Fig. 5: Example decomposition of total CH4 emissions for crude oil storage tanks.
From: Closing the methane gap in US oil and natural gas production emissions inventories
Total CH4 emissions (d) for crude oil storage tanks in petroleum systems (for a decomposition of CH4 emissions from condensate storage tanks in natural gas systems see Supplementary Fig. 33) are decomposed into several constituent parts and compared with corresponding factors in the Greenhouse Gas Inventory. Error bars reflect the 95% confidence interval based on the 2.5 and 97.5 percentile values extracted from the empirical distributions and filled squares and triangles represent the mean. Constituent parts include tank counts (a), the intentional emission factor (b), and the unintentional emission factor (c) (note the log scale for the right three panels). Intentional and unintentional emission factors are decomposed into emission factors (kg CH4 per emitting tank) and control rates (fraction of total tanks emitting). Intentional emissions are defined as flash CH4 released from uncontrolled storage tanks operating as designed. Unintentional emissions and the corresponding fraction-emitting value relate to emissions identified (at a screening value > 500 ppmv) at thief hatches, pressure-relief valves, and rusted holes. Note that, although both our activity data and the Greenhouse Gas Inventory activity data are based upon data from the Greenhouse Gas Reporting Program, our estimate of total tanks is different. This is because estimates of total well counts, which are used to extrapolate a population estimate for tanks, are slightly different (Supplementary Methods 5).
