Natural gas is a fossil fuel. Like other
fossil fuels
such as coal and oil,
natural gas
forms from the plants, animals, and microorganisms that lived millions of years ago.
There are several different theories to explain how
fossil fuels
are formed. The most prevalent theory is that they form underground, under intense conditions. As plants, animals, and microorganisms decompose, they are gradually covered by layers of s
oil
, sediment, and sometimes rock. Over millions of years, the organic matter is compressed. As the
organic
matter moves deeper into Earth’s crust, it encounters higher and higher temperatures.
The combination of
compression
and high temperature causes the carbon bonds in the
organic
matter to break down. This molecular breakdown produces thermogenic methane—
natural gas
.
Methane
, probably the most abundant
organic
compound on Earth, is made of carbon and hydrogen (CH4).
Natural gas
deposits are often found near
oil
deposits. Deposits of
natural gas
close to the Earth’s surface are usually dwarfed by nearby
oil
deposits. Deeper deposits—formed at higher temperatures and under more pressure—have more
natural gas
than
oil
. The deepest deposits can be made up of pure
natural gas
.
Natural gas
does not have to be formed deep underground, however. It can also be formed by tiny microorganisms called methanogens.
Methanogens
live in the intestines of animals (including humans) and in low-oxygen areas near the surface of the Earth. Landfills, for example, are full of decomposing matter that
methanogens
break down into a type of
methane
called biogenic
methane
. The process of
methanogens
creating
natural gas
(
methane
) is called methanogenesis.
Although most
biogenic
methane
escapes into the atmosphere, there are new technologies being created to contain and harvest this potential energy source.
Thermogenic
methane
—the
natural gas
formed deep beneath the Earth’s surface—can also escape into the
atmosphere
. Some of the gas is able to rise through permeable matter, such as porous rock, and eventually dissipate into the
atmosphere
.
However, most
thermogenic
methane
that rises toward the surface encounters geological formations that are too im
permeable
for it to escape. These rock formations are called sedimentary basins.
Sedimentary
basins
trap huge reservoirs of
natural gas
. In order to gain access to these
natural gas
reservoirs, a hole (sometimes called a well) must be drilled through the rock to allow the gas to escape and be harvested.
Sedimentary
basins
rich in
natural gas
are found all over the world. The deserts of Saudi Arabia, the humid tropics of Venezuela, and the freezing Arctic of the U.S. state of Alaska are all sources of
natural gas
. In the United States outside Alaska, basins are primarily around the states bordering the Gulf of Mexico, including Texas and Louisiana. Recently, the northern states of North Dakota, South Dakota, and Montana have developed significant drilling facilities in
sedimentary
basins
.
Types of Natural Gas
Natural gas
that is economical to extract and easily accessible is considered “conventional.” Conventional gas is trapped in
permeable
material beneath im
permeable
rock.
Natural gas
found in other geological settings is not always so easy or practical to extract. This gas is called “unconventional.” New technologies and processes are always being developed to make this unconventional gas more accessible and economically viable. Over time, gas that was considered “unconventional” can become conventional.
Biogas is a type of gas that is produced when
organic
matter
decomposes
without oxygen being present. This process is called anaerobic decomposition, and it takes place in
landfills
or where
organic
material such as animal waste, sewage, or industrial byproducts are decomposing.
Biogas
is biological matter that comes from plants or animals, which can be living or not-living. This material, such as forest residues, can be combusted to create a renewable energy source.
Biogas
contains less
methane
than
natural gas
, but can be refined and used as an energy source.
Deep Natural Gas
Deep natural gas is an unconventional gas. While most conventional gas can be found just a few thousand meters deep, deep natural gas is located in deposits at least 4,500 meters (15,000 feet) below the surface of the Earth. Drilling for deep natural gas is not always economically practical, although techniques to extract it have been developed and improved.
Shale
Shale
gas is another type of unconventional deposit.
Shale
is a fine-grained, sedimentary rock that does not disintegrate in water. Some scientists say
shale
is so im
permeable
that marble is considered “spongy” in comparison. Thick sheets of this im
permeable
rock can “sandwich” a layer of
natural gas
between them.
Shale
gas is considered an unconventional source because of the difficult processes necessary to access it: hydraulic fracturing (also known as fracking) and horizontal drilling.
Fracking
is a procedure that splits open rock with a high-pressure stream of water, and then “props” it open with tiny grains of sand, glass, or silica. This allows gas to flow more freely out of the well. Horizontal drilling is a process of drilling straight down into the ground, then drilling sideways, or parallel, to the Earth’s surface.
Tight Gas
Tight gas
is an unconventional
natural gas
trapped underground in an im
permeable
rock formation that makes it extremely difficult to extract. Extracting gas from “tight” rock formations usually requires expensive and difficult methods, such as
fracking
and acidizing.
Acidizing
is similar to
fracking
. An acid (usually hydrochloric acid) is injected into the
natural gas
well. The acid dissolves the tight rock that is blocking the flow of gas.
Coalbed Methane
Coalbed
methane
is another type of unconventional
natural gas
. As its name implies,
coalbed
methane
is commonly found along seams of
coal
that run underground. Historically, when
coal
was mined, the
natural gas
was intentionally vented out of the mine and into the
atmosphere
as a waste product. Today,
coalbed
methane
is collected and is a popular energy source.
Gas in Geopressurized Zones
Another source of unconventional
natural gas
is
geopressurized zones
.
Geopressurized zones
form 3,000-7,600 meters (10,000-25,000 feet) below the Earth’s surface.
These zones form when layers of clay rapidly accumulate and compact on top of material that is more porous, such as sand or silt. Because the
natural gas
is forced out of the
compressed
clay, it is deposited under very high pressure into the sand,
silt
, or other absorbent material below.
Geopressurized zones
are very difficult to mine, but they may contain a very high amount of
natural gas
. In the United States, most
geopressurized zones
have been found in the Gulf Coast region.
Methane Hydrates
Methane
hydrates
are another type of unconventional
natural gas
.
Methane
hydrates
were discovered only recently in ocean
sediments
and permafrost areas of the
Arctic
.
Methane
hydrates
form at low temperatures (around 0°C, or 32°F) and under high pressure. When environmental conditions change,
methane
hydrates
are released into the
atmosphere
.
The United States Geological Survey (USGS) estimates that
methane
hydrates
could contain twice the amount of carbon than all of the
coal
,
oil
, and conventional
natural gas
in the world, combined.
In ocean
sediments
,
methane
hydrates
form on the continental slope as bacteria and other microorganisms sink to the ocean floor and
decompose
in the
silt
.
Methane
, trapped within the
sediments
, has the ability to “cement” the loose
sediments
into place and keep the continental shelf stable. However, if the water becomes warmer, the
methane
hydrates
break down. This causes causes underwater landslides, and releases
natural gas
.
In
permafrost
ecosystems,
methane
hydrates
form as bodies of water freeze and water molecules create individual “cages” around each
methane
molecule. The gas, trapped in a frozen lattice of water, is contained at a much higher density than it would be in its gaseous state. As the ice cages thaw, the
methane
escapes.
Global warming, the current period of climate change, influences the release of
methane
hydrates
from both
permafrost
and ocean
sediment
layers.
There is a vast amount of potential energy stored in
methane
hydrates
. However, because they are such fragile geological formations—capable of breaking down and disrupting the environmental conditions around them—methods for extracting them are developed with extreme caution.
Drilling and Transportation
Natural gas
is measured in normal cubic meters or standard cubic feet. In 2009, the United States Energy Information Administration (EIA) estimated that the world’s proven
natural gas
reserves are around 6,289 trillion cubic feet (tcf).
Most of the reserves are in the Middle East, with 2,686 tcf in 2011, or 40 percent of total world reserves. Russia has the second-highest amount of proven reserves, with 1,680 tcf in 2011. The United States contains just over 4 percent of the world’s
natural gas
reserves. <
According to the EIA, total world consumption of dry
natural gas
in 2010 was 112,920 billion cubic feet (bcf). That year, the United States consumed a little more than 24,000 bcf, the most of any nation.
Natural gas
is most commonly extracted by drilling vertically from the Earth’s surface. From a single vertical drill, the well is limited to the gas reserves it encounters.
Hydraulic fracturing
, horizontal drilling, and
acidizing
are processes to expand the amount of gas that a well can access, and thus increase its productivity. However, these practices can have negative environmental consequences.
Hydraulic fracturing
, or
fracking
, is a process that splits open rock formations with high-pressure streams of water, chemicals, and sand. The sand props open the rocks, which allows gas to escape and be stored or transported. However,
fracking
requires huge quantities of water, which can radically reduce an area’s water table and negatively impact aquatic habitats. The process produces highly toxic and frequently radioactive wastewater that, if mismanaged, can leak and contaminate underground water sources used for drinking, hygiene, and industrial and agricultural use.
In addition,
fracking
can cause micro-earthquakes. Most of these temblors are far too tiny to be felt on the surface, but some geologists and environmentalists warn that the quakes may cause structural damage to buildings or underground networks of pipes and cables.
Due to these negative environmental effects,
fracking
has been criticized and banned in some areas. In other areas,
fracking
is a lucrative economic opportunity and providing a reliable source of energy.
Horizontal drilling is a way of increasing the area of a well without creating multiple expensive and environmentally sensitive drilling sites. After drilling straight down from the Earth’s surface, drilling can be directed to go sideways—horizontally. This broadens the well’s productivity without requiring multiple drilling sites on the surface.
Acidizing
is a process of dissolving acidic components and inserting them into the
natural gas
well, which dissolves rock that may be blocking the flow of gas.
After
natural gas
is extracted, it is most frequently transported through pipelines that can be from 2 to 60 inches in diameter.
The continental United States has more than 210 pipeline systems that are made up of 490,850 kilometers (305,000 miles) of transmission pipelines that transfer gas to all 48 states. This system requires more than 1,400
compressor
stations to ensure that the gas continues on its path, 400 underground storage facilities, 11,000 locations to deliver the gas, and 5,000 locations to receive the gas.
Natural gas
can also be cooled to about -162°C (-260°F) and converted into liquified
natural gas
, or LNG. In liquid form,
natural gas
takes up only 1/600 of the volume of its gaseous state. It can easily be stored and transported places that do not have pipelines.
LNG
is tranported by a specialized insulated tanker, which keeps the
LNG
at its b
oiling
point. If any of the
LNG
vaporizes, it is vented out of the storage area and used to power the transport vessel. The United States imports
LNG
from other countries, including Trinidad and Tobago and Qatar. However, the U.S. is currently increasing its domestic
LNG
production.
Consuming Natural Gas
Although
natural gas
takes millions of years to develop, its energy has only been harnessed during the past few thousand years. Around 500 BCE, Chinese engineers made use of
natural gas
seeping out of the Earth by building bamboo pipelines. These pipes transported gas to heat water. In the late 1700s, British companies provided
natural gas
to light streetlamps and homes.
Today,
natural gas
is used in countless ways for industrial, commercial, residential, and transportation purposes. The United States Department of Energy (DOE) estimates that
natural gas
can be up to 68 percent less expensive than electricity.
In residential homes, the most popular use for
natural gas
is heating and cooking. It is used to power home appliances such as stoves, air conditioners, space heaters, outdoor lights, garage heaters, and clothes dryers.
Natural gas
is also used on a larger scale. In commercial settings, such as restaurants and shopping malls, it is an extremely efficient and economical way to power water heaters, space heaters, dryers, and stoves.
Natural gas
is used to heat, cool, and cook in industrial settings, as well. However, it is also used in a variety of processes such as waste treatment, food processing, and refining metals, stone, clay, and petroleum.
Natural gas
can also be used as an alternative fuel for cars, buses, trucks, and other vehicles. Currently, there are more than 5 million
natural gas
vehicles (NGV) worldwide, and more than 150,000 in the United States.
Although NGVs initially cost more than gas-powered vehicles, they are cheaper to re-fuel and are the cleanest-running vehicles in the world. Gasoline- and diesel-powered vehicles emit harmful and
toxic
substances including arsenic, nickel, and nitrogen oxides. In contrast, NGVs may emit minute amounts of propane or butane, but release 70 percent less carbon monoxide into the
atmosphere
.
Using the new technology of fuel cells, the energy from
natural gas
is also used to generate
electricity
. Instead of burning
natural gas
for energy,
fuel cells
generate
electricity
with electrochemical reactions. These reactions produce water, heat, and
electricity
without any other
byproducts
or emissions. Scientists are still researching this method of producing
electricity
in order to affordably apply it to electric products.
Natural Gas and the Environment
Natural gas
usually needs to be processed before it can be used. When it is extracted,
natural gas
can contain a variety of elements and compounds other than
methane
. Water, ethane,
butane
,
propane
, pentanes, hydrogen sulphide, carbon dioxide, water vapor, and occasionally helium and nitrogen may be present in a
natural gas
well. In order to be used for energy, the
methane
is processed and separated from the other components. The gas that is used for energy in our homes is almost pure
methane
.
Like other
fossil fuels
,
natural gas
can be burned for energy. In fact, it is the cleanest-burning fuel, meaning it releases very few
byproducts
.
When
fossil fuels
are burned, they can release (or emit) different elements, compounds, and solid particles.
Coal
and
oil
are
fossil fuels
with very complex molecular formations, and contain a high amount of carbon, nitrogen, and sulfur. When they are burned, they release high amounts of harmful emissions, including nitrogen oxides, sulfur dioxide, and particles that drift into the
atmosphere
and contribute to air pollution.
In contrast, the
methane
in
natural gas
has a simple molecular make-up: CH4. When it is burned, it emits only carbon dioxide and water vapor. Humans exhale the same two components when we breathe.
Carbon dioxide and water vapor, along with other gases such as ozone and nitrous oxide, are known as greenhouse gases. The increasing amounts of
greenhouse gases
in the
atmosphere
are linked to
global warming
and could have disastrous environmental consequences.
Although burning
natural gas
still emits
greenhouse gases
, it emits almost 30 percent less CO2 than
oil
, and 45 percent less CO2 than
coal
.
Safety
As with any extractive activity, drilling for
natural gas
can lead to leaks. If the drill hits an unexpected high-pressure pocket of
natural gas
, or the well is damaged or ruptures, the leak can be immediately hazardous.
Because
natural gas
dissapates so quickly into the air, it does not always cause an explosion or burn. However, the leaks are an environmental hazard that also leak mud and
oil
into the surrounding areas.
If
hydraulic fracturing
was used to expand a well, the chemicals from that process can
contaminate
local aquatic habitats and drinking water with highly radioactive materials. The uncontained
methane
released in the air can also force people to temporary evacuate the area.
Leaks can also occur slowly over time. Until the 1950s, cast iron was a popular choice for distribution pipelines, but it allows a high amount of
natural gas
to escape. The
cast iron
pipes become leaky after years of freeze-thaw cycles, heavy overhead traffic, and strains from the naturally shifting s
oil
.
Methane
leaks from these distribution pipelines make up more than 30 percent of the
methane
emmissions in the U.S.
natural gas
distribution sector. Today, pipelines are made out of a variety of metals and plastics to reduce leakage.
Fast Fact
Natural Gas Consumers
In 2010, the latest date for which the U.S. Energy Information Administration supplies information, these nations consumed the most natural gas.
1. United States
2. Russia
3. Iran
4. China
5. Japan
Fast Fact
Oracular Seeps
Natural gas seeps, where the gas flows naturally to the surface, were revered as supernatural or spiritual sites by many ancient civilizations. One of the most famous of these seeps sits atop Mount Parnassus, near the town of Delphi, Greece. Around 1000 BCE, religious and spiritual leaders established a temple with a priestess who could tell the future. Millions of people, from ordinary citizens to political and military leaders, consulted the "Oracle of Delphi" for hundreds of years.
Fast Fact
Piping Up
The United states has 490,850 kilometers (305,000 miles) of interstate and intrastate pipelines to deliver natural gas all over the country.
Fast Fact
Proven Reserves
According to the U.S. Energy Information Administration, in 2011-2012, these nations had the biggest proven reserves of natural gas in the world. Data from some nations, including the United States, was not calculated.
1. Russia
2. Iran
3. Qatar
4. Saudi Arabia
5. Turkmenistan
Fast Fact
What's That Smell?
Raw natural gas is odorless. Companies that supply natural gas add an artificial smell to it, so people will know if there is a potentially dangerous leak. Most people recognize this as the "rotten egg" smell that comes from a gas stove or oven.