Offshore oil and gas platforms emit more methane than previously thought

Researchers from the University of Michigan (U-M) conducted a first-of-its-kind pilot study sampling air over offshore oil and gas platforms in the Gulf of Mexico. Their findings suggest the US government’s calculations are “too low”.

The research found that, for the full US Gulf of Mexico, oil and gas facilities emit "approximately one-half a teragram" [500 megatonnes] of methane each year. This compares with large emitting oil and gas basins such as the Four Corners region in the southwest US.

Furthermore, the study found the effective loss rate of produced gas is around 2.9 per cent. This is 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 recently, 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 Environmental Protection Agency (EPA) issues its US Greenhouse Gas Inventory; however, 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 the U-M findings:

• Errors in platform counts: Offshore facilities in state waters, of which there are in excess of 1,300, were missing from the US Greenhouse Gas Inventory.
• Persistent emissions from shallow-water facilities, particularly those primarily focused on natural gas, are higher than listed.
• 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 per cent of emissions, yet consist of less than one per cent of total platforms. If this emission process was identified, it could have provided an optimal mitigation opportunity, the researchers said.

Eric Kort, U-M associate professor of climate and space sciences and engineering, said EPA officials are currently amending their count of offshore platforms operating in the Gulf of Mexico. However, he added that 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. “By starting to identify and quantify the problem, with a particular focus on larger shallow water facilities, we can work towards finding optimal mitigation solutions.”

Kort added that shallow-water discrepancies warrant further investigation, as deepwater facilities may be sending some of the oil and gas they produce via pipeline to others located closer to shore.

U-M’s team conducted its sampling in 2018 with Scientific Aviation using a small research plane with enough room for a pilot and passenger in the two front seats, as well as scientific gear.

Tubes along the wings of the plane drew in the air that was pumped to the equipment for analysis of the amount of methane included as well as wind speed. Circling a single platform gave the researchers a clearer indication of how much methane that single source is emitting.

As well as 12 individual facilities, the flights also covered larger geographical areas. Flying downwind from clusters of five to 70 oil and gas facilities, and taking similar measurements, researchers can evaluate how well inventory estimates compare with large numbers of platforms.

“By measuring emissions from both individual facilities as well as many dozens of facilities we can compare results, evaluate different inventories, and generate a more statistically robust estimate of total emissions from the US Gulf of Mexico,” said Alan Gorchov Negron, a PhD candidate in climate sciences at U-M.

As a pilot study, Kort said the research “is promising but has gaps”, adding that greater statistical sampling and identification of the cause of high emissions can guide mitigation and improve reported emissions.

To further the work and fill in these gaps, new aerial sampling is in the works funded by the Alfred P Sloan Foundation, the U-M team added. The project, titled Flaring and Fossil Fuels: Uncovering Emissions & Losses (F³UEL), will mean more flights later over the next three years over the Gulf of Mexico, Alaska and California.

F³UEL’s progress can be tracked at http://graham.umich.edu/f3uel.