A group of researchers from Carnegie Mellon have released a study that estimates the "life cycle" greenhouse gas emissions for Marcellus shale gas.  The "life cycle estimates" are estimates of the total amount of greenhouse gas emissions for all activities associated with the production, treatment, transport, and ultimate use of shale gas for electricity production.  The researchers compared their life cycle estimates for Marcellus shale gas to similar estimates for coal and for natural gas produced by conventional means.  In contrast to the conclusions reached by a Cornell study, the Carnegie Mellon researchers concluded that Marcellus shale gas has life cycle greenhouse gas emissions that generally are significantly lower than the life cycle emissions for coal, and that are only slightly higher than those for natural gas produced from conventional wells that are not hydraulically fractured.

The Carnegie Mellon researchers estimated emissions for three greenhouse gases -- carbon dioxide, methane, and nitrous oxide -- and converted those emissions to "carbon dioxide equivalents" using the 100-year global warming potential (GWP) factors reported by the Intergovernmental Panel on Climate Change (IPCC).  The conversion is made because each type of greenhouse gas has a different amount of global warming potential.  For example, a molecule a methane is estimated to have 25 times more global warming effect than a molecule of carbon dioxide.  Thus, a molecule of carbon dioxide would count for one carbon dioxide equivalent, while a molecule of methane would count for 25 carbon dioxide equivalents. 

The "100-year" reference refers to the fact that the global warming effect is based on the global warming effect that emissions will have after 100 years have passed.  A specific time horizon must be chosen because methane will breakdown in the atmosphere over time, thereby decreasing its greenhouse gas effect over time.  Thus, the immediate greenhouse gas potential is different than that which will remain after 20 years, which is different than that which will remain after 100 years.  The 100-year greenhouse gas potential is used to obtain long range estimates of the greenhouse gas effect of emissions.  

The Carnegie Mellon researchers attempted to be comprehensive in the activities they chose to include in their life cycle analysis.  They included estimated emissions for various parts of the pre-drilling and pre-production process, including emissions from the operation of equipment used in constructing the well pad, equipment used in the drilling process, motors used in pumping fracturing fluid into the well for the fracking process itself, and trucks used is delivering water to the drill site for fracturing, as well as emissions associated with producing drilling mud, emissions from the process of venting or flaring during flowback and well completion, fugitive emissions that occur during treatment and transport of shale gas, and emissions from combustion when the gas ultimately is used to generate electricity in a power plant. 

The Carnegie Mellon researchers concluded that the life cycle greenhouse gas emissions for Marcellus shale gas emissions are about 3% higher than for natural gas produced from conventional wells, and are about 3% lower than liquefied natural gas imported to the U.S.  They estimated that the life cycle emissions for the use of Marcellus shale gas in power generation are much lower than for the use of domestic coal is most scenarios.  The one exception is a scenario in which one assumes that a power plant uses advanced carbon capture and sequestration (CCS) technology.  If CCS is used for both a natural gas-fired power plant and a coal-fired power plant, the estimated life cycle emissions for coal are slightly lower than for Marcellus shale gas.

The Carnegie Mellon researchers did not assume that so-called "green" completions or "reduced emissions" completions would be used during flowback and completion of a Marcellus well.  Such techniques would reduce emissions.  A couple of western states now require green completions, and some companies voluntarily are using green completions.  Further, the EPA has proposed regulations that would require green completions starting in March 2012.  If the Carnegie Mellon researchers had assumed that green completions are used, their estimates of life cycle greenhouse gas emissions for Marcellus shale gas would be closer to the life cycle estimates for natural gas produced from conventional wells, and would compare even more favorably relative to coal than when it is assumed that green completions are not used.

The Carnegie Mellon researchers' results contrast with those of a study by Cornell researchers, who concluded that shale gas has higher life cycle greenhouse gas emissions than those for the use of coal, whether one looks at a 20-year time horizon or a 100-year time horizon.

The Carnegie Mellon study was funded in part by the Sierra Club.