After water and {HEAD ~ a significant amount} of methane is recovered from {TAIL ~ the coal bed}, an inert gas such as nitrogen is injected into the coal bed formation 
After water and { HEAD ~ a significant amount } of methane is recovered from { TAIL ~ the coal bed } , an inert gas such as nitrogen is injected into the coal bed formation
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH