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I went back to dig up my old analysis on this from June 2018 after Alvarez et al. study in Science on upstream methane emissions in US oil & gas sector . Looks like I never shared it (I was in middle of finishing my dissertation at time), so here goes...
First on method: I estimate GHG benefits of U.S. coal to natural gas shift in electricity sector, including 1) direct emissions from coal & natural gas generators, 2) upstream coal mine methane emissions and 3) upstream natural gas methane emissions at different leak rates.
I calculate the net effect on CO2-equivalent emissions using the 100 year global warming potential impact and 20 year global warming potential (GWP). Please note all relevant caveats that apply to using GWP (see )
So on to results. These two tables summarize the impact of including upstream methane emissions from both coal mining and natural gas production at different leakage rates. The top table uses 100 year global warming potential and the bottom 20 year GWP. 
How to read this table?
1. Without accounting for upstream methane (CH4), I estimate that shift from coal to gas in US power generation from 2008-2017 cut CO2 by 236 million metric tons (MMT) or ~10% of 2008 total power sector CO2.
1. Without accounting for upstream methane (CH4), I estimate that shift from coal to gas in US power generation from 2008-2017 cut CO2 by 236 million metric tons (MMT) or ~10% of 2008 total power sector CO2.
2. If add in upstream CH4 emissions from coal mining, that amounts to another 0.5 MMT of CH4 reduced or 16 MMT of CO2-equivalent (CO2e) at 100 year GWP and 41 MMT CO2e at 20 year GWP. Ignoring Ch4 leaks in gas supply chain brings total to 252-277 MMT CO2e depending on GWP used.
3. Now let's start looking at the impact of methane leaks in upstream natural gas supply chain. Going across columns of the tables, I estimate impact of leakage rates up to 5% (e.g. 5 tons CH4 leak per 100 tons CH4 used for gas power generation).
I specifically highlight EPA inventory estimate of 1.4% & Alvarez et al. 2018 Science paper estimate of 2.3%...
(Note those estimates are for whole oil & gas sector. I'm assuming they apply proportionately to gas as oil, but that's an assumption for simplicity, so caveat emptor)
(Note those estimates are for whole oil & gas sector. I'm assuming they apply proportionately to gas as oil, but that's an assumption for simplicity, so caveat emptor)
4. So if we assume increasing leakage rates of 1.4% as per EPA inventory for oil & gas sector, methane leaks add 0.8 MMT of CH4 or 26.6/67.3 MMT of CO2e at 100/20 year GWPs. That "takes back" 4-11% of the initial reduction in coal-related emissions from coal to gas switch
5. If we assume the Alvarez et al. 2018 study's estimated leakage rate of 2.3%, then upstream gas leaks take back 11-28% of the gains from coal to gas switching, or 43-108 MMT CO2e depending on GWP rate used (top or bottom table).
6. If we assume leakage rates as high as 5%, then upstream methane leaks from gas supply chain would take back 32-82% of the emissions reductions from coal to gas switching.
Conclusions: Unless upstream leakage rate is higher than 5% (unlikely) and you're focused only on 20 year global warming impact, there's still a net climate gain from coal to gas switching in U.S. electricity sector. However, that gain is diminished if you consider methane leaks.
Additionally, if we assume growth of wind and solar generation from 2008-2017 offset coal generation as well, then wind + solar growth are responsible for driving down direct coal emissions by 276 MMT CO2. That's already bigger than direct impact of coal -> gas over same time
If I now add in upstream coal mine methane avoided by renewables, that total increases to 294-323 MMT CO2-e avoided by wind and solar, at 100 or 20 year GWP. That's 17% more CO2e avoided than coal to gas switch over same period assuming ZERO leaks of CH4 in gas supply chain.
Start accounting for drop in net CO2e reductions due to methane leaks, and it is clear that the combined growth of wind & solar in the U.S. power sector from 2008-2017 has had a larger net effect on GHG emissions declines than the hugely important shift from coal to gas.
That's my estimate anyway. Would love to see some real lifecycle assessment experts tackle this (rather than me during a couple of lunch breaks 10 months apart). /end.
