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