This short modelcalculates the impact of methane emissions on the CO2/boe of burning natural gas, compared against coal. With methane emissions fully controlled, burning gas is c60% lower-CO2 than burning coal.
However, taking natural gas to cause 120x more warming than CO2 over an immediate timeframe, the crossover (where coal emissions and gas emissions are equivalent) is 4% methane intensity. i.e., if 4-20% of methane is leaked, then the total warming from burning natural gas is equivalent to coal’s.