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