Subsequent to recovery of {HEAD ~ the methane} from {TAIL ~ the formation}, gaseous hydrogen is injected into the well and into the formation 
from
Subsequent to recovery of { HEAD ~ the methane } from { TAIL ~ the formation } , gaseous hydrogen is injected into the well and into the formation
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH