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is introduced into {TAIL ~ the formation} through the same wells in which nitrogen is injected and nitrogen and methane withdrawn 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introduced into { TAIL ~ the formation } through the same wells in which nitrogen is injected and nitrogen and methane withdrawn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