| Dilute brine flooding can either cause formation damage or improve oil recovery, mainly depending on the respective compositions of rock surface, brine and crude oil as well as their complex interactions. Previous studies showed that with other factors held constant, oil recovery sometimes was increased by manipulation of injection brine composition.;Studies on the effect of backpressure on waterflood oil recovery indicated that waterflooding without backpressure was associated with solution gas flooding and therefore recovered more oil than that with backpressure.;The effects on wettability and oil recovery by spontaneous imbibition of five commonly used chemicals added to invading brines at typical oil field concentrations were also investigated. No apparent influence was observed.;In the present work, two kinds of crude oils, two synthetic reservoir brines, and two kinds of rocks, including reservoir rocks, were employed to investigate the effects of brine composition on oil recovery. Waterflood results indicated that dilute brine flooding improved oil recovery to a certain degree. Different waterflood behaviors were observed from two different crude oil/brine/rock systems. Waterfloods run on M crude oil/M brine/Berea rock showed that oil recovery by waterflooding was significantly improved by injection of dilute brine at the outset of a waterflood. For the CS crude oil/CS brine/Berea sandstone systems, residual oil of reservoir brine flooding could be further recovered by switching injected reservoir brine to dilute brine. Mechanisms of oil recovery improvement as well as increases in oil recovery and injection pressure were system-dependent. Brine salinity, salinity contrast, content of divalent ions in brine, potentially mobile fines, polar components in crude oil and initial water saturation were all important to the improved oil recovery by manipulation of injection brine composition. |
