Abstract
Abstract. Natural gas infrastructure releases methane (CH4), a potent greenhouse gas, into the atmosphere. The estimated emission rate associated with the production and transportation of natural gas is uncertain, hindering our understanding of its greenhouse footprint. This study presents a new application of inverse methodology for estimating regional emission rates from natural gas production and gathering facilities in north-eastern Pennsylvania. An inventory of CH4 emissions was compiled for major sources in Pennsylvania. This inventory served as input emission data for the Weather Research and Forecasting model with chemistry enabled (WRF-Chem), and atmospheric CH4 mole fraction fields were generated at 3 km resolution. Simulated atmospheric CH4 enhancements from WRF-Chem were compared to observations obtained from a 3-week flight campaign in May 2015. Modelled enhancements from sources not associated with upstream natural gas processes were assumed constant and known and therefore removed from the optimization procedure, creating a set of observed enhancements from natural gas only. Simulated emission rates from unconventional production were then adjusted to minimize the mismatch between aircraft observations and model-simulated mole fractions for 10 flights. To evaluate the method, an aircraft mass balance calculation was performed for four flights where conditions permitted its use. Using the model optimization approach, the weighted mean emission rate from unconventional natural gas production and gathering facilities in north-eastern Pennsylvania approach is found to be 0.36 % of total gas production, with a 2σ confidence interval between 0.27 and 0.45 % of production. Similarly, the mean emission estimates using the aircraft mass balance approach are calculated to be 0.40 % of regional natural gas production, with a 2σ confidence interval between 0.08 and 0.72 % of production. These emission rates as a percent of production are lower than rates found in any other basin using a top-down methodology, and may be indicative of some characteristics of the basin that make sources from the north-eastern Marcellus region unique.
Highlights
The advent of hydraulic fracturing and horizontal drilling technology has opened up the potential to access vast reservoirs of previously inaccessible natural gas, shifting energy trends in the United States away from coal and towards natural gas (EIA, 2016b)
The Environmental Protection Agency (EPA)’s Greenhouse Gas Reporting Program data set for the year 2014 lists individual coal mines in the south-western portion of our domain as 8 of the top 10 CH4 emitting facilities across the entire United States. This large area source of CH4 can have an impact on CH4 concentrations hundreds of kilometres downwind and must be taken into account when winds are from the southwest (Fig. 7)
Using the model optimization technique presented in this study, we find a weighted mean natural gas emission rate from unconventional production and gathering facilities of 0.36 % of production with a 2σ confidence interval from 0.27 to 0.45 % of production
Summary
The advent of hydraulic fracturing and horizontal drilling technology has opened up the potential to access vast reservoirs of previously inaccessible natural gas, shifting energy trends in the United States away from coal and towards natural gas (EIA, 2016b). From a greenhouse gas (GHG) emissions perspective, natural gas has the potential to be a cleaner energy source than coal. During the process of extracting and distributing natural gas a percentage of the overall production escapes into the atmosphere through both planned releases and unintended leaks in infrastructure Though these emissions may be small from an economic perspective, their climatological impacts are not negligible (Alvarez et al, 2012; Schwietzke et al, 2014). Using the GWP20 value, it is estimated that a natural gas emission rate of greater than 3 % of total gas production would result in a natural gas power plant having a more negative impact on the climate than a coal-powered plant.
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