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How Natural Gas Works

Contents:
Conventional Natural Gas Resources
    Formation
    Production    
    Resources and Reserves
Unconventional Natural Gas Resources
Natural Gas Processing and Transportation
Uses of Natural Gas
Environmental Impacts of Natural Gas
The Future of Natural Gas

Natural gas is a versatile fossil fuel that we use for heating, cooking, electricity production, and transportation, and as an industrial feedstock. It currently makes up nearly one-quarter of the U.S. energy mix and continues to be a readily available domestic resource as a result of recent discoveries and advances in extraction technology.

Despite significant environmental concerns associated with its extraction and production, natural gas burns more cleanly than coal and oil and therefore offers advantages in reducing global warming emissions and improving public health.

However, natural gas is a fossil fuel whose emissions do contribute to global warming, making it a far less attractive climate solution than lower- and zero-carbon alternatives such as energy efficiency and renewable energy. During our nation’s transition to a low-carbon energy future, natural gas can play an important but limited role in the electricity and transportation sectorsif policies sufficient to minimize emissions and protect communities and public health are put in place.

Conventional Natural Gas Resources

Formation

Like oil, natural gas is a product of decomposed organic matter, typically from ancient marine microorganisms, deposited over the past 550 million years.  This organic material mixed with mud, silt, and sand on the sea floor, becoming gradually buried over time.  Sealed off in an oxygen-free environment and exposed to increasing amounts of heat and pressure, the organic matter underwent a thermal breakdown process that converted it into hydrocarbons.

The lightest of these hydrocarbons exist in the gaseous state under normal conditions and are known collectively as natural gas.  In its pure form, natural gas is a colorless, odorless gas composed primarily of methane.  Methane, the simplest and lightest hydrocarbon, is a highly flammable compound consisting of a carbon atom surrounded by four hydrogen atoms (chemical formula: CH4). 

Once natural gas forms, its fate depends on two critical characteristics of the surrounding rock: porosity and permeability.  Porosity refers to the amount of empty space contained within the grains of a rock.  Highly porous rocks, such as sandstones, typically have porosities of 5 percent to 25 percent, giving them large amounts of space to store fluids such as oil, water, and gas.  Permeability is a measure of the degree to which the pore spaces in a rock are interconnected.  A highly permeable rock will permit gas and liquids to flow easily through the rock, while a low-permeability rock will not allow fluids to pass through.

After natural gas forms, it will tend to rise towards the surface through pore spaces in the rock because of its low density compared to the surrounding rock.  Most of the natural gas deposits we find today occur where the gas happened to migrate into a highly porous and permeable rock underneath an impervious cap rock layer, thus becoming trapped before it could reach the surface and escape into the atmosphere.

Production

Conventional natural gas deposits are commonly found in association with oil reservoirs, with the gas either mixed with the oil or buoyantly floating on top.  Natural gas can be located with seismic methods similar to those used for petroleum exploration.  Gas prospectors set off an explosion on the surface to generate seismic waves in the underlying rock.  By measuring the travel times of these waves through the Earth at acoustic receivers known as "geophones," geophysicists can construct a picture of the subsurface structure and identify likely gas deposits.  To extract the gas, vertical wells are drilled to penetrate the overlying impermeable cap rock and reach the reservoir.  Natural buoyancy then brings the gas to the surface, where it can be processed and sent to homes and industry.


Figure 1: Natural gas production from conventional fields in the United States. 

Resources and Reserves

The United States is endowed with substantial natural gas resources, and new discoveries have revised estimates of their size sharply upward in the past few years.  In 2007, the U.S. Energy Information Administration (EIA) estimated that the U.S. possesses 1,533 trillion cubic feet of natural gas that could be recovered using current technology,[1]  enough to supply the nation’s needs at the 2008 rate of consumption for over 65 years. According to the latest assessment from the Potential Gas Committee, the recent expansion of production from unconventional gas deposits has raised the U.S. gas resource base to 2,074 trillion cubic feet, a 35 percent increase.[2]   This latest figure suggests that the U.S. gas supply could last for 90 years at current consumption rates.  Of the total U.S. gas resource, 237.7 trillion cubic feet of gas are classified as “reserves,” the amount that can currently be extracted economically.[3]

Despite large reserves, U.S. consumers still use more gas each year than is produced domestically.  Canadian gas fields supply most of the balance through pipelines. The remainder is delivered as liquefied natural gas from overseas via supertankers.
 
The largest known gas reserves in the world are found in Russia, which has seven times the reserves of the United States.  Iran and Qatar each have three to four times as much gas as the U.S., and significant reserves are also present in Saudi Arabia, Abu Dhabi, Nigeria, and Venezuela.[4]   The total world recoverable gas resource is estimated at 16,200 trillion cubic feet, enough to meet 150 years of demand at the 2009 rate of consumption.[5] The U.S. EIA projects a 50 percent rise in natural gas consumption by 2030, with growth in Asia driving increased demand.[6]

Unconventional Natural Gas

Despite decades of extraction and use, the estimated size of the U.S. natural gas resource has steadily risen since the 1990s, largely buoyed by the increased feasibility of extracting gas from unconventional deposits. Unconventional natural gas, which includes shale gas, tight gas, coal bed methane, and methane hydrates, is more difficult and costly to exploit than conventional deposits.  Such sources could help close the growing gap between domestic production and consumption in the United States but present greater environmental challenges in their production.

  • Shale Gas. Unlike conventional gas, which resides in highly porous and permeable reservoirs and can be easily tapped by standard vertical wells, shale gas remains trapped in its original source rock, the organic-rich shale that formed from the sedimentary deposition of mud, silt, clay, and organic matter on the floors of shallow seas.

    The first well in the United States drilled specifically to produce natural gas tapped into a shale gas deposit in Fredonia, New York in 1821.[7]  Because of the very low permeability of these shales, however, conventional extraction using vertical wells proved not to be cost effective, as more easily exploited deposits were found elsewhere.

    Today, shale gas is the fastest growing natural gas resource in the United States and worldwide as a result of several recent developments.  Advances in horizontal drilling technology allow a single well to pass through larger volumes of a shale gas reservoir and thus produce more gas.  The development of hydraulic fracturing technology (also known as hydrofracturing, hydrofracking, or simply fracking) has also improved access to shale gas deposits. This process requires injecting large volumes of water mixed with sand and fluid chemicals into the well at high pressure to fracture the rock, increasing permeability and production rates.  In addition to these technological advances, high natural gas prices during most of the last decade have provided further incentive to develop the shale gas resource, though gas prices have declined dramatically since the 2008 recession.

    To extract shale gas, a production well is drilled vertically until it reaches the shale formation, at which point the wellbore turns to follow the shale horizontally.  As drilling proceeds, the portion of the well within the shale is lined with steel tubing, called the “casing.”  After drilling is completed, small explosive charges are detonated to create holes in the casing at intervals where hydraulic fracturing is to occur.  In a hydraulic fracturing operation, the fracturing fluid is pumped in at a carefully controlled pressure to fracture the rock out to several hundred feet from the well.  Sand mixed with the fracturing fluid acts to prop these cracks open when the fluids are subsequently pumped out.  After fracturing, gas will flow into the well bore and up to the surface, where it is collected.

    Current estimates of the U.S. shale resource base vary widely, reflecting continued uncertainty about how much gas will ultimately prove recoverable. Most estimates fall between 420 trillion to 870 trillion cubic feet,[8] representing 18 and 38 years of consumption at 2008 levels, respectively. These deposits are located throughout the United States, typically where conventional gas resources also occur. Recently, the Marcellus Shale, stretching from eastern Ohio through West Virginia, Pennsylvania, and New York, and the Barnett Shale in Texas have experienced strong interest and sharp increases in new well drilling. More than 20,000 shale wells have been drilled nationwide in the last ten years.[9]

  • Tight Gas Sandstone. Tight gas refers to natural gas that has migrated into a reservoir rock with high porosity but low permeability.  These types of reservoirs are not usually associated with oil and commonly require horizontal drilling and hydraulic fracturing to increase well output to cost-effective levels.

  • Coalbed Methane. Natural gas is often colocated with petroleum, but it can also be found trapped within coal deposits.  Methane has traditionally posed a hazard to underground coal miners, as the highly flammable gas is released during mining activities.  Otherwise inaccessible coal seams can also be tapped to collect this gas, known as coalbed methane, by employing similar well-drilling and hydraulic fracturing techniques as are used in shale gas extraction. As of 2008, slightly less than 10 percent of U.S. natural gas reserves, or 20.8 trillion cubic feet, were in coalbed methane deposits,[10] mostly in the western United States.

    Coalbed methane deposits have also attracted interest for their potential for carbon sequestration.  Injecting carbon dioxide (CO2) into hard-to-mine coal seams would cause the CO2 to displace the methane locked within the coal, enhancing the recovery of the natural gas resource while storing the CO2 where it would not contribute to global warming.

  • Methane Hydrates. Methane hydrates, which consist of methane molecules trapped in a cage of water molecules, occur as crystalline solids in sediments in arctic regions and below the floor of the deep ocean.  Although they look like ice, methane hydrates will burn if lit.

    Methane hydrates are the most abundant unconventional natural gas source and the most difficult to extract.  Methane hydrates are conservatively estimated to hold twice the amount of energy found in all conventional fossil fuels, but the technical challenges of economically retrieving the resource are significant.  There is also a significant risk that rising temperatures from global warming could destabilize the deposits, releasing the methane—a potent greenhouse gas—into the atmosphere, and further exacerbating the problem.[11,12]

  • Biogenic Gas. Certain types of bacteria, known as methanogens, can produce methane, the chief component of natural gas, in the process of breaking down organic matter in an oxygen-free environment.  This type of gas is call “biogenic” to differentiate it from the “thermogenic” or fossil gas produced from organic material buried in the Earth's crust at high temperatures and pressures.  The properties of biogenic methane are identical to those of thermogenic methane.

    Livestock manure, food waste, and sewage are all potential sources of biogenic gas, or biogas, which is usually considered a form of renewable energy. One study has estimated that the U.S. technical potential from livestock manure alone could supply 1 percent of the country’s energy needs and lead to a 4 percent reduction in U.S. greenhouse gas emissions.[13]  Already, dozens of U.S. farmers, particularly in the Midwest, have invested in anaerobic digesters and generators to produce electricity and heat (and extra farm revenue) from livestock wastes.  Small-scale biogas production is a well-established technology in parts of the developing world, particularly Asia, where farmers collect animal manure in vats and capture the methane given off while it decays.

    Landfills offer another under-utilized source of biogas. When municipal waste is buried in a landfill, bacteria break down the organic material contained in garbage such as newspapers, cardboard, and food waste, producing gases such as carbon dioxide and methane. Rather than allowing these gases to go into the atmosphere, where they contribute to global warming, landfill gas facilities can capture them, separate the methane, and combust it to generate electricity, heat, or both. In 2007, more than six billion kilowatt-hours of electricity were generated with landfill gas, a 20 percent increase from 2003[14] and enough to power about 550,000 typical U.S. homes.[15]

Natural Gas Processing and Transportation

Gas Flaring

In the early days of petroleum exploration, natural gas was not considered a useful product because of the difficulties in transporting it to markets.  As a result, gas was simply burned off at the well or vented into the atmosphere. 

Even today, flaring and venting continue in locations where local markets and gas transportation infrastructure are lacking, or where the gas itself is contaminated with other incombustible gases.  The World Bank estimates that 5.2 trillion to 6 trillion cubic feet of gas are flared or vented worldwide each year.[16]

Figure 2: Natural gas well flare and pipelines.  Worldwide, 5.2 trillion to 6 trillion cubic feet of gas are flared annually. Image source: Shell International B. V.

Processing

Although the natural gas we use in our homes and power plants contains more than 90 percent methane, several other substances may be mixed in with the raw gas first extracted from a well.  These substances can include water, carbon dioxide, hydrogen sulfide, liquid hydrocarbon condensate, and heavier gaseous hydrocarbons such as ethane, propane, and butane.  Most of these components are separated from the methane at a processing facility, with the hydrocarbon byproducts recovered for other uses and the water, CO2, and other compounds disposed of as waste.

Because pure natural gas is odorless, a sulfur-based compound with a rotten-egg smell is added before it is piped to homes so that leaks can be noticed easily.

Transportation

The United States has developed an extensive and complex network of pipelines to transport natural gas from production areas to end users. One of the first major U.S. pipelines was built in 1891 to carry gas from central Indiana to Chicago.  Little further construction happened until after World War II, but the 1950s and 1960s saw the rapid expansion of a national gas pipeline network that continues to grow today.[17]   As of 2008, the lower 48 states had just over 300,000 miles of large transmission pipelines for shipping natural gas all over the country.[18]   More than one million additional miles of smaller, low-pressure distribution pipelines deliver the gas to individual homes and businesses.[19]   Today, pipelines supply more than 98 percent of the United States’ natural gas needs from sources in North America.

Liquefied natural gas, or LNG, offers a means of transporting natural gas across long distances where pipelines are not available.  To produce LNG, natural gas is compressed and cooled to around minus 260 oF, a step that converts the gas to a liquid and reduces its volume by a factor of 600.  LNG can then be shipped in specially-constructed tankers for overseas transport.  Upon reaching its destination, the LNG is unloaded at a receiving terminal, returned to a gaseous state, and sent through local pipelines to end-users.[20]

LNG requires substantial infrastructure to cool and compress the natural gas to liquid, ship it overseas, and return the LNG to gaseous form at the receiving end.  In addition, the production of LNG and subsequent refrigeration during transport is highly energy-intensive.  For these reasons, only recent increases in natural gas prices have allowed LNG to compete with gas delivered through pipelines.  Today, LNG accounts for a small but growing fraction of the natural gas trade.  In 2008, LNG accounted for 1.5 percent of the U.S. natural gas supply,[21] but the U.S. Department of Energy (DOE) expects a nearly sixfold increase in the amount of LNG imported by 2030.[22]  Three-quarters of imported LNG came from Trinidad and Tobago, with the remainder from Egypt, Norway, Nigeria, and Qatar.[23]

In 2000, the United States had only four LNG import terminals.  By 2010, three new terminals had been constructed, and several additional projects or expansions of existing facilities have been approved or proposed.[24]  The growth in LNG shipments to the U.S. has also provoked safety concerns, particularly where LNG terminals are situated near densely settled areas (Figure 3).  In the wake of the Sept. 11, 2001, terrorist attacks, LNG deliveries have faced tight security and stricter regulations as policy makers have debated the risks of an attack on LNG facilities or ships.


Figure 3: An LNG tanker passing through densely-populated Boston Harbor under security escort.  Image source: ferc.gov / U.S. Coast Guard

Uses of Natural Gas

Natural gas is a versatile, clean-burning, and efficient fuel that sees use in a wide variety of applications today.  The Chinese first used natural gas two thousand years ago, sending it through bamboo pipes and burning it to evaporate seawater and produce salt.[25]   In the 19th and early 20th centuries, natural gas was used primarily for street and building lighting, providing what was known as gaslight.  Today, improved distribution of gas has made possible a wide variety of uses in homes, businesses, factories, and power plants.  In 2008, for example, the U.S. consumed 23.2 trillion cubic feet of natural gas, nearly one-quarter of U.S. energy consumption and the energy equivalent of almost 190 billion gallons of gasoline.[26]


Figure 4: Historical usage of natural gas in the United States. Residential and commercial consumption have remained relatively constant over the past 35 years, while use for electricity generation has expanded greatly since the 1990s. Data source: U.S. EIA


Figure 5: Natural gas consumption by sector in the United States in 2008.  Data source: U.S. EIA.

Electric Power

The fastest growing use of natural gas today is for the generation of electric power.  Natural gas power plants usually generate electricity in gas turbines (which are derived from jet engines), directly using the hot exhaust gases of fuel combustion.  Single-cycle gas turbines generally convert the heat energy from combustion into electricity at efficiencies of 35 to 40 percent.  Higher efficiencies of 50 percent or more are possible in natural gas “combined-cycle” (NGCC) plants.  NGCC plants first use the combustion gases to drive a gas turbine, after which the hot exhaust from the gas turbine is used to boil water into steam and drive a steam turbine (Figure 6).


Figure 6: Schematic diagram for a natural gas combined-cycle power plant.  First, the natural gas is combusted in a gas turbine connected to a generator.  The hot exhaust gases are then run through a heat exchanger to generate steam for a steam turbine.  NGCC plants can reach efficiencies above 50 percent, compared to 30-35 percent for coal-fired power plants.  Figure source: global-greenhouse-warming.com

Low natural gas prices in the 1990s stimulated the rapid construction of gas-fired power plants.  In 2003, natural gas passed coal as the energy source with the largest installed electricity generation capacity in the U.S.  While natural gas-fired plants are among cheapest power plants to construct, their operating costs are generally higher than those of coal-fired power plants because the fuel is more expensive.  However, natural gas plants have greater operational flexibility than coal plants because they can be fired up and turned down rapidly.  For these two reasons, many natural gas plants are used primarily to provide peaking capacity at times when electricity demand is especially high, such as the summer months when air conditioning is widely used.  During much of the year, these natural gas “peaker” plants are idle, while coal-fired power plants typically provide base load power.  An MIT study calculated that increased utilization of existing natural gas power plants to displace coal-fired power could reduce the electric sector's carbon emissions by 22 percent in the near term.[27]  Today, coal still provides most of the nation’s electricity, despite the higher installed capacity of natural gas.  Natural gas’ contribution to electricity generation is rapidly growing, however, from only 9 percent in 1988 to 20 percent in 2008.[28]


Figure 7: New electric generation capacity installed in 2003-2009.  Low gas prices in the 1990s encouraged a massive expansion of natural gas generating capacity that continues today.


Figure 8: Historical net electricity generation (electric sector only), 1950-2008.  The contribution of natural gas to electricity generation has more than doubled since the late 1980s.  Data source: U.S. EIA

Heating and Cogeneration

Residential and commercial uses account for over a third of U.S. natural gas consumption, as gas is used in buildings for space and water heating and for cooking.  About half of all U.S. homes use natural gas for heating,[29] and 70 percent of new homes are built with gas heating systems.  Home furnaces can reach efficiencies of over 90 percent.[30]   Measures to increase building efficiency are widely considered the most cost-effective way to reduce the amount of natural gas we use.  One study estimated that an ambitious program to improve building performance through means such as high-efficiency insulation, windows, furnaces, water heaters, and other appliances could save 234 trillion cubic feet of natural gas over the next 50 years.[31]

Natural gas can also be used to produce both heat and electricity simultaneously, a technology called “cogeneration” or “combined heat and power” (CHP).[32]  Cogeneration systems are highly efficient, able to put 75 to 80 percent of the energy in gas to use.  “Trigeneration” systems, which provide electricity, heating, and cooling, can reach even higher efficiencies.  A 2009 UCS report indicates that CHP use could more than triple by 2030 if policies are enacted to make steep cuts in carbon emissions.[33]

Industrial and Other Uses

Natural gas sees a broad range of other uses in industry, as a source of both heat and power and as an input for producing plastics and chemicals.  Most hydrogen gas (H2) production, for example, comes from reacting high temperature water vapor (steam) with methane.  Today, the resulting hydrogen is mostly used to produce ammonia for fertilizer, one of the most important industrial products derived from natural gas.

Hydrogen produced from natural gas can itself be used as a fuel.  The most efficient way to convert hydrogen into electricity is using a fuel cell, which combines hydrogen with oxygen to produce electricity, water, and heat.  Although the process of reforming natural gas to hydrogen still has associated carbon dioxide emissions, the amount released for each unit of electricity generated is much lower than for a combustion turbine.

Compressed natural gas (CNG) has seen limited use as a transportation fuel, mostly in public transit. CNG, which is compressed at over 3,000 psi to one percent of the volume the gas would occupy at normal atmospheric pressure, can be burned in an internal combustion engine that has been appropriately modified. Just 0.1 percent of the natural gas consumed in the United States in 2008 powered vehicles, but this still represents the energy content of more than 5 million barrels of oil.

Compared to gasoline, CNG vehicles emit far less carbon monoxide, nitrogen oxides (NOx), and particulates.  The main disadvantage of CNG is its low energy density compared with liquid fuels.  A gallon of CNG has only a quarter of the energy in a gallon of gasoline.[34]  CNG vehicles therefore require big, bulky fuel tanks, making CNG practical mainly for large vehicles such as buses and trucks.

Implications of Competing Uses

The wide range of uses for natural gas makes it a critical resource for the United States and world economies.  This versatility also means that changes in natural gas demand for one use can affect gas prices for many other applications.  During the 1990s, natural gas prices in the United States were generally low and stable.  The major expansion of natural gas use in power plants led to steady increases in gas prices for all uses, including home heating and industry.  Since the early 2000s, gas prices have been notable for their volatility (Figure 9).  After spiking to record levels in 2005 and 2008, prices after the global economic downturn are at their lowest in eight years.
 
Part of this volatility stems from the difficulties in transporting gas where pipeline infrastructure is not already in place.  Because of this limitation, there is no worldwide market price for natural gas, and local prices can be heavily dependent on regional production and availability.


Figure 9: Monthly prices for natural gas at the wellhead and for residential customers since 1990.  After being consistently low throughout the 1990s, gas prices have experienced great volatility in the 2000s.  Source: U.S. EIA.

The dependence on regional gas supplies can also threaten countries’ energy security.  Much of Eastern and Central Europe’s natural gas supply comes from Russia and passes through pipelines in several different countries on its way westward.  Repeated disputes between Russia and Ukraine, for instance, have led to gas shutoffs that have caused shortages in countries as distant as France and Italy.[35]

Environmental Impacts of Natural Gas

Global Warming Emissions 

Although natural gas is a hydrocarbon fossil fuel, the global warming emissions from its combustion are much lower than those from coal or oil. Natural gas produces 43 percent fewer carbon emissions than coal for each unit of energy delivered, and 30 percent fewer emissions than oil.

The full global warming impact of natural gas also includes "upstream" emissions from drilling gas wells, building pipelines, and processing raw gas.  Much of these upstream emissions, however, consist of leakage of natural gas itself from pipelines and storage facilities.[36]  A 1997 EPA study estimated that 1.4 percent of all gas produced in the United States is lost between the well and customer.[37]  Methane, the main component of natural gas, is itself a strong warming agent. Compared with CO2, methane is 25 times more effective at trapping heat over a 100-year timescale, and 72 times more effective over a 20-year timescale,[38].  Further measurements are needed to establish exactly how much gas is being lost.  However, efforts to reduce even relatively small amounts of methane leakage from the gas pipeline network, as well as from sources such as coal mines, landfills, and livestock, can clearly result in major greenhouse gas reductions, with the added benefit that the captured methane can be used to produce energy. 

Air pollution

Cleaner burning than other fossil fuels, the combustion of natural gas produces negligible amounts of sulfur, mercury, and particulates.  Burning natural gas does produce nitrogen oxides (NOx), which are precursors to smog, but at lower levels than gasoline and diesel used for motor vehicles. DOE analyses indicate that every 10,000 U.S. homes powered with natural gas instead of coal avoids the annual emissions of 1,900 tons of NOx, 3,900 tons of SO2, and 5,200 tons of particulates.[39,40] Reductions in these emissions translate into public health benefits, as these pollutants have been linked with problems such as asthma, bronchitis, lung cancer, and heart disease for hundreds of thousands of Americans.[41]

Land Use and Wildlife

Like most energy sources, natural gas production inevitably disturbs the natural landscape, as land must be cleared for roads, drilling equipment, processing facilities, and pipelines. Gas exploration, drilling, and production can create traffic congestion, road damage, dust, and noise in local communities.  These activities can also harm wildlife through noise, pollution, and habitat destruction and fragmentation. The advent of horizontal drilling technology, used extensively in unconventional gas production, has greatly reduced the surface footprint of drilling operations by allowing multiple wells to be drilled from a single well pad. However, much of the development of the U.S. shale gas resources is occurring in locations where oil and gas production has not previously taken place (in some cases in wilderness areas), requiring infrastructure to be built from scratch. Noise and traffic abatement measures, as well as careful regulation of drilling in environmentally sensitive locations such as wetlands, waterways, and endangered species’ habitats, are necessary to minimize these impacts. 


Figure 10: Shale gas drilling operation in the Marcellus Shale in Upshur County, West Virginia.  Image source: WVSORO

Water use and Pollution

Natural gas extraction can have a significant impact on local water resources. Recently developed hydraulic fracturing techniques used in unconventional gas production have added new dimensions to conventional water management challenges for the gas industry because of the quantity of water and nature of the fluids involved. Drilling and hydrofracturing a single horizontally-drilled gas shale well typically requires a total of two to four million gallons of water. The fluid injected into a well to fracture the surrounding rock is about 99 percent water and sand, with the remainder comprised of some subset of over 200 chemicals used to enhance the fracturing process by reducing friction, preventing corrosion, and killing bacteria.[42] These fracturing chemicals often include toxic substances, such as methanol, that can contaminate drinking water if present in even small amounts.[43]  Considerable care is necessary to transport, store, and handle these chemicals safely and avoid releases into water supplies.

The U.S. Department of Energy has claimed that the potential for toxic chemicals to enter drinking water supplies during hydraulic fracturing is low, assuming proper development and monitoring of the well.[44]  With a few important exceptions, shale gas deposits where hydraulic fracturing is employed are typically thousands of feet deeper than freshwater aquifers.  Fracturing fluids would have to penetrate multiple overlying rock layers to reach aquifers from the gas-bearing shale where hydraulic fracturing takes place. There is some concern that the hydraulic fracturing process itself could compromise these layers, but the greater risk appears to be in failures of the metal casings and cement barriers used to isolate gas wells from groundwater supplies.[45]  A casing failure in 2007 in a shale gas well in Bainbridge, Ohio, allowed natural gas to leak into nearby residential drinking water wells, where it caused an explosion that severely damaged one home and forced the evacuation of 19 others.[46]

Gas production also poses serious challenges with respect to wastewater disposal.  Even conventional gas wells yield unwanted water that comes to the surface along with the gas.  This “produced water,” as it is known, can carry with it naturally-occurring dissolved solids, heavy metals, and hydrocarbons in quantities that make it unsuitable for human consumption and difficult to dispose of safely.  When hydraulic fracturing is used for shale gas or coalbed methane extraction, the wastewater disposal issues are compounded by the huge volumes of fracture fluids involved. After a hydraulic fracturing operation, well operators pump the fracturing fluids out of the well along with any naturally occurring produced water. This “flowback” water is usually highly saline and may contain toxic fracturing chemicals and small quantities of naturally-occurring radioactive minerals. Gas companies often temporarily store flowback water in open-air pits with impermeable liners to avoid seepage, but unexpected precipitation can cause these pits to overflow. Covered holding tanks, used in some locations, offer a more secure temporary storage option.[47]

The ultimate disposal method for flowback water varies from state to state.  Where possible, the wastewater is directly injected into underground saline aquifers that are already unsuitable for drinking.  In the Marcellus Shale region, where the occurrence of saline aquifers is limited, produced water is usually sent to municipal wastewater treatment plants before being released back into surface water bodies.  Increasing shale gas production could strain the capacity of local treatment facilities, which are not always equipped to handle the volumes of fluids involved in shale gas production or the high salinity of the water produced. The large water requirements of hydrofracturing make it essential that research is done to ensure that overall water use is reduced, flowback water is recycled where possible, and wastewater is properly treated or disposed. 

Shale gas development is relatively new, and many of the potential environmental effects from the fluids used in hydraulic fracturing and the disposal of produced water are not completely understood.  The Energy Policy Act of 2005 specifically exempted hydraulic fracturing from regulation under the Safe Drinking Water Act.  At the state level, regulations covering the hydraulic fracturing process and the disclosure of the chemicals used vary widely among states.  The EPA and other organizations are undertaking much needed efforts to more fully characterize the environmental risks of hydraulic fracturing, secure more complete and public disclosure of the chemicals used in fracturing fluids, and examine non-toxic or less toxic alternatives.  Above all, stronger regulation of shale gas production, whether under existing clean water laws or new regulations, will be critical for minimizing its environmental impacts.

Earthquakes

Although the process of hydraulically fracturing gas shales does generate large numbers of micro-earthquakes, these mostly measure around -1.5 on the Richter scale and are barely detectable at the surface.   Nevertheless, scientists continue to explore the potential for hydraulic fracturing to induce larger earthquakes.

The Future of Natural Gas

A convergence of factors is driving our society towards greater reliance on natural gas as a source of energy.  An increased focus on the potential reductions in carbon emissions and air pollution from burning natural gas instead of coal or oil have made natural gas an environmentally attractive alternative to other fossil fuels.  Concurrently, improved techniques for extracting unconventional sources of gas have dramatically raised estimates of the U.S.’s available gas resource.

Because energy produced from natural gas has much lower associated carbon emissions than these other fossil fuels, natural gas could act as a “bridge” fuel to a low-carbon energy future.  Particularly in the electric sector, natural gas has the potential to ease our transition to renewable energy.

In the short term, renewable energy added to the grid may displace natural gas use, because natural gas power typically has the highest operating costs.  In the long term, increased amounts of renewable energy are likely to encourage the use of natural gas as a complementary source of power. The integration of large amounts of renewable energy sources into the electrical generation mix will pose some challenges for the nation’s electric system because of the inherent variability of solar and wind power. Natural gas plants have the operational flexibility to vary their production rapidly, allowing them to provide reliability to the electric power system as it transitions to greater shares of renewable generation.

Natural gas is by no means a panacea for the environmental problems caused by our energy use.  There is broad agreement among climate scientists that carbon reductions of about 80 percent will be needed to avert the worst effects of climate change, so simply switching to natural gas from coal and oil will not ultimately bring about the necessary reductions.  In addition, the development of our newly-discovered shale gas resource will disturb areas previously untouched by oil and gas exploration and raise serious water management and quality challenges.  Some researchers have also suggested that abundant shale gas resources could delay the transition to renewables by providing a cheap, plentiful alternative.[48]  Given the competing uses of natural gas and the vagaries of regional supplies, increased dependence on natural gas also exposes our economy to its frequent price volatility.

Overall, the increased use of natural gas over coal and oil will produce real and substantial reductions in global warming emissions and improvements in public health. As gas use expands, the natural gas industry must also minimize the environmental effects of its extraction and production. If used wisely and efficiently, natural gas can help our economy effectively transition toward even cleaner, more sustainable sources of energy like wind, solar, geothermal, and bioenergy.

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