Life cycle greenhouse gas (GHG) emissions from Marcellus Shale natural gas are 20-50% lower than coal for electric generation, according to a new Carnegie Mellon University (CMU) study.
The findings challenge a recent Cornell University study that claimed shale gas produced not only more emissions than conventional gas, but also more than coal or oil (see Shale Daily, April 21; April 13).
The CMU study estimated that Marcellus gas produces 11% more GHG emissions than conventional gas before combustion, and only 3% more including combustion. It also concluded that Marcellus emissions are comparable to those from imported liquefied natural gas.
The study looked at carbon dioxide (CO2), methane and nitrous oxide emissions. It considered emissions from well site construction, drilling and hydraulic fracturing (hydrofracking), as well as midstream and downstream activities such as processing, transmission and combustion.
The authors submitted the report for publication in January and included citations from 2011.
The researchers estimate that a typical Marcellus well averages 68 grams of CO2e (CO2 equivalent) per megajoule (MJ) of natural gas produced over its life cycle. The researchers said that figure would need to be around 125 grams of CO2e/MJ before Marcellus gas produced more GHG emissions than coal.
While that suggests that natural gas generally produces lower emissions than coal, the researchers found that coal combustion techniques that incorporated carbon capture and storage technology produced slightly less emissions than natural gas. "This implies that the upstream emissions for natural gas life cycle are higher than the upstream emissions from coal, once efficiencies of power generation are taken into account," the researchers concluded.
The study estimates that with a hydraulic fracturing event (and its associated flaring and venting emissions) contributing 1.5 g CO2e/MJ to the shale gas emissions estimate, "more than 25 fracturing events would need to occur in a single well" before it would equal coal emissions.
The researchers said their estimates of life-cycle emissions "are subject to considerable uncertainty, particularly for the production rates and well lifetime."
The study compares estimates of Marcellus emissions against the national average for natural gas emissions in 2008, before significant shale development began in Pennsylvania. The researchers used existing industry, academic and government data to compile the study.
The Cornell conclusion depended in part on methane leaks during development. Methane, the main ingredient in natural gas, is a more potent GHG than CO2.
The CMU study found that flaring and venting are the largest sources of GHG emissions in the "preproduction" phase for development, but are "not substantial contributors to the life cycle estimates, which are dominated by the combustion emissions of the gas." The CMU study did not consider "external leaks" outside the well, such as those from "poorly installed well casing."
The study only considered the Marcellus and the researchers said other shale plays could generate different results. They also noted that "green completion" techniques and capturing the gas that is currently vented or flared could reduce emissions in the early stages of development.
The study was published the journal Environmental Research Letters and authored by CMU civil and environmental engineering professors Mohan Jiang, W. Michael Griffin, Chris Hendrickson, Paulina Jaramillo, Jeanne VanBriesen and Aranya Venkatesh. It included input from the CMU Tepper School of Business and was funded in part by the Sierra Club.