Measurement of fugitive methane emissions from an abandoned coal exploration hole in Queensland, Australia using a Quantum Gas LiDAR.

Rapid reduction in global methane emissions has been identified as the fastest approach to reduce future global warming and emission reduction requires accurate identification of sources, and reliable emission quantification. This study addresses an unaccounted source of methane by providing the first longer-term quantification of substantial fugitive methane emissions from an abandoned coal exploration borehole in Queensland, Australia, revealing a mean emission rate of 235 t/a. Depending on the Global Warming Potential applied, this mean rate is equivalent to 6580 or 19,768 t COe. The surveyed borehole, classified as a "super-emitter" (>25 kg/h), exhibited substantial temporal variability in methane flux with the 5th and 95th percentile of 22.8 t/a and 438 t/a, respectively. The survey used a trailer mounted Quantum Gas LiDAR system to enable continuous monitoring over seven days and six nights in variable weather conditions. Exploratory data analysis identified a negative correlation with atmospheric pressure as a likely meteorological driver of emission rate variability, although other factors such as subsurface (groundwater) pressure changes may also influence emission rates. The study highlights that legacy coal exploration infrastructure, if improperly sealed, can act as anthropogenic pathways for uncontrolled methane release. These findings indicate the need for surveys of a larger sample of coal exploration holes across Queensland's coal basins to understand the frequency and rates of emissions, to investigate emissions from other holes, such as water bores, and further investigation of emission dynamics considering geological characteristics and nearby activities. This research indicates the possible significance of legacy coal exploration bores as an under-reported and unaccounted source of methane emissions, with potential implications for coal basins globally.