oil (Figure 13-2). However, unless a natural
gas pipeline has been built, these deposits cannot be