Argonne study finds shale gas GHG lifecycle emissions 6% lower than natural gas, 23% lower than gasoline and 33% lower than coal; upstream methane leakage a key contributor
The base results from a study by a team at the Center for Transportation Research, Argonne National Laboratory indicate that shale gas life-cycle greenhouse gas (GHG) emissions are 6% lower than conventional natural gas, 23% lower than gasoline, and 33% lower than coal. However, the range in values for shale and conventional gas overlap, so there is a statistical uncertainty whether shale gas emissions are actually lower than those of conventional natural gas.
The study also highlights that upstream CH4 (methane) leakage and venting is a key contributor to the total upstream emissions of natural gas pathways, and can significantly reduce the life-cycle benefit of natural gas compared to coal or petroleum. The study found that shale gas (SG) well completion and workover emissions are a much more significant factor as compared to a conventional natural gas (NG) pathway. The study is published in the ACS journal Environmental Science & Technology.
In the United States, there has been a rapid increase in natural gas (NG) production from shale formations due to recent advancements in drilling technologies, such as horizontal drilling and hydraulic fracturing. In horizontal drilling, a well is drilled down to the depth of the play and turned approximately 90 degrees to run laterally through the formation. This allows for greater access to the play and can increase production on a per-well basis. Due to the low permeability of shale, producers hydraulically fracture the formation to enable better flow of NG. The fracture fluid is typically water-based and contains proppants to maintain fracture openings once pumping of the fluid has ceased. The development of this resource has generated interest in expanding NG usage in areas such as electricity generation and transportation. However, the environmental impacts (e.g., water quality, air quality, global climate change) of shale gas (SG) production and use are currently being debated as the impacts of these new technologies have just started to be examined.
...In this analysis we examined the current state of knowledge regarding the key CH4 emission sources from shale gas, conventional NG, coal, and petroleum to estimate up-to-date GHG emissions and to understand the uncertainties involved in calculating their life-cycle GHG impacts. We used the GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) model, which can analyze more than 100 fuel pathways, to perform our simulations. We updated the latest version, GREET 1.8d, to include shale gas production and have revised the existing pathways for NG, coal, and petroleum. Through this effort we have also identified data gaps that need to be addressed in future GHG assessments of the natural gas life cycle.—Burnham et al.
The Argonne-developed GREET is widely used to examine life-cycle energy and emission effects of different transportation fuels and advanced vehicle technologies. The Argonne team here used GREET to estimate the GHG emissions from feedstock recovery, fuel production, and fuel use as well as from transportation and distribution of feedstocks and fuels.
In this study, they expanded the system boundary to include infrastructure establishment, including gas well drilling and completion. The GREET model calculates CO2, CH4, and N2O emissions for each stage from both fuel combustion and non-combustion sources such as leaks.
|Life-cycle GHG emissions per vehicle kilometer traveled for passenger cars and transit buss for both 100-year and 20-year time horizons. Credit: ACS, Burnham et al. Click to enlarge.|
Based on the findings, the team emphasized the volume of gas vented during completions and workovers of shale gas wells needs to be examined with and without technologies and practices that can reduce emissions. This, in turn will require a better understanding of the volumes of both fracking fluids and natural gas being released during the flowback and how those volumes vary during the process, they said.
They also suggested that:
The number of workovers typically performed during the lifetime of shale gas wells needs further examination as the decision to do a workover will be based on the economics of the well, likely depending on factors such as the age of the well, expected improvement in production after workover, and the wellhead price of NG.
Greater transparency is needed on the percentage of completions and workovers implementing REC (reduced emissions completion) technologies. A survey of flaring practices for wells with and without RECs by examining state regulations and industry practices would provide greater certainty of the emissions from shale gas.
For conventional wells, the volume of gas vented during liquid unloadings (removing liquids that gradually build up and block flow in wet gas wells) needs to be calculated for the various technologies implemented to removing liquids, along with a survey of the prevalence of each technology in practice would provide much greater certainty to these emissions. This survey should also examine the percentage of conventional NG and shale wells requiring liquid unloadings as not all wells undergo this process.
The number of unloadings required over the lifetime of a well is a factor that causes significant uncertainty and should be examined in detail. This data should differentiate the unloadings required in different basins/geologic formations as well as in different wells within the same basin. The number of unloadings required as function of the age of the well would also provide relevant information when trying to create an inventory of these emissions.
Flaring practices should also be examined for liquid unloading operations by examining state regulations and industry practices.
Large-scale shale gas production is a relatively new phenomenon. Environmental management in general and GHG emission reduction in particular need to be exercised in order for shale gas (and conventional NG) to be produced sustainably. The partnership of the natural gas industry and EPA under the NG STAR program has helped reduce CH4 emissions but further efforts could be taken to address remaining environmental issues of natural gas production and transmission. With this context, our analysis, among other analyses, provides some insight on critical stages that industry and government agencies could work together on to reduce the environmental footprint of natural gas.—Burnham et al.
Andrew Burnham, Jeongwoo Han, Corrie E. Clark, Michael Wang, Jennifer B. Dunn, and Ignasi Palou-Rivera (2011) Life-Cycle Greenhouse Gas Emissions of Shale Gas, Natural Gas, Coal, and Petroleum. Environmental Science & Technology doi: 10.1021/es201942m