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The Newest Question in the Continuing Debate about Shale Gas and the Environment

By Eleanor Saunders

Among the environmental controversies sparked by the use of horizontal drilling and hydraulic fracturing to extract shale gas, the most basic one concerns the quantity of greenhouse gas emitted over the fuel’s entire life-cycle – from the initial stage of drilling and completing wells through the intermediate steps of processing, transporting, storing and distributing gas to the end point of its combustion at power plants.

Until recently, this issue played a far less prominent role in the shale-gas debate than concerns about the possible harm to water supplies, air quality or the character of rural communities.[1] But over the last months, the contribution of shale gas wells to greenhouse-gas emissions has come to the fore.

Some new analyses suggest that shale gas may be responsible for significantly more emissions of methane over its full life-cycle than so-called conventional natural gas supplies. [2] Methane is a potent greenhouse gas that makes up 70 to 90 percent of unrefined gas and has an impact on climate that is at least twenty-five times greater than carbon dioxide’s (CO2), when calculated over a period of 100 years.

Adding to the implications of the new studies are predictions that the portion of total U.S. gas production from shale formations will rise quickly, from about 16 percent in 2009 to about 45 percent in 2035.[3] Thus, emissions from shale wells would correspondingly grow, an increase whose impact is magnified by methane’s powerful effect on climate.

These findings could challenge the reputation of natural gas as an environmentally preferable fossil fuel, less deleterious to the climate and human health than coal. This reputation has led many respected authorities – at the Massachusetts Institute of Technology and the Center for Strategic and International Studies, for example – to recommend using gas as a ‘bridge fuel’ until clean-energy technologies are mature enough to replace fossil fuels or remove their carbon emissions, a possibility if emergent carbon-capture-and-storage processes turn out to be economically and environmentally viable.

Disagreements over the climate impact of shale gas are particularly important to the approximately 15 states where laws and policies continue to require reduced levels of greenhouse-gas emissions.[4]  Because about 80 percent of the gas in current U.S. supplies comes from conventional wells, and natural-gas power plants produce on average about 45 percent fewer greenhouse-gas emissions than coal-fired plants, several states have designated their emissions level as the benchmark by which to evaluate proposals for new generation. [5] Part of the lower emissions level has to do with the efficiency of natural-gas plants, part with a chemical transformation that occurs during combustion so that only CO2, and not methane, is emitted from the plants.

With natural gas generating 23 percent of U.S. electricity compared to coal’s 43 percent, the potential for increasing its share in electricity markets is huge. Thus, a great deal is riding on the outcome of the debate about shale gas’s greenhouse-gas emissions from both economic and environmental perspectives.

Conflicting Analyses

The current emissions controversy was initially triggered by two analyses, one from U.S. Environmental Protection Agency (EPA) and the other from a team of researchers at Cornell University.

The first salvo was a quiet one. Without fanfare, EPA revised its estimate of methane emissions from shale-gas wells in a background technical support document to its November 2010 final rule for greenhouse-gas reporting by the petroleum and gas industries.  Based largely on data gathered from industry members of its Natural Gas STAR Program, EPA increased the numbers associated with intentional venting and inadvertent leakage of methane by 120 percent.

The next salvo, which came from a peer-reviewed study published in April 2011 by Cornell University researchers, stirred up far stronger reactions because it directly challenges the value of natural gas as a bridge fuel.  The study estimates that between 3.6 and 7.9 percent of the methane in shale gas escapes into the atmosphere from a combination of intentional venting and unintentional leakage over the productive life of a well, an amount that is at minimum 30 percent greater and possibly over 50 percent greater than conventional gas, according to conditions at individual well sites.  Depending upon the time frame used to calculate the impact of these emissions on climate – 100 versus 20 years — shale gas could put as much or even more greenhouse gas into the atmosphere than coal, the authors contend.

Both EPA and the Cornell researchers note that the methane emissions could be reduced through improved industry practices.  A variety of available technologies decrease vented and fugitive emissions by as much as 90 percent, and the expense of installing them would be offset relatively quickly through sales of the captured methane, according to information from partners in EPA’s Natural Gas STAR Program, a voluntary group exploring “green” industry practices.[6]

But despite the acknowledgement that a remedy exists, the response from industry and other energy experts to EPA’s revised figures and the Cornell study has been heated. The most comprehensive critique comes from IHS CERA, an energy consulting company that works with industry clients.

In their assessment, the IHS CERA authors raise three main objections to EPA’s analysis.  They contend that the agency misinterprets industry data points about methane emissions, assumes the ratio of venting versus flaring a well’s emissions prior to its production of saleable gas is higher than it actually is,[7] and mistakenly presumes that the greater productivity of shale-gas wells means that more methane emissions occur. EPA does not consider that increased productivity may imply that fewer wells are drilled, thereby reducing emissions, and it wrongly posits a venting rate far higher than safety allows and good industry practice supports, IHS CERA says.  Based on IHS CERA’s calculations, actual shale-gas methane emissions are only one-third what EPA estimates. The critique also says that Cornell study commits many of the same errors.

Most other substantive analyses, which appeared around the same time as the IHS CERA study, have entered the fray by addressing to what degree prior analyses were on target or not.[8] On balance, more of them conclude that EPA and the Cornell team overestimated emissions to at least some degree.

Only one study, published in August 2011 by a team of researchers from Carnegie Mellon University, put together its own independent data points, specific to the context of Marcellus shale gas, in an attempt to improve the accuracy of results. The study occupies the interesting position of receiving funding from the Sierra Club, a non-profit environmental organization, yet concluding that the difference between emissions from Marcellus shale-gas wells and conventional domestic wells is much less than the EPA and the Cornell authors found.

The team assembled a wide range of information on emissions associated with Marcellus well site investigation, preparation, drilling and completion – what the group terms the pre-production phase. They make the reasonable assumption that the intermediate steps from processing through distribution and the final phase of combustion at power plants is identical for shale and conventional gas, though they also claim to have more accurate information for these parts of the life cycle, based on another Carnegie Mellon study. The researchers also try to account for uncertainties in their data by constructing a range of possible emissions scenarios, and then employing special statistical methods that take into account the impact of these uncertainties on their final figures. The study concluded by comparing the full life cycle results for the Marcellus shale gas to the average life cycle emissions for conventional domestic natural gas.

According to its calculations, Marcellus wells cause an 11 percent increase in greenhouse-gas emissions relative to average domestic gas, exclusive of combustion, and a 3 percent increase when the emissions from combustion are incorporated into the life cycle analysis, figures that are considerably lower than the results reported by the EPA and the Cornell study.

The Carnegie Mellon team also compared their results to the life cycle emissions associated with domestic coal and imported liquid natural gas.  Relative to coal, both conventional natural gas and Marcellus shale gas have lower life cycle emissions, the study concludes.  But if carbon capture and storage were to remove and sequester the greenhouse gases from power generation, then gas, and especially Marcellus shale gas, would have a carbon footprint greater than coal, it says.  The finding indicates that coal has “upstream” greenhouse emissions that are lower than gas, the researchers say.

Of particular importance is the study’s conclusion that relative to imported liquefied natural gas (LNG), emissions from domestic shale gas are about three percent lower. Since the Energy Information Agency had previously expected imports of LNG to rise, and that prediction has turned out to be wrong by virtue of the increasing role played by domestic shale gas, the Carnegie Mellon research injects an important new element into the debate.

Where the Debate Stands Now

Although it might look like the final answer is in, the question is still short of reaching resolution. And the reason behind lingering doubts is the one thing about which all sides agree: that a great deal of uncertainty remains about the accuracy of information on shale-well methane emissions.

Here, for example, are the limitations with which Timothy Skone of the National Energy Technology Laboratory qualifies his analysis of emissions.  They include uncertainties about

  • Gas production rates, gas composition and flaring rates, due to variation from formation to formation;
  • The effectiveness of  “green” – that is, environmentally preferable – methods and technologies;
  • The quantity of fugitive emissions around well heads;
  • The greenhouse-gas emissions associated with the production and treatment of hydrofracking fluids; and
  • The emissions from changes in land use and building access roads to shale-gas well sites.

It should be noted that Skone’s final estimate of shale-gas emissions is lower than estimates from EPA and Cornell, but he and the Cornell researchers wholly agree that available data suffers from serious shortcomings and further study is needed.

Even IHS CERA’s more strident critique acknowledges the lack of comprehensive data and recognizes that EPA’s new rules for data collection could be “beneficial to all who seek to understand greenhouse-gas emissions from the industry.”  The authors also do not appear to object to EPA’s new source performance standards, which would regulate air emissions during the completion phase of hydraulically fractured gas wells, requiring “green” practices for flaring and capturing methane.  Their only bone of contention is their belief that EPA’s rules do not differ from common industry practice.

With that much uncertainty at the core of the debate, it is difficult to imagine resolving the question unless and until industry reporting of about shale well emissions allows for more valid analyses to take place. If results from accurate emissions total show real benefits over coal and LNG, then at least some of the currently skeptical stakeholders might be more open to accepting the other environmental risks of shale gas.


[1] See Opportunities and Challenges Posed by the Marcellus Shale on Green Matters for background on the debates about the environmental and quality-of-life impacts of exploiting shale formations, and for an explanation of the techniques of hydraulic fracturing and horizontal drilling being used to extract gas from shale formations.
[2] Conventional gas refers to reserves that can be exploited through the standard techniques associated with vertically drilled wells that tap directly into gas reservoirs as opposed to the deep and long horizontal wells made productive by hydraulically fracturing shale. Unrefined natural gas from either source is comprised of about 70 to 90 percent methane, whereas refined natural gas is almost purely methane.
[3]
See the Energy Information Agency’s Executive Summary from its Annual Energy Outlook 2011.  
[4]
See the interactive map of states’ Greenhouse Gas Emissions Targets at the Pew Center for Global Climate Change website.  Of the states listed as having targets, nine – CA, CT, MA, ME, MD, MN, NJ, OR and WA – have enacted laws that set emissions reduction targets; three – NH, RI and VT – are signatories to a regional climate established by the association of New England Governors and Eastern Canadian Premiers; and two – CO and NY –have executive orders that still seem to be in force to set reduction targets. The New England states, NY, MD and DE are also signatories to the Regional Greenhouse Gas Initiative, which established an emissions cap for power plants. NJ Gov. Chris Christie plans to remove his state from the agreement at the end of this year. Although other states on the Pew map have had either executive orders or targets set by executive level agencies, it is not clear that these policies still guide policy as a result of electoral changes.
[5]
Beginning with California, a number of U.S. states such as Washington, Oregon and Massachusetts have designated natural gas’s emissions levels – approximately 1135 lbs/MWh of carbon-dioxide equivalent — as the limit for any new power plants, which, if exceeded, would require plant operators to find ways to offset the excess emissions.  In many additional states, utilities have been replacing coal generation with natural gas generation as a way of reducing other air pollutants, such as nitrogen oxides, sulfur dioxide and mercury, because gas emits less of them, too.
[6]
General Electric also has produced a report on technologies that could reduce global flaring and venting of gas by five percent per year, an amount equivalent to 30 percent of consumption in the European Union and 23 percent in the United States, the company says. 
[7]
Flaring or burning changes the emissions from methane into less potent CO2.
[8]
See especially TJ Skone’s slide presentation from a lecture given at Cornell in May 2011, Life Cycle Greenhouse Gas Analysis of Natural Gas Extraction and Delivery in the United States; JD Hughes July 2011 Life Cycle Greenhouse Gas Emissions from Shale Gas Compared to Coal and a joint Deutsche Bank/Worldwatch Institute analysis published in August 2011, Comparing Life Cycle Greenhouse Gas Emissions from Natural Gas and Oil.

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