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This is not a newly recognized problem; unintended gas releases from coal-bed methane production has been known since the 1980’s. I investigated and actually co-authored a paper that was presented to a professional society. Basically, this is the release scenario:
Coal-bed methane is trapped on coal particles under hydrostatic pressure. When the water is pumped out, the gas is released and can be captured with a gas well. (Previous to industrial drilling and extraction, many homeowners in the area had water wells completed in or with open casing passing through coal formations which is why gas accumulates in their well house and can explode if not properly vented.)
With the advent of large-scale drilling and extraction, and pumping of water to release the gas, multiple pathways were exposed that allowed gas to move to the surface outside the drilled well. One of those was explained above. Or the released gas may migrate into naturally occurring rock fractures in the coal beds and make its way along bedding planes and eventually to the surface through natural fractures.
Yet another pathway involves the presence of adjacent historic gas wells drilled much deeper than the shallow coal-gas wells. These wells were completed in the 1950-1960 time frame and many did not have cement opposite the coal formations or the cement may be degraded. When gas is released from the coal beds, it moves up along the outside of the older casing and can migrate into shallow water bearing aquifers. Gas well surface casing may prevent it from getting to the surface, but the cement surrounding these older casings may also be degraded allowing release to the atmosphere.
Several other avenues to the surface may work to release the methane. Once in the shallow aquifer, it may come to the surface through fractures, domestic water wells, or cathodic protection wells. The later are usually PVC-cased wells drilled to shallow water and contain an electrode to protect metal pipelines from external corrosion. These wells are usually open to the surface and provide a direct route for gas migration.
So the issue is not so simply addressed and I do not know whether the majority of the fugitive gas is from migration through natural fractures or from man-made pathways. In either case, removing the source(s) of the problem is not easy and short of shutting down the industry may not be possible. Of course, that is what the radical enviros want to do and they will grab on to whatever research may support their quest.