Methane venting from uncontrolled production storage tanks at conventional oil wells—Temporal variability, root causes, and implications for measurement

Uncontrolled oil production storage tanks are important but poorly understood sources of methane (CH4) emissions in the upstream oil and gas sector. This study reports and analyzes directly measured, temporally varying CH4 emission rates, total gas vent rates, and vent gas CH4 fractions from storage tanks at eight active upstream oil production sites in Alberta, Canada. Using a built-for-purpose optical mass flux meter (VentX) supplemented by an ultrasonic flow meter and quantitative optical gas imaging camera where possible, mean vent rates (whole gas) among tanks in the study ranged from 37 to 598 m3/d; however, at some individual tanks, instantaneous flow rates could vary significantly from 0 to over 4,000 m3/d for minutes at a time, while unsteady CH4 volume fractions varied by up to 41% absolute. Root cause analysis revealed the limits of estimating vented emissions from oil production volumes using an assumed gas–oil ratio, especially in cases where produced gas from wells fully or partially bypasses separators. The analysis of the acquired data also demonstrated how 1-h duration vent measurements recommended in some regulations are insufficient to reliably estimate emissions from unsteady tanks. These two factors are the likely reason for significantly underreported vent rates in the present sample and are thought to be a key cause of the mismeasurement/underestimation of tank venting that has driven persistent gaps between bottom-up inventories and top-down measurements. Finally, detailed statistical analyses were completed to suggest minimum sampling durations and instrumentation requirements for direct measurements of tanks and minimum sample sizes for discrete (“snapshot”) surveys of both individual tanks and multitank surveys under different scenarios. Results show that caution is warranted when interpreting snapshot measurements of individual tanks, but aggregate emissions of multiple tanks should be accurately measurable with readily achievable sample sizes. These results are expected to be especially valuable to ongoing efforts seeking to develop robust protocols for gas certification and measurement, reporting, and verification (MRV) of CH4 emissions in the oil and gas sector.