Quantification and assessment of methane emissions from offshore oil and gas facilities on the Norwegian continental shelf

Abstract

Abstract. The oil and gas (O&G) sector is a significant source of methane (CH4) emissions. Quantifying these emissions remains challenging, with many studies highlighting discrepancies between measurements and inventory-based estimates. In this study, we present CH4 emission fluxes from 21 offshore O&G facilities collected in 10 O&G fields over two regions of the Norwegian continental shelf in 2019. Emissions of CH4 derived from measurements during 13 aircraft surveys were found to range from 2.6 to 1200 t yr−1 (with a mean of 211 t yr−1 across all 21 facilities). Comparing this with aggregated operator-reported facility emissions for 2019, we found excellent agreement (within 1σ uncertainty), with mean aircraft-measured fluxes only 16 % lower than those reported by operators. We also compared aircraft-derived fluxes with facility fluxes extracted from a global gridded fossil fuel CH4 emission inventory compiled for 2016. We found that the measured emissions were 42 % larger than the inventory for the area covered by this study, for the 21 facilities surveyed (in aggregate). We interpret this large discrepancy not to reflect a systematic error in the operator-reported emissions, which agree with measurements, but rather the representativity of the global inventory due to the methodology used to construct it and the fact that the inventory was compiled for 2016 (and thus not representative of emissions in 2019). This highlights the need for timely and up-to-date inventories for use in research and policy. The variable nature of CH4 emissions from individual facilities requires knowledge of facility operational status during measurements for data to be useful in prioritising targeted emission mitigation solutions. Future surveys of individual facilities would benefit from knowledge of facility operational status over time. Field-specific aggregated emissions (and uncertainty statistics), as presented here for the Norwegian Sea, can be meaningfully estimated from intensive aircraft surveys. However, field-specific estimates cannot be reliably extrapolated to other production fields without their own tailored surveys, which would need to capture a range of facility designs, oil and gas production volumes, and facility ages. For year-on-year comparison to annually updated inventories and regulatory emission reporting, analogous annual surveys would be needed for meaningful top-down validation. In summary, this study demonstrates the importance and accuracy of detailed, facility-level emission accounting and reporting by operators and the use of airborne measurement approaches to validate bottom-up accounting.

Satellites Detect a Methane Ultra-emission Event from an Offshore Platform in the Gulf of Mexico

“…The satellites limitations to detect CH 4 over water has led to a lower number of measurement-based, top-down studies, mostly performed from airplanes or ships, resulting in temporally constrained emission characterization and making it challenging to monitor intermittent emission events. − …”

Flaring efficiencies and NOx emission ratios measured for offshore oil and gas facilities in the North Sea

“…measured much larger median CH4 emissions of 120 g CH4 s -1 (range: 20-360 g CH4 s -1 ) from four facilities in the North Sea. Riddick et al (2019) measured CH4 emissions using a shipborne platform, reporting median emissions of 6.8 g CH4 s -1 (214 tonnes CH4 yr -1 ) across eight facilities, in exceptional agreement with Foulds et al (2022). In the southern North Sea, Pühl et al (in prep) measured median emissions of 10 g CH4 s -1 from a sample of UK and Dutch oil and gas platforms.…”

“…A full description of the FGGA for measurements onboard the FAAM aircraft was reported by O'Shea et al (2013), with a modified instrumental setup (used after January 2019) described by Shaw et al (2022). Raw CO2 and CH4 mole fraction data were corrected for small effects associated with water vapour dilution and spectroscopic error.…”

“…For a full description of data correction, calibration and validation, refer to O' Shea et al (2013) and Pitt et al (2019). CH4 and CO2 data were measured at 1 Hz for flights conducted in 2018, and at 10 Hz for flights conducted in 2019 (Foulds et al, 2022;Shaw et al, 2022). 10 Hz measurements were time-averaged onto a 1 Hz grid for consistency between datasets.…”

“…Offshore oil and gas facilities in the North and Norwegian Seas have been the subject of several studies complementary to the work presented here. Foulds et al (2022) measured CH4 emission fluxes from 21 offshore facilities on the Norwegian continental shelf, finding mean emissions of 211 tonnes CH4 yr -1 (6.7 g CH4 s -1 ) per facility. Wilde et al (2021a) https://doi.org/10.5194/acp-2022-679 Preprint.…”