A national estimate of US underground natural gas storage incident emissions

The 2015 Aliso Canyon storage well blowout was widely reported as the worst natural gas leak in the history of the United States (U.S.) and released similar to 0.1 million metric tons of methane (CH4), a potent greenhouse gas. Although storage well fugitive emissions are estimated in the U.S. Environmental Protection Agency's annual Greenhouse Gas Inventory, the inventory does not include historical estimates of anomalous large emission events other than Aliso Canyon or smaller incident related CH4 releases. A total of 129 underground natural gas storage (UGS) incident-related events between 1940 and 2016 were compiled from various federal/state agencies and literature reviews. Incident emissions were estimated based on best available information, such as direct operator reports, the monetary cost of gas lost, or modeling of the escaping gas at sonic speeds. There are 387 active UGS fields in three types of reservoirs: salt caverns, aquifers, and depleted oil and gas (O&G) fields. 65% of events were in the depleted O&G fields, which account for 79% of storage fields. Texas recorded the highest number of incidents (20), 14 of which were in salt dome reservoirs. The incident emissions showed a heavy-tailed emission pattern with CH4 releases up to 29 billion cubic feet (8.2 x 10(8)m(3)). The top seven events contributed 98% of the total estimated/measured CH4 emissions.