Subsequent to storage of the hydrogen, {HEAD ~ a pressure gradient} is established from the formation to {TAIL ~ the surface} to withdraw previously introduced hydrogen to the surface 
is established to
Subsequent to storage of the hydrogen , { HEAD ~ a pressure gradient } is established from the formation to { TAIL ~ the surface } to withdraw previously introduced hydrogen to the surface
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH