Significant reductions in ethane emissions in the Denver-Julesburg Basin from 2015 to 2021 from oil and natural gas operations

Abstract

Given the low or nonexistent ethane (C2H6) signature in biogenic emissions, top-down studies of methane (CH4) emissions use wellhead gas composition to determine the associated ethane-to-methane (C2/C1) ratio, which is then used to differentiate oil and natural gas (O&NG) and biogenic sources and to attribute the contribution of CH4 emissions from oil and gas (O&G) sites. However, this ratio may vary within and across different basins, emitting sources, and facility configurations. Understanding these variations is essential for accurately attributing CH4 emissions to different O&G sectors and sources and to subsequently inform policy decisions and emissions mitigation strategies. This study investigates stack test data from Fourier Transform Infrared Spectroscopy (FTIR) analysis of the exhaust gas for 10 four-stroke rich burn (4SRB) engines, 54 four-stroke lean burn (4SLB) with pre-chamber, and 104 4SLB without pre-chamber provided by past research studies and O&G operators. Stack tests are conducted to determine the compressor driver's (engine) compliance with emissions limits. Fuel gas is fed into natural gas (NG) fired engines for compression purposes and stack tests determine the amount of specific pollutants in the exhaust gas. Fuel gas composition and stack test data are used to calculate the fuel and exhaust gas C2/C1 ratios and the CH4 and C2H6 destruction efficiencies for these engine categories. The results show that engines preferentially combust heavier hydrocarbons over CH4, evidenced by consistently higher destruction efficiency (DE) for C2H6 than CH4 across all engine types. Additionally, recent design modifications in O&G production sites, including tankless facility configurations, may have led to a reduction in CH4 emissions and a more pronounced decrease in C2H6 emissions. In the Denver-Julesberg (DJ) basin in Colorado, estimated emissions from 2015 to 2021 show a 73.9% increase in NG production, stable CH4 emissions, and a 54.2% reduction in C2H6 emissions. The analysis also reveals a shift in emissions from the production sector to the midstream sector and suggests that when the wellhead C2/C1 ratio was used 22 to attribute CH4 emissions from the oil and gas sector, top-down methods may have overestimated CH4 emissions by an average of 159.65% in 2015, and by 10.86 % in 2021.