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