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Oil and gas wells (OGWs) with integrity failures can be a conduit for methane and contaminant leakage to groundwater aquifers, surface water bodies and the atmosphere. Recent reviews have addressed OGW leakage but focused on specific types of wells (conventional/unconventional) or specific geographic regions. Here, we conduct a literature review and focus on factors and policies affecting leakage of active and abandoned OGWs, studies quantifying OGW methane emissions, and leakage repair and emission reduction options. Of the 38 factors reviewed here in published literature, studies find that 15 (39%) factors, including geographic location, well deviation, casing quality and plugging status consistently affect OGW leakage. For 15 (39%) factors, including surface casing depth, well elevation and land cover, one or two studies show that they do not affect OGW leakage. For the remaining eight (21%) factors, including well age, studies show conflicting results. Although increased frequency of well monitoring and repair can lead to reduced OGW leakage, several studies indicate that monitoring and repair requirements are not always enforced. Moreover, we find that while 27 studies quantify OGW methane emissions to the atmosphere at the oil and gas wellhead scale, there still are major gaps in the geographical distribution of the collected data, especially for abandoned and orphaned wells. Although studies measuring abandoned wells may include measurements from orphaned wells, these measurements are not separated by well status (orphaned/abandoned), which is important for policy makers aiming to plug thousands of orphaned wells. To repair OGW leakage, we find that most studies focus on well cement and casing repair, and other studies focus on improving the cement mixture to avoid the need for repairs. Alternatives to cement and casing repair for methane emission reductions, such as soil methane oxidation to reduce leakage from OGWs may be effective, but their widespread applicability requires further study. Overall, our review of factors affecting OGW leakage can be used to guide OGW leakage monitoring and repair policies to target wells with high leakage potential, thereby reducing climate and environmental impacts.

Abstract. Direct measurements of methane emissions at the component level provide the level of detail necessary for the development of actionable mitigation strategies. As such, there is a need to understand the magnitude of component-level methane emission sources and to test methane quantification methods that can capture methane emissions at the component level used in national inventories. The static chamber method is a direct measurement technique that has been applied to measure large and complex methane sources, such as oil and gas infrastructure. In this work, we compile methane emission factors from the Intergovernmental Panel on Climate Change (IPCC) Emission Factor Database in order to understand the magnitude of component-level methane flow rates, review the tested flow rates and measurement techniques from 40 controlled-release experiments, and perform 64 controlled-release tests of the static chamber methodology with mass flow rates of 1.02, 10.2, 102, and 512 g h−1 of methane. We vary the leak properties, chamber shapes, chamber sizes, and use of fans to evaluate how these factors affect the accuracy of the static chamber method. We find that 99 % of the component-level methane emission rates from the IPCC Emission Factor Database are below 100 g h−1 and that 77 % of the previously available controlled-release experiments did not test for methane mass flow rates below 100 g h−1. We also find that the static chamber method quantified methane flow rates with an overall accuracy of +14/-14 % and that optimal chamber configurations (i.e., chamber shape, volume, and use of fans) can improve accuracy to below ±5 %. We note that smaller chambers (≤20 L) performed better than larger-volume chambers (≥20 L), regardless of the chamber shape or use of fans. However, we found that the use of fans can substantially increase the accuracy of larger chambers, especially at higher methane mass flow rates (≥100 g h−1). Overall, our findings can be used to engineer static chamber systems for future direct measurement campaigns targeting a wide range of sources, including landfills, sewerage utility holes, and oil and natural gas infrastructure.

<p>Measurements have shown that abandoned oil and gas wells emit methane to the atmosphere, but the estimates of methane emissions at the national scales remain highly uncertain. Here, we provide an overview of available measurement data and studies investigating factors linked to high methane-emitting abandoned wells. We then analyze abandoned oil and gas well data in Canada and the United States to estimate methane emissions for both countries and evaluate uncertainties in the national estimates. Available measurement data indicate that average methane emission rates used as emission factors vary by 3 orders of magnitude or more, even after accounting for plugging status. Plugging status has been shown to be an important predictor of high methane emitting wells; however, there may be other important factors such as age, depth, fluid type and geographical region. Such well attribute data are not consistently available for many abandoned and orphaned oil and gas wells in Canada and the United States. Overall, there is a need for additional measurements of methane emissions from abandoned oil and gas wells and compilation of well attributes to reduce uncertainties in national estimates.</p>