Leaks and other losses of gas occur throughout the fossil gas system, due to a multitude of diverse processes.
Across the country—from North Dakota to Texas, and from California to Pennsylvania—there is leakage in gas fields, where gas is extracted from rock deep underground. There’s leakage along the network of transmission pipelines that carries gas to cities. And there’s leakage in the cities, from distribution pipelines that snake under the streets and up to buildings, and from appliances inside buildings.
Recent research has filled in many gaps in our knowledge, showing that the U.S. gas system is significantly leakier than had been thought. (For more information, see the page on implications of methane leakage.)
There are seven main stages in the process of extracting gas from rock deep underground and ultimately delivering it to customers. All these steps are responsible for some methane leakage, but the leakage rates vary between the different stages, and from region to region. Adding up the leakage from all of these steps provides an estimate of the full life cycle leakage for gas.
Leakage from production areas—from well sites, and also gathering and processing steps described below—is the single largest contributor to methane leakage from the oil and gas system. But for many cities, leakage within the cities adds substantially to the life cycle emissions from use of gas, particularly in the residential and commercial sectors.
The U.S. extracts more fossil gas than any other country—more than one-fifth of all the gas extracted worldwide. There are nearly one million active oil and gas wells in the country. Some wells produce only gas, but more often they produce a mix of gas, oil, and other compounds known as natural gas liquids. The extraction process involves drilling a well, and often fracking it (using hydraulic fracturing) to open paths for the oil and gas to escape the rock. Methane leakage can occur from the well itself, or from other equipment on the well site.
Pipelines known as gathering lines run from each well site, carrying gas away, usually to gas processing facilities. Gathering pipelines can leak methane due to damage to pipelines. But most of the leakage from gathering is estimated to occur from equipment along the pipelines, such as compressors that boost the pressure of the gas to move it along the pipelines. In the Gas Index results, leakage from gathering pipelines is included within the category of production, since the pipelines are located within production areas.
Natural gas is composed mostly of methane, the simplest hydrocarbon molecule, with only one carbon atom. But most gas at the wellhead contains other hydrocarbon molecules known as natural gas liquids, such as ethane (with two carbon atoms) and propane (with three carbon atoms). Much of the gas extracted in the U.S. contains around 10-20% natural gas liquids, so it is sent to processing facilities, where most of the natural gas liquids are removed and sold separately. Gas processing facilities then output consumer-grade natural gas, which is composed nearly entirely (about 95%) of methane. Gas processing facilities also leak some methane, but their leakage is estimated to be much smaller than leakage from extraction sites and gathering pipelines. In the Gas Index results, leakage from gas processing plants is included within the category of production, since the plants are generally located within production areas.
Consumer-grade gas is sent over long distances through transmission pipelines, which form an extensive network nationwide. As with gathering lines, transmission pipelines themselves can leak some methane, but most of the leakage is from equipment that is part of the pipeline system, primarily compressors. In the U.S., about half of gas is sold directly from transmission lines, primarily to large industrial plants or power plants. The remaining gas is sold in cities, where it enters the distribution pipeline system to reach various consumers.
The gas distribution pipeline system is made up of distribution mains, which typically run under city streets, and service lines, which branch off of distribution mains, and run up to each building that receives gas. A new study using extensive measurements of methane leakage in cities shows that distribution mains leak substantial amounts of methane, about five times more than previously recognized. Distribution pipelines can leak due to damage from natural processes, such as earthquakes. They are also often damaged by people digging, such as for construction projects, and accidentally hitting a pipeline.
For consumers, gas meters are one of the more visible parts of the gas system, since they are one of the few parts of the system that is above ground. Every building with gas service has at least one gas meter, and large buildings with many customers can have many gas meters. Customers’ gas meters can leak a substantial amount of methane. A recent major study found commercial sector gas meters were leaking about six times more than EPA had estimated, and also found large differences in the leakage rate between regions of the country.
Any leakage that occurs after gas passes through the meter as “behind-the-meter” leakage, or “beyond-the-meter” leakage. Such leakage has been found from pipelines that carry gas within buildings, as well as from appliances that burn gas, such as furnaces, water heaters, and ovens. For example, when a stove or other appliance is starting up, there can be a moment when a pulse of gas leaks out before it ignites. Also, when turning off appliances, there can be leakage of unburned gas.
In addition to the seven stages above, researchers have measured methane emissions from whole cities, using flights overhead and measurements from towers located in and around the cities. They have also estimated how much of the methane emissions come from natural gas, as opposed to other sources such as landfills or other biological decomposition. These citywide methane emissions can’t be attributed to a specific stage of the gas system; the methane could be coming from leakage of distribution pipelines, customer gas meters, and/or buildings (behind-the-meter leakage).
If these citywide measurements indicate that there is more methane leakage from a city’s gas system than the Gas Index estimated, then these additional emissions are also included in the total for the city. For the cities with such measurements, the measured leakage rates are all similar to or much higher than the Gas Index model estimates for those cities.
