Coalbed Biogenic Methane: Insights on the “Blind Spots” in Mitigation of Emissions

Biogenic or microbial methane (CH4) emissions, believed to be the main driver of the recent surge in global atmospheric CH4 emissions, have altered monitoring, measurement, and mitigation of fossil-fuel emissions. As of 1981, over 20% of the world’s natural gas reserves were biogenic in origin. Additional biogenic CH4 reserves from coal have been discovered since 1981 mixed (40–80%) with thermogenic CH4. Biogenic CH4 accumulates up to 100% in coal reservoirs in the Powder River Basin (PRB), USA. Biogenic CH4 is generated by microbial breakdown of fossil organic matter as an early-stage (primary) type during burial over geologic time and is rarely preserved. Also, biogenic CH4 is generated as a late-stage (secondary) type from recent geologic to present times and is commonly preserved. Late-stage biogenic CH4 is sustained by nutrients and microbes in meteoric/surface waters discharged into coal aquifers. Groundwater is pumped from wells in coal aquifers to desorb and produce CH4 and dewater coal mines. The co-produced water with dissolved CH4 is discharged into diverse surface aquatic systems. The emission factors (EFs) of co-produced water are 2.0522 × 10−9 Gg CH4/gal of water in the PRB and 2.0694 × 10−3 Gg CH4/well in the Black Warrior Basin, U.S. Accurate data on biogenic CH4 emissions from coal sources is a major gap in the accounting of current global groundwater-driven CH4 whose average flux is estimated to be 3.9 ± 6.2 mmol/m2/day or accounting for up to 70% of CH4 emissions from surface aquatic systems. Biogenic CH4 emissions from coal mining and coalbed gas extractions and related infrastructures are overlooked because the focus has been on coalmine methane (CMM) emissions. CMM data from ground-based measurements is highly variable and used by the Intergovernmental Panel on Climate Change three-tier system to estimate EFs for national inventories. However, 90% of CMM emissions are attributable to a small group of the most coal-consuming-and-producing countries but fails to capture other coal sources worldwide. This created gaps and “blind spots” in “unstructured” low-concentration, diffused biogenic CH4 emission data. These key “blind spots” include sources from flooded, abandoned coal mines; coalbed methane (CBM) co-produced water with dissolved CH4 and infrastructures/facilities; and groundwater drawdown from water withdrawals during coal mining and CBM extraction. Also, a critical “blind spot” is the mixing of biogenic CH4 emissions from subsurface coals with biogenic CH4 generated at the surface from wetlands, agriculture, and landfills/wastes, which grew 85% from 2008 to 2020. Limited understanding of the mixing of biogenic CH4 from diverse sources and their contributions to global methane requires accurate attribution of overlapping isotopic signatures (δ13CCH4 and δD). This paper addresses knowledge gaps in coalbed biogenic CH4 emissions by a systematic review of the literature and specific study cases, which provided insights on key “blind spots” in their mitigation.