Abstract
Many researchers have observed an increase in ultimate oil recovery by injecting low salinity brine in laboratory and field experiments.
In this work, linear laboratory corefloods were conducted to study the effect of low salinity waterflooding on oil recovery rate, residual oil saturation and relative permeability. We conducted 21 different waterfloods using nine brines, three crude oils, six outcrop Berea cores and two oil reservoir cores. Every core used was at least 11 long and pressure taps were use to measure pressure drop across sub-sections of the core.
Increase in ultimate recovery of crude oil with decrease in salinity of injected brine was observed in several, but not all, cases. These results suggest that that injecting low salinity brine changes the wetting state of some cores from mixed to water-wet and that this change improves the ultimate oil recovery. This hypothesis of a changing wettability is supported by the following observations: (1) When low salinity injection produced higher oil recovery, the end-point water relative permeability kw° decreased and post-waterflood end-point relative permeability to oil kro° increased. (2) Injecting low salinity brine produced a persistent wettability alteration (toward water-wet) that eliminated oil recovery salinity dependence in subsequent floods. (3) The oil recovery salinity dependence noted in (2) could be restored by extended aging (thus restoring mixed-wetness). (4) Injecting low salinity brine improved recovery by delaying breakthrough, suggesting an improvement in displacement stability.
