injecting into {HEAD ~ the formation} through {TAIL ~ the injection} well a drive fluid to push the surfactant slug towards a production well; and 
through
injecting into { HEAD ~ the formation } through { TAIL ~ the injection } well a drive fluid to push the surfactant slug towards a production well ; and
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH