When energy companies extract shale gas, they emit only a fraction more methane into the atmosphere than companies doing conventional gas drilling, according to a new study.
That fraction -- about 216 gigagrams of methane in 2010 -- was due to hydraulic fracturing, a technique in which drillers inject pressurized water, sand and chemicals to fracture shale rock and release trapped gas. Fracking accounted for 3.6 percent of the 6,002 gigagrams of methane emitted overall by natural gas operations in 2010.
The implication is that shale gas drilling operations leak most of their methane from much of the same points as conventional gas drilling operations: pipelines, compressor stations, valves and other point sources. These account for about 96.4 percent of the emissions from a gas production site, the study finds.
The study, by researchers at the Massachusetts Institute of Technology, was published this week in Environmental Research Letters. The work did not receive any industry funding, although the research institute, MIT Energy Initiative, collaborates extensively with industry.
"The majority of GHG [greenhouse gas] emissions are not related to the 'unconventional' nature of shale gas; rather it is the entire natural gas system, including compressor stations, gathering pipelines, transportation and distribution systems that should be better assessed for potential emissions reductions," wrote Sergey Paltsev, co-author of the study and assistant director for economic research at the MIT Joint Program on the Science and Policy of Global Change, in an email.
The researchers studied methane emissions from the time a well is first fracked to the ninth day of its life -- a process industry refers to as "completion." During this time, some injected water and brine returns to the surface, together with methane. Operators choose to either capture this gas and route it to a pipeline, burn it by flaring or vent it into the atmosphere where it can wreak climate havoc.
The scientists used production data that companies file with the states to calculate methane leakage during fracking and completion. They looked at the Barnett, Fayetteville, Haynesville, Marcellus and Woodford shales, and assumed that the wells would progressively produce more gas until peak production on day nine.
At this point, the data get a bit fuzzy. Depending on who is asked, companies either almost completely capture or flare their methane during completions, or almost completely vent the gas to the atmosphere. U.S. EPA assumes that half the gas is flared and half is vented.
In the MIT study, the authors assume that 70 percent is captured, 15 percent is flared and 15 percent is vented. They term this "current field practice" and say it is based on "extensive discussions with industry, EPA and other relevant groups regarding actual field practice." They are not more transparent on those discussions in their supplementary information.
Using these percentages, they calculate that the shale gas industry emitted 216 gigagrams of methane from fracking and completion in 2010. In comparison, the natural gas industry as a whole emitted, from well to your gas stove, some 10,259 gigagrams that year.
Robert Howarth, professor at Cornell University and author of a controversial study last year on methane leakage from gas operations, challenged the "current field practice" breakdown. He said he would rather wait for results of a study on methane emissions from gas sites being conducted by the Environmental Defense Fund to make up his mind (EnergyWire, Oct. 15). Howarth had assumed in his study than most of the gas produced during fracking and completion is vented to the atmosphere.
The new study is different in its broader goals than other research, including Howarth's study, which looked at shale gas wells throughout their entire 30-year lifetime (Greenwire, April 11, 2011). It is also different from a study out of the National Oceanic and Atmospheric Administration in which scientists measured the methane emitted by oil and gas fields overall, without looking at fracking or well completions specifically (Greenwire, Feb. 14).
Since 2010, EPA has released new rules requiring that operators either flare all their gas or capture it through a process called "green completions." The MIT study suggests the rule does not entirely address the issue of methane leakage from natural gas operations in the United States.
"Overall, we have found that fugitive methane emissions associated with shale gas-related hydraulic fracturing are not insignificant, and there should be a continued focus on minimizing them," wrote Francis O'Sullivan, co-author of the paper and executive director of the Energy Sustainability Challenge Program at the MIT Energy Initiative, in an email. "However, we also point out that it is incorrect to suggest that shale gas-related hydraulic fracturing has substantially altered the overall level of fugitive emissions from the natural gas system."