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