Annual emissions of methane have risen by almost 10% in the past two decades. Atmospheric concentrations of methane reached 1,875 parts per billion in 2019 which is more than 2.5 times above pre-industrial levels. The primary anthropogenic sources of methane are the fossil fuel industry and agriculture, especially livestock production (enteric fermentation). Over the past decades there has been a shift away from coal to natural gas for power generation driven in part by the perception that carbon emissions from natural gas are less than those from coal for the same amount of energy. However, it has been argued that if methane emissions from the oil and gas supply chain were factored into the calculation, the carbon advantage of natural gas would be reduced or even erased. A recent study of nine natural gas basins in the U.S. has focused on estimating methane emissions from the U.S. oil and natural gas supply chain.
For several decades natural gas has been perceived as an economical way to reduce green house gas emissions from electric power generation. It is generally asserted that natural gas power plants, which are responsible for nearly a third of power generation in the US, produce 50% less emissions than coal-fired plants for the same amount of power. However, the natural gas that is emitted by the supply chain, which includes all operations associated with natural gas production, processing, and transport, was not included in the GHG footprint calculation for natural gas. Nor were emissions from local distribution and end use of natural gas.
It is important to include all emissions of natural gas in the GHG footprint calculation because the GHG advantage of natural gas has been used as a justification for the large scale shift from coal to natural gas over the past decades. Robert W. Howarth and others have argued that if emissions of methane from shale gas wells, which represent about 75% of U.S. natural gas production, were included it would be found that natural gas from shale formations produces more GHG warming than coal per unit of power generation over 25 years. In other words when emissions are included in the GHG footprint of natural gas, the carbon emissions advantage over coal is significantly reduced and the GHG benefits of the switch to natural gas overestimated.
New assessment of emissions from the oil and gas supply chain
In 2012 the Environmental Defense Fund (EDF) initiated a series of 16 independent projects to examine the areas that make up the oil and gas supply chain: production; gathering lines and processing facilities; long-distance pipelines, storage and local distribution; as well as some consumers using natural gas, commercial trucks and refueling stations. Based on this work a new study synthesizes existing data to provide an improved overall assessment of emissions from the oil and natural gas supply chain . This assessment does not address emissions from local distribution and end use of natural gas.
Measurements of methane emissions are either top-down or bottom-up. The top-down approach uses aircraft and satellites to estimate aggregate emissions from all contributing sources across large geographies. Bottom-up studies attempt to estimate emissions from emissions from individual pieces of equipment, operations, or facilities that are expected to represent the majority of emissions from the natural gas supply chain. These measurements are generally conducted on-site or downwind using handheld sensors.
For the recent study nine natural gas production areas were measured using overflights. The nine basins are geographically distributed across the U.S. and represent about 33% of natural gas, 24% of oil production, and 14% of all wells. For this analysis the results of recent facility-scale measurements have been used to estimate methane emissions from the U.S. oil and natural gas supply chain. These were then compared to top-down measurements recorded by overflights.
The bottom-up methodology used was different from the conventional component-by-component approach in that it relied on measurements of facility-level emissions which captured emissions from abnormal as well as normal operations. The national bottom-up estimate of total methane emissions in 2015 from the U.S. oil and natural gas supply chain is 13 megatonnes of methane per year. When the bottom-up estimates were compared with the top-down data for the nine basins for which aerial measurements had been reported, agreement was found between the two approaches to within estimated uncertainty ranges.
13 megatonnes of methane per year is 2.3% of U.S. annual gross natural gas production of 33 trillion cubic feet or 934 billion cubic metres.
Comparison with EPA estimate
The measurements using bottom-up and top-down methodologies for 2015 are significantly higher than the corresponding estimate in the U.S. Environmental Protection Agency’s Greenhouse Gas Inventory. The bottom-up estimate is 63% higher than the EPA estimate.
The study argues that the source of the discrepancy is the sampling methods used in conventional inventories of methane emissions. It is suggested that the conventional approach systematically underestimate total emissions because high emissions caused by malfunctioning equipment and operations are not included. The difference between the EPA estimates and the estimates derived from facility-level measurements is that the EPA approach does not adequately sample abnormal operating conditions.
There is a growing consensus that a small number of facilities are responsible for the majority of methane emissions. A 2016 study found that just a few natural gas wells account for more than half of the total volume of leaked methane gas in the United States. An extensive aerial infrared camera survey of 8000 production sites in seven U.S. oil and gas basins found that 4% of surveyed sites had one or more observable methane high-rate emission plumes. Furthermore the large variability in methane emissions for similar equipment and facilities in different geographies supports the hypothesis that equipment malfunction is a major source of emissions.
This study, using both top-down and bottom-up methodologies, has concluded that methane emissions from the U.S. oil and natural gas supply chain in 2015 amounted to about 2.3% of gross U.S. gas production. Emissions of this magnitude increase the GHG footprint of natural gas power generation and erode the carbon advantage of natural gas over coal.
This estimate is about 60% higher than the EPA Gas Inventory estimate. The discrepancy is ascribed to the EPA’s approach to inventorying methane emissions which does not adequately sample emissions from malfunctioning equipment and operations. Independent evidence indicates that abnormal operations are a major contributor to methane emissions.
Methane and the greenhouse-gas footprint of natural gas from shale formations, Robert W. Howarth, Renee Santoro and Anthony Ingraffea, Climate Change, 2011, Climatic Change DOI 10.1007/s10584-011-0061-5
Aerial surveys of elevated hydrocarbon emissions from oil and gas production sites, D. R. Lyon, R. A. Alvarez, D. Zavala-Araiza, A. R. Brandt, R. B. Jackson, S. P. Hamburg, Environ. Sci. Technol. 50, 4877–4886 (2016). doi:10.1021/acs.est.6b00705pmid:27045743
Chart of atmospheric concentration of methane 1980 to 2020 Nature 23 July 2020