Correction to Measurements of Methane Emissions from Natural Gas Gathering Facilities and Processing Plants: Measurement Results

Facility-level methane emissions were measured at 114 gathering facilities and 16 processing plants in the United States natural gas system. At gathering facilities, the measured methane emission rates ranged from 0.7 to 700 kg per hour (kg/h) (0.6 to 600 standard cubic feet per minute (scfm)). Normalized emissions (as a % of total methane throughput) were less than 1% for 85 gathering facilities and 19 had normalized emissions less than 0.1%. The range of methane emissions rates for processing plants was 3 to 600 kg/ h (3 to 524 scfm), corresponding to normalized methane emissions rates <1% in all cases. The distributions of methane emissions, particularly for gathering facilities, are skewed. For example, 30% of gathering facilities contribute 80% of the total emissions. Normalized emissions rates are negatively correlated with facility throughput. The variation in methane emissions also appears driven by differences between inlet and outlet pressure, as well as venting and leaking equipment. Substantial venting from liquids storage tanks was observed at 20% of gathering facilities. Emissions rates at these facilities were, on average, around four times the rates observed at similar facilities without substantial venting. ■ INTRODUCTION Methane is the primary component of natural gas; it is also a potent greenhouse gas (GHG). The Environmental Protection Agency (EPA) estimates that the natural gas system contributes 23% of U.S. anthropogenic methane emissions. However, there are discrepancies between recent studies and EPA GHG inventories in some natural gas producing areas. The EPA GHG inventories largely rely on data collected in the early 1990s and may not reflect recent changes in technology, operations, and regulation. New measurements are needed to characterize methane emissions from the natural gas system. This study investigates the methane emissions from natural gas gathering and processing (G&P) facilities, which, collectively, gather natural gas from production wells, remove impurities, and deliver it to interand intrastate pipeline networks. We define gathering and processing as the equipment and pipeline network between the sales points at well pads and downstream delivery points. This includes gathering pipelines, and the equipment at gathering facilities: compressors (driven by electric motors (“motors”) and/or natural gas-fired internal combustion engines (“engines”) or turbines), dehydration systems to remove water, and treatment systems to remove hydrogen sulfide and/or carbon dioxide. Processing plants often house this equipment on a larger scale, acting as central nodes in a system of smaller gathering facilities. Processing plants also separate natural gas liquids (NGLs) (such as ethane, propane, butane, and heavier hydrocarbons) from methane. For this study, processing plants are defined as the facilities that meet the 40 CFR Part 60 Subpart KKK definition of “natural gas processing plant” based on the presence of NGL extraction. Facilities that only fractionate NGLs were not included in this study. This paper presents facility-level measurements of methane emission rates at 130 G&P facilities (114 gathering and 16 processing). A mobile laboratory was used to perform downwind tracer flux measurements, which is an established technique to estimate the total emissions of methane (or other Received: October 28, 2014 Revised: January 23, 2015 Accepted: January 28, 2015 Published: February 10, 2015 Article