Waterflood Performance by Injection of Brine With Different Salinity for Reservoir Cores

Both laboratory experiments and field tests indicate that waterflooding with low salinity brine can improve oil recovery. The brine composition and the extent of dilution along with the oil phase and rock properties are all important in the process. Two consolidated reservoir cores with permeability of about 600 md were waterflooded up to 5 cycles by injection of high salinity formation brine of 29,690 ppm, low salinity brine of 1,479 ppm, and two concentrations of sodium chloride. Each core was tested with two crude oils. Either the more viscous crude oil or a viscous mineral oil was used to establish the desired initial water saturation at which mixed wettability was subsequently induced by adsorption from crude oil at reservoir temperature. Displacement tests showed that tertiary recovery from mixed wettability cores by injection of the low salinity brine was significantly higher (7 to 14% OOIP) than given by injection of the high salinity brine. Increased oil recovery was accompanied by slight change in the pH of the effluent brine and sharp increase in the pressure drop across the cores followed by slower decline. No clay was observed in the production stream and the oil/brine interface in the oil/brine separator was clear. No tertiary response occurred when the injection brine was changed to 8000 ppm NaCl solution. Further lowering to 1500 ppm NaCl resulted in 13% OOIP recovery. An additional 4% OOIP recovery was obtained by injection of low salinity brine. For a core that gave strong tertiary response, after replacement of crude oil by refined oil there was only slight recovery and small pressure response to low salinity brine flooding, even though the core was mixed wet because an adsorbed film still remained on the rock surface. Reversion to crude oil as the oil phase restored strong tertiary recovery and pressure response. A large increase in oil recovery was observed for secondary recovery by injection of low versus high salinity brine. Increase in pressure drop across the core was closely related to increase in oil recovery for both secondary and tertiary modes.