Question about Wellhead Casing

[polo.gaputan]

Hi,

I have a question about measuring fugitive methane emissions from abandoned or "orphan" oil and gas wells.

ACR (American Carbon Registry) published a methodology encompassing measuring fugitive emissions by connecting to the wellhead casing. I was wondering if this would be representative of fugitive methane emissions (leaks due to improper plugging of the well). From my understanding, the wellhead casing is not directly connected to the production tubing and any gas from the casing is due to any leaks from the tubing and the casings.

Here is a link to the methodology: https://acrcarbon.org/methodology/plugging-orphaned-oil-and-gas-wells/

Thanks for the help in advance.