Methane emission flux estimation from offshore oil and gas platforms with a dispersion model and airborne measurements.

Accurate quantification of methane emission fluxes from the oil and gas sector remains challenging. Previous methods can encounter issues associated with atmospheric boundary layer dynamics, and the presence of multiple overlapping emission sources. We evaluate a methodology to estimate methane emission fluxes using the commercially available dispersion model ADMS6 and airborne measurements. It takes into consideration many parameters including meteorology variables such as boundary layer stability and high-accuracy atmospheric dynamics measurements from the aircraft. Assumptions about the source type are needed for accurately simulating plume dispersion behaviour. The first method uses a single modelled plume concentration enhancements with a fixed mass flux input, and plume concentrations measured with the FAAM aircraft. The emission flux is scaled using the ratio between the modelled and observed enhancements. For the second method, we generate multiple modelled plumes with varying emission fluxes, creating different potential scenarios. By comparing these simulations to the observed plume, we identify the most accurate fit and extract the corresponding emission rate directly from the model inputs. We then evaluate the methodologies using several emission case scenarios sampled by the FAAM Airborne Laboratory. The study focuses on offshore oil and gas extraction facilities such as the uncontrolled TOTAL ELGIN gas platform methane accidental release in 20121, and fugitive emissions from gas facilities on the Norwegian continental shelf2. The results from the methods are compared with the flux values determined with the more established mass-balance methodology1-4 and sources of uncertainties are discussed. 1. Lee, James D. et al. (Mar. 2018). “Flow rate and source reservoir identification from airborne chemical sampling of the uncontrolled Elgin platform gas release”. In: Atmospheric Measurement Techniques 11.3, pp. 1725–1739. ISSN: 1867-8548. DOI: 10.5194/amt-11-1725-2018. 2. France, James L. et al. (Jan. 2021). “Facility level measurement of offshore oil and gas installations from a medium-sized airborne platform: method development for quantification and source identification of methane emissions”. In: Atmospheric Measurement Techniques 14.1, pp. 71–88. ISSN: 1867-8548. DOI: 10.5194/am-14-71-2021. 3. Foulds, Amy et al. (Apr. 2022). “Quantification and assessment of methane emissions from offshore oil and gas facilities on the Norwegian continental shelf”. In: Atmospheric Chemistry and Physics 22.7, pp. 4303–4322. ISSN: 1680-7324. DOI: 10.5194/acp-22-4303-2022. 4. Pühl, M et al. (2023). “Aircraft-based mass balance estimate of methane emissions from offshore gas facilities in the Southern North Sea”. In: Atmospheric Chemistry and Physics Discussions 2023, pp. 1–32. DOI: 10.5194/acp-2022-826.