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