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