The average methane content in British coal seams is 4.7 m3 per ton of coal (Jackson and Kershaw, 1996); this methane,
if released to the atmosphere, has a global warming potential about 5% of that of the CO2 from burning the coal.

158If 5% of the natural gas leaks, it’s equivalent to a 40% boost in carbon dioxide. Accidental methane pollution has nearly
eight times as big a global-warming effect as the CO2 pollution that would arise from burning the methane; eight
times, not the standard “23 times,” because “23 times” is the warming ratio between equal masses of methane and
CO2. Each ton of CH4 turns into 2.75 tons of CO2 if burned; if it leaks, it’s equivalent to 23 tons of CO2. And 23/2.75
is 8.4.

Further reading: World Energy Council [yhxf8b]

Further reading about underground coal gasification: [e2m9n]