Glinting sunlight reveals methane emissions from offshore platforms

18 Aug 2022

Offshore emissions: the locations of targeted platforms are shown as diamonds. The average persistence adjusted emission rate for each source is shown in terms of both the circle radii and infill colours (yellow and the smallest circles correspond to a 500 kg/h emission rate, while cerise and the largest circles correspond to 2000 kg/h and higher). (Courtesy: AK Ayasse et al 2022 Environ. Res. Lett. 17 084039).

Methane emissions from offshore oil and gas platforms can be systematically mapped using a sun-glint-based remote sensing method, researchers from the US have shown. Their new approach could help inform efforts to reduce methane release and improve national emissions inventories.

A powerful greenhouse gas, methane is a significant contributor to climate change. Previous research has established that at least 20% of human-related methane emissions are from oil and gas production. These can stem both from normal operations and malfunctions or leaks.

While methane release from onshore oil and gas facilities are well studied, emissions from offshore platforms are poorly understood, despite these facilities contributing to some 30% of all oil and gas production. Current estimates of methane emissions tend to be unreliable – and they fail to account for skewed emissions where a small fraction of equipment is responsible for a large proportion of emissions.

Offshore challenges

Observational studies, meanwhile, are difficult given the remote locations of offshore platforms. Boats are often unable to get close enough to platforms and lack the ability to accurately detect elevated emissions plumes. Aircraft equipped with gas analysers can detect methane but tend to be unable to locate sources with the required accuracy. Aircraft and satellites armed with imaging spectrometers offer higher spatial resolution – but they struggle with trace gas detection over the ocean because water is a very dark surface in the methane absorption bands.

To address these shortcomings atmospheric scientist Alana Ayasse of the University of Arizona and Carbon Mappers and colleagues have demonstrated the potential of a remote sensing method that works by capturing the glint of the Sun on the water’s surface. This provides sufficient reflected radiance to discern a methane signal.

“We achieve this by banking the aeroplane at the right time and place, so that the angle of the sensor – mounted to the plane – is at the same angle as the Sun and is in alignment with the target,” Ayasse explains.

Louisiana study

In 2021 the team used this technique to analyse over time the emissions from more than 150 offshore, shallow water oil/gas wells and production platforms in the Gulf of Mexico, off the coast of Louisiana. The survey covered around 8% of all such facilities in the region.

Not only did the researchers demonstrate the efficacy of the sun glint method for the remote detection of methane release, but they were also able to reveal that the emissions from the offshore platforms appear generally to be both higher relative to production and more persistent than those from onshore oil and gas basins. Furthermore, the emissions were highly skewed, the team noted, with most being derived from storage tanks and vent booms.

This work is a big step towards full-scale operational monitoring of offshore productions over large areas globallyAlana Ayasse

“While there have been a few previous one-off experimental detections of methane over the ocean, this work is a big step towards full-scale operational monitoring of offshore productions over large areas globally,” explains Ayasse. This capacity, she says, is vital for informing emissions reduction efforts. For example, the researchers point out that the normal operation of a pressure relief valve could be responsible for intermittent methane emissions from a storage tank – but a more persistent release could be indicative that a valve is stuck and needs repair.

“We have demonstrated with pilot programmes in California that sharing high-resolution methane data with onshore oil and gas operators can directly lead to voluntary leak repair action,” says Ayasse. “Long-term mitigation requires many actors and many moving parts, but having good data is fundamental to it all.”

Satellite deployment

Atmospheric physicist Debra Wunch of the University of Toronto, who was not involved in the study, says the research provides further evidence that, in order to make progress on reducing methane release, reported emissions need to be verified and monitored. “Using glint measurements over water will allow us to use the next generation of methane satellites to include offshore oil and gas production in our atmospheric monitoring, a previously difficult source of emissions to monitor.”

Grant Allen, an atmospheric physicist at the University of Manchester says, “The study confirms the findings of previous measurement-led field projects, which have consistently found that a small number of facilities (onshore and offshore) typically account for the large majority of methane emissions – so-called super-emitter facilities. Often, the reasons for this may be due to poor operational practice, or some potentially unidentified or unwanted venting (called fugitive emission). Identifying super emitters in this way can help to target fast interventions to prevent further emissions and lead to more targeted emissions policy and regulation.”

Accurate inventories

Allen also points out that directly measuring methane emissions can help us identify errors in national greenhouse gas emissions inventories and operator-reported emissions estimates. The former is important for holding governments to account on climate emissions reduction targets and allowing us to accurately model emission and climate change trajectories. He concludes, “Measurement-led studies such as this help to keep our emissions inventories as honest as possible”.READ MORE

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With their initial study complete, the researchers are now looking to return to the Gulf of Mexico to survey a larger population of offshore infrastructure in order to improve assessments of methane loss rates in the region. This includes deep-water platforms, whose production is different to their shallow-water counterparts.

“We are also looking forward to launching the first two Carbon Mapper satellites in 2023,” Ayasse adds. These, she explains, “are designed to provide more complete and resistant global monitoring of methane emissions from major offshore oil and gas production areas which otherwise remain largely invisible”.

The study is described in Environmental Research Letters.

Ian Randall is a science writer based in the UK.

from physicsworld.com   28/8/2022