Methane emissions from oil and gas platforms in the Bohai Sea, China

Abstract

Although oil and gas explorations contribute to atmospheric methane (CH4) emissions, their impact and influence along the shelf seas of China remain poorly understood. From 2012 to 2017, we conducted four ship-based surveys of CH4 in the seawater column and boundary layer of the Bohai Sea, China, and further measured CO2 and several meteorological parameters. The average observed CH4 mixing ratios in the boundary layer and its concentrations in seawater column were 1950 ± 46 ppb in November 2012 (dissolved CH4 was not observed in this survey), 2222 ± 109 ppb and 13.0 ± 5.9 nmol/L in August 2014, 2014 ± 20 ppb and 5.4 ± 1.4 nmol/L in February 2017, and 1958 ± 25 ppb and 5.3 ± 3.8 nmol/L in May 2017, respectively. The results demonstrated that the CH4 emissions from the oil and gas platforms accounted for approximately 72.5 ± 27.0% of the increase in the background atmospheric CH4 in the local area. The remaining emissions were attributed to land–sea air mass transportation. Conversely, the influence of the air–sea exchange was negligible, measuring within the 10−3 ppb range. For carbon balance calibration, the mean flaring efficiency of the oil-associated gas based on the enhancement of CO2 (ΔCO2) and enhancement sum of CO2 and CH4 (ΔCO2 + ΔCH4) was 98.5 ± 0.5%. Furthermore, the CH4 emission rate from the oil and gas platforms was 0.026 ± 0.017 Tg/year, which was approximately 7.2 times greater than the sea-to-air CH4 flux over the entire Bohai Sea area. Thus, oil and gas platforms must be recognized as important artificial hotspot sources of atmospheric CH4 in the Bohai Sea.