Generating Representative Methane Emissions from Offshore Oil and Gas Production Facilities Using Bottom-Up Methods

With an increase in methane quantification requirements and recent developments in technology, quantification of methane emissions from offshore oil and gas production infrastructure has become more important, and studies reporting emissions more numerous. While reported emissions range from those below method quantification limits to thousands of tons of methane per hour, there is currently no clear understanding on what expected emission from offshore facilities should be. To investigate if current methods generate realistic emission estimates, we will create bottom-up models that simulate two prototypical facilities operating in the Gulf of Mexico and compare the calculated emissions to actual emissions reported by published studies. Prototypical facility type 1 are those fixed platforms operating closer to shore in shallower water and comprise typically older, lower-producing platforms with less processing equipment, no compressor, the oil is piped to shore and usually unmanned 24 hours per day. Using the bottom-up model, total emissions from a prototypical facility type 1 are estimated at 9.2 kg CH4 h-1, largest emissions from fugitive emissions, pneumatic controllers and chemical injection pumps, and correspond to a loss of 1.9 % of the average facility production of 480 kg CH4 h-1. Prototypical facility type 2 platforms operate in deeper water, farther from shore, are newer, have higher production rates from more well heads, have more processing equipment, oil storage tanks, compressors and power generation, and are usually manned 24 hours per day. Total emissions from prototypical type 2 facilities are estimated at 42.2 kg CH4 h-1 (loss of 1.2 % of the average facility production) with the largest emissions from the liquid storage tanks, water storage tanks, and compressors. Fugitive emissions are estimated at 23.4 kg CH4 h-1 with 13 fugitive emissions per facility with an average emission rate of 1.8 kg CH4 h-1. The measured average emission from 13 type 1 facilities was 17.6 kg CH4 h-1 with a median production loss estimated at 8 %, 76 % higher than this study’s bottom-up estimate. As there are few moving parts and no storage tanks on type 1 facilities, the largest unknown emission source are fugitives and we therefore suggest that 6 % of production is currently lost as fugitive emissions from type 1 facilities. The measured average emission from 20 type 2 facilities was 35.5 kg CH4 h-1 with a median production loss estimated at 2.4 %, 50% higher than the bottom-up estimate. Using emission factors that consider the long-tail emission distribution in part reconciles the difference between modelled and measured emission estimates but we suggest the current the fugitive emission estimate may be an underestimate and more data on the number and size of fugitive emissions would help to reconcile the modelled and measured emission estimate. We suggest a bottom-up approach that uses production data coupled with facility equipment could be used to identify facilities that have unusually large measured emissions, caused by methodological failure or larger than expected fugitive emissions, which should be targeted for further evaluation resulting in remeasurement or identification of source type (maintenance event or fugitive) so that a more accurate estimates can be made on the absolute emission.