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Informing methane emissions inventories using facility aerial measurements at midstream natural gas facilities


Increased interest in greenhouse gas (GHG) emissions, including recent legislative action and voluntary programs, has increased attention on quantifying, and ultimately reducing, methane emissions from the natural gas supply chain. While inventories used for public or corporate GHG policies have traditionally utilized bottom-up (BU) methods to estimate emissions, the validity of such inventories has been questioned. To align with climate initiatives, multiple reporting programs are transitioning away from BU methods to utilizing full-facility measurements using airborne, satellite or drone (top-down (TD)) techniques to inform, improve, or validate inventories. This study utilized full-facility estimates from two independent TD methods at 15 midstream natural gas facilities in the U.S.A., and were compared with a contemporaneous daily inventory assembled by the facility operator, employing comprehensive inventory methods. Methods produced multiple full-facility methane estimates at each facility, resulting in 801 individual paired estimates (same facility, same day), and robust mean estimates for each facility. Mean estimates for each facility, aggregated across all facilities, differed by 28% [10% to 43%] for the first deployment and nearly 2:1 (49% [32% to 68%]) the second deployment. Estimates from the two TD methods statistically agreed in 12% (97 of 801) of the paired measurements. These data suggest that one or both methods did not produce accurate facility-level estimates, at a majority of facilities and in aggregate across all facilities. Operator inventories, which included extensions to capture sources beyond regular inventory requirements and to integrate local measurements, estimated significantly lower emissions than the TD estimates for 96% (1535 of 1589) of the paired comparisons. Significant disagreement is observed at most facilities, both between the two TD methods and between the TD estimates and operator inventory. Overall results were coupled with two case studies where TD estimates at two pre-selected facilities were coupled with comprehensive onsite measurements to understand factors driving the divergence between TD and BU inventory emissions estimates. In 3 of 4 paired comparisons between the intensive onsite estimates and one of the TD methods, the intensive on-site work did not conclusively diagnose the difference in estimates. In these cases, the preponderance of evidence suggests that the TD methods mis-estimate emissions an unknown fraction of the time, for unknown reasons. The results presented here have two implications. Firstly, these findings have important implications for the construction of voluntary and regulatory reporting programs that rely on emission estimates for reporting, fees or penalties, or for studies using full-facility estimates to aggregate TD emissions to basin or regional estimates. Secondly, the TD full-facility measurement methods need to undergo further testing, characterization, and potential improvement specifically tailored for complex midstream facilities.


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Embargo expires: 12/29/2024.



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