Environmental Impacts of Natural Gas

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Global warming emissions

Natural gas is a fossil fuel, though the global warming emissions from its combustion are much lower than those from coal or oil.

Natural gas emits 50 to 60 percent less carbon dioxide (CO2) when combusted in a new, efficient natural gas power plant compared with emissions from a typical new coal plant [1]. Considering only tailpipe emissions, natural gas also emits 15 to 20 percent less heat-trapping gases than gasoline when burned in today’s typical vehicle [2].

Emissions from smokestacks and tailpipes, however, do not tell the full story.

The drilling and extraction of natural gas from wells and its transportation in pipelines results in the leakage of methane, primary component of natural gas that is 34 times stronger than CO2 at trapping heat over a 100-year period and 86 times stronger over 20 years [3]. Preliminary studies and field measurements show that these so-called “fugitive” methane emissions range from 1 to 9 percent of total life cycle emissions [4].

Whether natural gas has lower life cycle greenhouse gas emissions than coal and oil depends on the assumed leakage rate, the global warming potential of methane over different time frames, the energy conversion efficiency, and other factors [5]. One recent study found that methane losses must be kept below 3.2 percent for natural gas power plants to have lower life cycle emissions than new coal plants over short time frames of 20 years or fewer [6]. And if burning natural gas in vehicles is to deliver even marginal benefits, methane losses must be kept below 1 percent and 1.6 percent compared with diesel fuel and gasoline, respectively. Technologies are available to reduce much of the leaking methane, but deploying such technology would require new policies and investments [7].

Air pollution

Photo: Mscalora/Wikimedia Commons

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 [7]. 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 [9].

However, despite these benefits, unconventional gas development can affect local and regional air quality. Some areas where drilling occurs have experienced increases in concentrations of hazardous air pollutants and two of the six “criteria pollutants” — particulate matter and ozone plus its precursors — regulated by the EPA because of their harmful effects on health and the environment [9]. Exposure to elevated levels of these air pollutants can lead to adverse health outcomes, including respiratory symptoms, cardiovascular disease, and cancer [11]. One recent study found that residents living less than half a mile from unconventional gas well sites were at greater risk of health effects from air pollution from natural gas development than those living farther from the well sites [12].

Land use and wildlife

The construction and land disturbance required for oil and gas drilling can alter land use and harm local ecosystems by causing erosion and fragmenting wildlife habitats and migration patterns. When oil and gas operators clear a site to build a well pad, pipelines, and access roads, the construction process can cause erosion of dirt, minerals, and other harmful pollutants into nearby streams [13].

A study of hydraulic fracturing impacts in Michigan found potential environmental impacts to be “significant” and include increased erosion and sedimentation, increased risk of aquatic contamination from chemical spills or equipment runoff, habitat fragmentation, and reduction of surface waters as a result of the lowering of groundwater levels [14].

Water use and pollution

Unconventional oil and gas development may pose health risks to nearby communities through contamination of drinking water sources with hazardous chemicals used in drilling the wellbore, hydraulically fracturing the well, processing and refining the oil or gas, or disposing of wastewater [15]. Naturally occurring radioactive materials, methane, and other underground gases have sometimes leaked into drinking water supplies from improperly cased wells; methane is not associated with acute health effects but in sufficient volumes may pose flammability concerns [16]. The large volumes of water used in unconventional oil and gas development also raise water-availability concerns in some communities.

Groundwater

There have been documented cases of groundwater near oil and gas wells being contaminated with fracking fluids as well as with gases, including methane and volatile organic compounds. One major cause of gas contamination is improperly constructed or failing wells that allow gas to leak from the well into groundwater. Cases of contamination have been documented in Ohio and Pennsylvania [17].

Another potential avenue for groundwater contamination is natural or man-made fractures in the subsurface, which could allow stray gas to move directly between an oil and gas formation and groundwater supplies.

In addition to gases, groundwater can become contaminated with hydraulic fracturing fluid [18]. In several cases, groundwater was contaminated from surface leaks and spills of fracturing fluid. Fracturing fluid also may migrate along abandoned wells, around improperly sealed and constructed wells, through induced fractures, or through failed wastewater pit liners [19].

Surface Water

Unconventional oil and gas development also poses contamination risks to surface waters through spills and leaks of chemical additives, spills and leaks of diesel or other fluids from equipment on-site, and leaks of wastewater from facilities for storage, treatment, and disposal. Unlike groundwater contamination risks, surface water contamination risks are mostly related to land management and to on- and off-site chemical and wastewater management.

The EPA has identified more than 1,000 chemical additives that are used for hydraulic fracturing, including acids (notably hydrochloric acid), bactericides, scale removers, and friction-reducing agents. Only maybe a dozen chemicals are used for any given well, but the choice of which chemicals is well-specific, depending on the geochemistry and needs of that well [20]. Large quantities — tens of thousands of gallons for each well — of the chemical additives are trucked to and stored on a well pad. If not managed properly, the chemicals could leak or spill out of faulty storage containers or during transport.

Drilling muds, diesel, and other fluids can also spill at the surface [21]. Improper management of flowback or produced wastewater can cause leaks and spills. There is also risk to surface water from deliberate improper disposal of wastewater by bad actors.

Water Use

The growth of hydraulic fracturing and its use of huge volumes of water per well may strain local ground and surface water supplies, particularly in water-scarce areas. The amount of water used for hydraulically fracturing a well can vary because of differences in formation geology, well construction, and the type of hydraulic fracturing process used [22]. The EPA estimates that 70 billion to 140 billion gallons of water were used nationwide in 2011 for fracturing an estimated 35,000 wells [23]. Unlike other energy-related water withdrawals, which are commonly returned to rivers and lakes, most of the water used for unconventional oil and gas development is not recoverable. Depending on the type of well along with its depth and location, a single well with horizontal drilling can require 3 million to 12 million gallons of water when it is first fractured — dozens of times more than what is used in conventional vertical wells [24]. Similar vast volumes of water are needed each time a well undergoes a “work over,” or additional fracturing later in its life to maintain well pressure and gas production. A typical shale gas well will have about two work overs during its productive life span [25].

Earthquakes

Hydraulic fracturing itself has been linked to low-magnitude seismic activity—less than 2 moment magnitude (M) [the moment magnitude scale now replaces the Richter scale]— but such mild events are usually undetectable at the surface [26]. The disposal of fracking wastewater by injecting it at high pressure into deep Class II injection wells, however, has been linked to larger earthquakes in the United States [27]. At least half of the 4.5 M or larger earthquakes to strike the interior of the United States in the past decade have occurred in regions of potential injection-induced seismicity [28]. Although it can be challenging to attribute individual earthquakes to injection, in many cases the association is supported by timing and location of the events [29]. 

References:

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[29] Van der Elst 2013.

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