In its recently released Inventory of US Greenhouse Gas Emissions and Sinks: 1990 – 2011 (earlier post), the US Environmental Protection Agency (EPA) reported that methane (CH4) emissions from the field production of natural gas have declined by 36% from 2007 to 2011 (from 83.1 to 53.4 Tg CO2 eq), after having increased by 43% from 1990 through 2006. Reasons the agency sited for this trend include factors such as increased voluntary reductions, as well as the effects of the recent global economic slowdown.
The finding may have an impact on future regulation of fracking for natural gas production, as opponents of the technology have pointed to higher rates of methane leak as being an argument against it.
Methane is more than 20 times as effective as CO2 at trapping heat in the atmosphere—i.e., a global warming potential of 21. (The figure includes the direct effects and those indirect effects due to the production of tropospheric ozone and stratospheric water vapor.) Anthropogenic sources of CH4 include natural gas and petroleum systems, agricultural activities, landfills, coal mining, wastewater treatment, stationary and mobile combustion, and certain industrial processes.
During the period from 2007 to 2011, gross natural gas production in the US increased from 24,663,656 to 28,479,026 million cubic feet, an increase of 15.5%, according to the US Energy Information Administration (EIA). Within that, from 2007 to 2011, natural gas production from fracking grew 327% from 1,990,145 million cubic feet to 8,500,983 million cubic feet.
Overall, EPA reported a slightly less than 1% drop in methane (CH4) emissions from 2010 to 2011 (from 592.7 to 587.2 Tg or million metric tons CO2 equivalent.) CH4 emissions represented 8.8% of total reported US GHG emissions (6,702.3 Tg CO2 eq) in 2011.
Natural gas systems—wells, processing facilities, transmission and distribution pipelines—were the largest anthropogenic source category of CH4 emissions in the US in 2011 with 144.7 Tg CO2 eq (25%).
The methane and non-combustion CO2 emissions from natural gas systems are generally process related, with normal operations, routine maintenance, and system upsets being the primary contributors. Emissions from normal operations include: natural gas engines and turbine uncombusted exhaust; bleed and discharge emissions from pneumatic devices; and fugitive emissions (leak) from system components.
Routine maintenance emissions originate from pipelines, equipment, and wells during repair and maintenance activities. Pressure surge relief systems and accidents can lead to system upset emissions.
There are four major stages to natural gas systems, EPA noted: field production; processing; transmission and storage; and distribution.
Field production. Wells withdraw raw gas from underground formations. Emissions arise from the wells themselves, gathering pipelines, and well-site gas treatment facilities such as dehydrators and separators. Emissions from pneumatic devices, gas wells with liquids unloading, and gas well completions and refracturing (workovers) with and without hydraulic fracturing account for the majority of CH4 emissions, according to EPA, while flaring emissions account for the majority of the non-combustion CO2 emissions.
Emissions from field production accounted for approximately 37% of CH4 emissions and about 33% of non-combustion CO2 emissions from natural gas systems in 2011.
Processing. In this stage, natural gas liquids and various other constituents from the raw gas are removed, resulting in “pipeline quality” gas, which is injected into the transmission system. Fugitive CH4 emissions from compressors, including compressor seals, are the primary emission source from this stage.
Processing plants account for about 14 percent of CH4 emissions and approximately 66 percent of non-combustion CO2 emissions from natural gas systems.
Transmission and Storage. Natural gas transmission involves high-pressure, large-diameter pipelines that transport gas long distances from field production and processing areas to distribution systems or large volume customers such as power plants or chemical plants. Compressor station facilities move the gas throughout the United States transmission system. Fugitive CH4 emissions from these compressor stations and from metering and regulating stations account for the majority of the emissions from this stage.
CH4 emissions from the transmission and storage sector account for approximately 30% of emissions from natural gas systems, while CO2 emissions from transmission and storage account for less than 1% of the non-combustion CO2 emissions from natural gas systems.
CH4 emissions from this source decreased by 11% from 1990-2011 due to increased voluntary reductions (e.g., replacement of high bleed pneumatics with low bleed pneumatics, replacement of wet seals with dry seals).
Distribution. Distribution pipelines take the high-pressure gas from the transmission system at “city gate” stations, reduce the pressure and distribute the gas through primarily underground mains and service lines to individual end users. There were more than 1,231,000 miles of distribution mains in 2011, an increase of approximately 287,000 miles since 1990.
Distribution system emissions, which account for approximately 19% of CH4 emissions from natural gas systems and less than 1% of non-combustion CO2 emissions, result mainly from fugitive emissions from gate stations and pipelines. An increased use of plastic piping, which has lower emissions than other pipe materials, has reduced emissions from this stage.
Some other significant trends in US emissions of CH4 reported by EPA include:
Emissions from enteric fermentation—the second largest anthropogenic source of CH4 emissions in the United States—increased by 4.6 Tg CO2 eq. (3.5%) in 2011 since 1990. This increase in emissions from 1990 to 2011 in enteric generally follows the increasing trends in cattle populations.
Landfills—the third largest anthropogenic source of CH4 emissions in the United States—accounted for 17.5% of total CH4 emissions (103.0 Tg CO2 eq.) in 2011. From 1990 to 2011, CH4 emissions from landfills decreased by 44.7 Tg CO2 Eq. (30.3%), with small increases occurring in some interim years.
In 2011, CH4 emissions from coal mining were 63.2 Tg CO2 Eq., a 9.2 Tg CO2 eq. (12.6%) decrease under 2010 emission levels. The overall decline results from the mining of less gassy coal from underground mines and the increased use of CH4 collected from degasification systems.
Methane emissions from manure management increased by 65.3% since 1990, from 31.5 Tg CO2 Eq. in 1990 to 52.0 Tg CO2 Eq. in 2011. The majority of this increase was from swine and dairy cow manure, since the general trend in manure management is one of increasing use of liquid systems, which tends to produce greater CH4 emissions.
Inventory of US Greenhouse Gas Emissions and Sinks: 1990 – 2011 (EPA 430-R-13-001)