Aircraft-based mass balance estimate of methane emissions from the offshore oil industry in Angola

Atmospheric methane (CH₄) concentrations have more than doubled since the beginning of the industrial age, making CH₄ the second most important anthropogenic greenhouse gas after carbon dioxide (CO₂). The oil and gas (O&G) sectors are one of the major anthropogenic CH₄ sources accounting for 22% of global anthropogenic CH₄ emissions. The METHANE-To-Go Africa (MTGA) scientific aircraft campaign in September 2022 was conducted as part of UNEP’s International Methane Emissions Observatory (IMEO). During the campaign, we conducted the first large scale methane measurements of the O&G sector in West Africa. The study provides an initial empirical understanding of the magnitude and location of emissions in this important but previously unobserved source region. The emissions of O&G facilities were determined using an aircraft-based mass balance method.

The entire emissions of the Angolan offshore O&G sector and the liquid natural gas (LNG) plant were observed to be in the range of emissions reported by the Angolan operators. This is much less than the estimates from scientific emission inventories like EDGAR and CAMS-GLOB-ANT.

For the regional scale emission estimates, the Angolan O&G facilities are aggregated in blocks, a local operator-wise separation of assets. Most blocks have low emissions of methane. We observed medium emissions at one block and high emissions at two blocks. These three blocks are close to the coast, in shallow water, and the facilities are generally older than further out at sea.

Often the emissions of individual facilities or groups of facilities could be discerned from the mass balance flights. We deduced emission estimates for 31 individual facilities and 10 groups of facilities. The emission estimates on different days are consistent for all facilities, showing little temporal variation. The generally older shallow-water facilities show higher emissions than the deep and ultra-deep water facilities, which have a higher oil production.

The additional trace gases CO₂, SO₂, NO_y and aerosol particles were also observed from the aircraft. This data is used to further investigate the source of CH₄ emissions: flaring, fugitives, venting, or burning of fuel gas. The CH₄-CO₂ ratio indicates that most CH₄ emissions result from fugitives and venting, not flaring. Ten different flare exhaust plumes were sampled at close distance. The flaring observations will be further analyzed including information on gas composition from the operators.

Overall, this study gathered a unique dataset in its coverage providing extraordinarily comprehensive measurements of the CH₄ emissions from the O&G industry off the coast of West Africa.