Quantitative airborne assessment of gas flaring combustion efficiency in the Bakken Shale

The Bakken shale formation in North Dakota is a prolific source of oil and natural gas, producing 361 million barrels of oil in 2014. Drilling activities in the Bakken largely focus on oil, though co-production of natural gas is abundant. Up to 1/3 of the natural gas produced in this play is not captured, but is instead flared. The US EPA considers flare combustion efficiency to be 98%, indicating fugitive emissions of 2% of flared natural gas due to incomplete combustion. Studies of flaring combustion efficiency have been primarily laboratory-based, with minimal real-world analysis in the field. A recent study [Caulton et al., 2014] analyzed ten flares in the Bakken and found extremely high combustion efficiency of over 99.8%.Differences between this field study and the EPA standard have potentially significant implications for methane emissions from flaring, but given the small sample size, further in-field sampling is needed to quantify combustion efficiency in real conditions and assess why it may be different from laboratory studies.Here we will present a study on flaring combustion efficiency of methane and ethane in the Bakken field using continuous in-situ airborne observations of methane, ethane, and carbon dioxide. Over thirty flare plumes were observed during a three-week period in the Bakken formation in May 2014 in a range of wind conditions. For each flare we calculate the destruction efficiency and emission factors for methane and ethane. Preliminary results suggest combustion efficiency comparable with the EPA value of 98%, notably lower than the previous study. In addition, we calculate the corresponding mass flux of methane from incomplete flare combustion for the entire field using gas flare volume information and compare to the total emissions from the field calculated using our flight data and a mass-balance approach. Using the combustion efficiency from our study suggests methane emissions from incomplete combustion during flaring could be responsible for almost 10% of the total field emissions, contrasted with <1% if the Caulton numbers are used.