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