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