Preferably, hydrogen is introduced into {TAIL ~ the formation} through the same wells in which {HEAD ~ nitrogen} is injected and nitrogen and methane withdrawn 
Preferably , hydrogen is introduced into { TAIL ~ the formation } through the same wells in which { HEAD ~ nitrogen } is injected and nitrogen and methane withdrawn
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH