Study Published in Proceedings of National Academy of Sciences
July 21, 2014 — A new study published in the Proceedings of the National Academy of Sciences says estimates of life cycle greenhouse gas (GHG) emissions from electricity generated from shale gas are similar to those from conventionally-produced natural gas, and both energy sources, on average, emit approximately half the GHG emissions of coal-powered electricity. However, under certain circumstances, the emissions for conventional and shale gas electricity can reach levels approaching best-performing coal-fired plants.
"Published studies and articles about greenhouse gas emissions have come to widely differing conclusions; harmonization helps to clarify the existing knowledge on this important topic," National Renewable Energy Laboratory Senior Scientist and lead author Garvin Heath said. "With a more 'apples-to-apples' comparison provided by harmonization, a clear tendency emerges in the published literature. We see that life cycle GHG emissions from electricity generated from shale gas are similar to those from conventionally-produced natural gas. Both energy sources, on average, emit approximately half the GHG emissions of coal-powered electricity when considering emissions from the smoke stack and those upstream through the supply chain. Further, through the development of novel probability distribution functions, we find assumptions about lifetime production of wells and the practice of liquids unloading to have the greatest influence on the results."
Study authors—Heath and Patrick O'Donoughue of NREL and Douglas Arent and Morgan Bazilian of the Joint Institute for Strategic Energy Analysis—emphasized the importance of actual measurements of GHG emissions to advance understanding of energy options.
"Harmonization of prior estimates of GHG emissions provides greater precision, but does not fully address the questions of accuracy of our knowledge of GHG emission sources in the natural gas supply chain, questions we highlighted in our recent article
in Science magazine synthesizing 20-plus years of research on methane leakage from natural gas systems," Arent said. "As called for in that article and this one, verified measurements of emissions from components and activities throughout the natural gas supply chain, and robust analysis of lifetime well production and the prevalence of practices to reduce emissions, can help create a more robust understanding of our energy options."
The study team screened hundreds of published life cycle assessments for quality and modern relevance, including dozens related to conventionally produced natural gas and shale gas. In addition, the team conducted sensitivity analysis on three important activities in the production of shale gas—well completion, well recompletion, and liquids unloading. Previous research had found these activities to be significant to life cycle GHG emissions. When considering these additional factors, shale gas life cycle GHG emissions could approach the range of best-performing coal-fired generation.
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