Use of Light Alkane Fingerprints in Attributing Emissions from Oil and Gas Production

Spatially resolved emission inventories were used with an atmospheric dispersion model to predict ambient concentrations of methane, ethane, and propane in the Eagle Ford oil and gas production region in south central Texas; predicted concentrations were compared to ground level observations. Using a base case inventory, predicted median propane/ethane concentration ratios were 106% higher (95% CI: 83% higher–226% higher) than observations, while median ethane/methane concentration ratios were 112% higher (95% CI: 17% higher–228% higher) than observations. Predicted median propane and ethane concentrations were factors of 6.9 (95% CI: 3–15.2) and 3.4 (95% CI: 1.4–9) larger than observations, respectively. Predicted median methane concentrations were 7% higher (95% CI: 39% lower–37% higher) than observations. These comparisons indicate that sources of emissions with high propane/ethane ratios (condensate tank flashing) were likely overestimated in the inventories. Because sources of propane and ethane emissions are also sources of methane emissions, the results also suggest that sources of emissions with low ethane/methane ratios (midstream sources) were underestimated. This analysis demonstrates the value of using multiple light alkanes in attributing sources of methane emissions and evaluating the performance of methane emission inventories for oil and natural gas production regions.