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