Where EPA reported 712.6 thousand metric tons of methane emissions from gas well workovers (including hydraulic re-fracturing) in its 2010 national inventory of GHGs, the study estimated emissions of 197.3 thousand metric tons.
Prepared by URS Corp. and The LEVON Group, the study was sponsored by the American Petroleum Institute and America’s Natural Gas Alliance (collectively, API/ANGA) in response to EPA’s revised calculation methodology for GHG emissions from natural gas systems.
The revised methodology was first adopted in 2011 for preparation of the 2009 national inventory and resulted in a 204% increase to estimated GHG emissions for the natural gas production sector.
The API/ANGA study analyzed data for nearly 91,000 natural gas wells and is being touted as the most comprehensive data set compiled for natural gas operations.
The data did not include measured emissions and, therefore, the study does not challenge the EPA’s assumptions regarding emission rates for various emission activities.
However, the API/ANGA data do indicate that the frequency and duration of these activities are much lower than assumed by EPA.
For hydraulic re-fracturing events, EPA had assumed a re-fracture rate of 10%, meaning 10% of fracture-completed gas wells are re-fractured every year. In contrast, the API/ANGA study found an average well re-fracture rate of 1.6% to 2.3%.
The study scope included emission activities other than those associated with the re-fracturing of natural gas wells. In particular, API/ANGA found that EPA appears to be overestimating the frequency and duration of liquid unloading events resulting in emissions to the atmosphere. EPA’s 2010 national inventory estimated 4.5 million metric tons of methane from unloading events.
In comparison, the API/ANGA study found only 0.64 million metric tons of emissions. Coupled with the hydraulic fracturing results discussed above, the study estimated total production sector emissions of 4.4 million metric tons of methane versus EPA’s estimate of 8.8 million metric tons.
The API/ANGA study was not prepared as a challenge to the EPA’s recently-issued New Source Performance Standards (Subpart OOOO) and National Emission Standards for Hazardous Air Pollutants (Subparts HH and HHH) for the oil and gas sector.
Therefore, the study does not discuss the VOC or HAP emission assumptions underlying those rules. Nevertheless, the API/ANGA study highlights the limited information regarding upstream emissions upon which EPA has historically relied and generally supports the positions taken by parties challenging the rules.
This article was prepared by Bob Greenslade (bgreenslade@fulbright.com / 303 801 2747) from Fulbright’s Environmental Law Practice.
