Closing the gap: Explaining persistent underestimation of US oil and natural gas production methane inventories

Abstract:

Methane (CH₄) emissions from the oil and natural gas (O&NG) system are an important contributor to greenhouse gas emissions. In the US, recent studies have emphasized the ~1.5x-2x divergence between official Environmental Protection Agency (EPA) greenhouse gas inventory (GHGI) estimates of CH₄ emissions from O&NG and those estimated from field measurements at different spatial scales. Site-level studies have isolated production as the dominant contributor to this divergence. Based on updated synthesis of component-level emission data, we develop a new inventory model for CH₄ emissions using a bootstrap resampling approach. Our component-level model closes the gap between the GHGI production CH₄ emissions estimate and recent site-level synthesis studies. In contrast to site-level studies, we can isolate differences between our study and the GHGI at the equipment level. We that find equipment leaks and tank venting and malfunction-related emissions are the largest contributors to the divergence between our modelled results and the GHGI. To further understand this divergence, we decompose EPA equipment-level emissions factors into the underlying component-level data. This decomposition shows that EPA inventory methods are based on measurements that are systematically lower than modern emissions measurements, and that the methods used in inventories to account for large emitters contribute to systematic underestimation. If EPA adopted our proposed method, it could provide for more accurate inventory estimates, helping the GHGI to more accurately guide methane mitigation policy priorities.