Yale School of Forestry & Environmental Studies
Marcellus Shale Gas Well Share

Hydraulic Fracturing "cheat sheet" for Peer-reviewed Literature

Though shale gas extraction with the use of hydraulic fracturing has been underway in the U.S. for about a decade, peer-reviewed literature looking at its impacts has only begun to be published. Some of the articles that were among the first published on the environmental impacts, and remain among the most talked about, are described here.

Natural gas has been touted as a bridge to less carbon-intensive energy options. In the late 1990s and 2000s it was found that the U.S. domestic natural gas supply is much larger than once believed because the combined technology of horizontal drilling and hydraulic fracturing now allows for the extraction of previously inaccessible natural gas from shale formations. In recent years, however, hydraulic fracturing has become a highly charged issue due to the risks the process poses to the environment and human health. Some topics that have become particularly polarizing in cost-benefit discussions of shale gas include the greenhouse gas footprint and effects on water quality.

Dr. R. Howarth, a biogeochemist at Cornell University, is the primary author of one of the first peer-reviewed articles published on the impacts of shale gas extraction. His March 2011 letter, “Methane and the greenhouse-gas footprint of natural gas from shale formations,”concluded that due to the fugitive emissions of methane—methane that is vented or leaks into the atmosphere over the lifetime of a well—the greenhouse gas footprint of shale gas is larger than that of conventional gas, oil, and, over a twenty-year time horizon, also larger than that of coal.[i] In discussions of greenhouse gas footprints, distinction among time horizons is critical as various greenhouse gases remain in the atmosphere for different periods of time. Methane only resides in the atmosphere for approximately twenty years whereas carbon dioxide remains in the atmosphere for much longer. (Keep in mind, however, that methane is a more potent greenhouse gas than carbon dioxide because an individual methane molecule traps more heat than a carbon dioxide molecule). Howarth’s conclusions were significant because they challenged the “bridge fuel” characterization of shale gas.

Two articles published shortly after Howarth’s paper contradicted his conclusions because they provide significantly different estimates for fugitive emissions. A second, independent analysis of greenhouse gas emissions was conducted by researchers at Carnegie Mellon University, and focused on gas extraction from the Marcellus Shale.[ii] The Carnegie Mellon study, “Life cycle greenhouse gas emissions of Marcellus shale gas,” used a fugitive emissions estimate of two percent, which is small compared with Howarth’s range of 3.6 to 7.9 percent. This reduced fugitive emissions estimate in the Carnegie Mellon paper contributed to their opposing conclusion that natural gas from the Marcellus Shale has lower life cycle greenhouse gas emissions than coal.

The second article that challenged Howarth’s conclusions was an official comment submitted by another Cornell professor, Dr. L. Cathles.[iii] The Cathles response attacks Howarth’s paper for “significantly” overestimating fugitive emissions and ignoring the use of green completions, or application of technology that has been used for years by industry to reduce and, ideally, eliminate fugitive emissions. This disagreement in the primary literature obscures from policy makers a clear choice among fossil fuels.

One of the most common questions concerning hydraulic fracturing has proven to be challenging to answer; does the process of shale gas extraction contaminate drinking water supplies? Shale gas extraction involves several steps that have the potential to contaminate drinking water. Wells are drilled directly through drinking water aquifers, but fresh water resources are protected from the borehole by several concentric layers of steel pipes cemented in place, called casings. At the surface, “frac fluid” is prepared by mixing chemicals, including biocides and corrosion inhibitors, with water and sand. The frac fluid is then pumped down the well at high enough pressures to fracture the shale. Finally, a portion of the frac fluid returns to the surface and is stored at the drilling site until it is transported for treatment or disposal. Freshwater contamination could occur during any of these steps from accidents or an operator’s failure to follow industry best practices. However, there is also little empirical evidence available to discern whether a well that is fractured without accident and following best practices is capable of contaminating drinking water.

Duke University researchers published a study on drinking water, “Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing,” in May 2011.[iv] This study analyzed water samples from homes with private drinking water wells in varying proximity to active shale gas wells. Their analysis found that the closer a home was to a shale gas well, the higher the average and maximum methane concentrations in drinking water wells. Importantly, this study pointed out that methane is often naturally present in drinking water, and is not hazardous at low concentrations. However, some of the concentrations found in wells close to the active shale gas sites were well above the action level for hazard mitigation established by the Department of the Interior. Through isotopic fingerprinting the study also found that methane in wells closer to active drilling sites was from a deeper methane source than methane in wells at least one kilometer away from an active shale gas site. This suggests that methane contamination in wells close to active shale gas wells is coming from depths close to those being mined by the gas companies.

Formal comments submitted in response to the Duke article criticized the study on several grounds.[v],[vi] Some found that the sample size of drinking water wells was too small, and that pre-drill, or baseline, data are needed to compare with the data collected by Duke researchers to truly prove the impact of shale gas extraction on drinking water. Other critiques, including those from the gas industry,[1],[vii] opined that the study did not adequately establish that the source of methane was the Marcellus Shale formation, where fracturing was focused, or that the mechanism by which the methane reached the wells was related to gas extraction processes.

In contrast to the field of climate change science, consensus among scientists in the literature has yet to emerge concerning the environmental impacts of shale gas extraction. As researchers continue to pursue investigations of impacts, the question of whether to adhere to the “precautionary principle” or the “proactionary principle” in management decisions about hydraulic fracturing looms. Advocates of the precautionary principle would argue that in the absence of scientific consensus about the potential harm from hydraulic fracturing, the onus to prove that hydraulic fracturing is safe lies with the gas industry. Further, they would find that in the absence of evidence that hydraulic fracturing is safe, precautions such as halting ongoing mining activities should be taken to protect the environment. Across the aisle, proponents of the proactionary principle would say that precautionary actions are costly and unnecessary if there is no proof there is harm to mitigate. Though clear answers to scientific and management questions remain elusive, it is clear that academic efforts to determine the true environmental costs of shale gas extraction should be supported.

[1] In the interest of providing a complete picture of the reaction to this Duke University paper, the shale gas industry response was included despite the fact that this cited industry paper is not peer-reviewed.

[i] Robert W. Howarth, Renee Santoro, and Anthony Infraffea. “Methane and the greenhouse-gas footprint of natural gas from shale formations,” Climatic Change 106, no.4 (2011): 679-690.
[ii] Mohan Jiang, W. Michael Griffin, Chris Hendrickson, Paulina Jaramillo, Jeanne VanBriesen, and Aranya Venkatesh. “Life cycle greenhouse gas emissions of Marcellus shale gas,” Environmental Research Letters 6, no. 3 (2011).
[iii] Lawrence M. Cathles III, Larry Brown, Milton Taam, and Andrew Hunter. “A commentary on ‘The greenhouse-gas footprint of natural gas in shale formations’ by R.W. Howarth, R. Santoro, and Anthony Ingraffea,” Climatic  Change 113, no. 2 (2012): 525-535.
[iv] Stephen G. Osborn, Avner Vengosh, Nathaniel R. Warner, and Robert B. Jackson. “Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing,” Proceedings of the National Academy of Sciences 108, no. 20 (2011): 8172-8176.
[v] Samuel C. Schon. “Hydraulic fracturing not responsible for methane migration,” Proceedings of the National Academy of Sciences 108, no. 37 (2011): E664.
[vi] Tarek Saba, and Mark Orzechowski. “Lack of data to support a relationship between methane contamination of drinking water wells and hydraulic fracturing,” Proceedings of the National Academy of Sciences 108, no. 37 (2011): E663.
[vii] Industry article (not peer-reviewed): Lisa J. Molofsky, John A. Connor, Shahla K. Farhat, Albert S. Wylie Jr., and Tom Wagner. “Methane in Pennsylvania water wells unrelated to Marcellus shale fracturing,” Oil and Gas Journal (December 5, 2011), Copyright of PennWell Corporation.