Quantification and assessment of methane emissions from offshore oil and gas facilities on the Norwegian continental shelf
Authors: Foulds Amy , Allen Grant , Shaw Jacob T. , Bateson Prudence , Barker Patrick A. , Huang Langwen , Pitt Joseph R. , Lee James D. , Wilde Shona E. , Dominutti Pamela , Purvis Ruth M. , Lowry David , France James L. , Fisher Rebecca E. , Fiehn Alina , Pühl Magdalena , Bauguitte Stéphane J. B. , Conley Stephen A. , Smith Mackenzie L. , Lachlan-Cope Tom , Pisso Ignacio , Schwietzke Stefan 
Published: April 2022

Abstract. The oil and gas (O&amp;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&amp;G facilities collected in 10
O&amp;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.