Q844. What factors influenced the trends in GHG emissions associated with natural gas liquids supply?
A844. For suppliers of natural gas liquids, the CO2 associated with natural gas liquids (NGLs) supplied to the U.S. economy (i.e., ethane, propane, butane, isobutane and pentanes plus) has gradually increased from 212 million metric tons (MMT) in 2011 to 363 MMT in 2018. This increase is due to increased production of natural gas in areas that are high in natural gas liquids; increased U.S. demand for NGL products; increased production due to construction of new fractionation plants; and increased exports due to the construction of new pipelines and terminals. 
The reported CO2 was also affected by changes in the default emission factors. For suppliers of natural gas liquids, the default emission factors used for calculating the CO2 for ethane, propane, butane and isobutene were revised in 2013. The default emission factors for propane, butane and isobutane were increased by a few percent over those used in prior to 2013, while the default emission factor for ethane was decreased by over 30 percent. The impact these changes had on the total CO2 reported by an NGL fractionator depends on the mixture of products the plant supplies and whether the fractionator used the default value or a measured value. Since most NGL fractionators supply ethane, the reported CO2 across the industry was lower beginning in 2013 than would have been reported if the factors had not been updated.
 U.S. Energy Information Administration, Petroleum Supply Annual, September 2018.