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