Study: Offshore Platforms Release More Methane Than EPA Estmates

Offshore energy-producing platforms in U.S. waters of the Gulf of Mexico are emitting twice as much methane than previously thought, according to a new study from the University of Michigan.

Researchers conducted a first-of-its-kind pilot study sampling air over offshore oil and gas platforms in the Gulf of Mexico. They found that, for the full U.S. Gulf of Mexico, oil and gas facilities emit approximately one-half a teragram of methane each year, comparable with large emitting oil and gas basins like the Four Corners region in the southwest U.S. The effective loss rate of produced gas is roughly 2.9 percent, similar to large onshore basins primarily focused on oil, and significantly higher than current inventory estimates.

Offshore harvesting accounts for roughly one-third of the oil and gas produced worldwide, and these facilities both vent and leak methane. Until now, only a handful of measurements of offshore platforms have been made, and no aircraft studies of methane emissions in normal operation had been conducted. 

Each year the EPA issues its U.S. Greenhouse Gas Inventory, but its numbers for offshore emissions are not produced via direct sampling. The study, published in Environmental Science and Technology, identified three reasons for the discrepancy between EPA estimates and their findings:

• Errors in platform counts: Offshore facilities in state waters, of which there are in excess of 1,300, were missing from the U.S. Greenhouse Gas Inventory.

• Persistent emissions from shallow-water facilities, particularly those primarily focused on natural gas, are higher than inventoried.

• Large, older facilities situated in shallow waters tended to produce episodic, disproportionally high spikes of methane emissions. These facilities, which have more than seven platforms apiece, contribute to nearly 40 percent of emissions, yet consist of less than one percent of total platforms. If this emission process were identified, it could provide an optimal mitigation opportunity, the researchers said.

Research leader Eric Kort, a University of Michigan associate professor, said EPA officials are already making adjustments to correct their count of offshore platforms operating in the Gulf of Mexico. But emissions estimates, particularly for shallow waters, still need adjustments.

"We have known onshore oil and gas production often emits more methane than inventoried. With this study we show that this is also the case for offshore production, and that these discrepancies are large," Kort said.