Low-salinity waterflooding (LSW) is an enhanced technique that improves oil recovery by lowering and optimizing the salinity of injected water. Previous research demonstrated that the initial water saturation and oil aging during LSW affect oil recovery. This work aims to validate and quantify these effects.
This laboratory study investigated the effect of injected brine, initial water saturation, and crude oil aging on relative permeability and low-salinity waterflooding performance in sandstone reservoirs. A set of comprehensive coreflood tests have been conducted using 3″ Bandera sandstone cores to examine the effect of initial water saturation and injection brine on the relative permeability curves. Additionally, six coreflood experiments were conducted to examine the effect of the connate water salinity variation on the relative permeability end points.
Four spontaneous imbibition experiments were performed using 6 in. length Buff Berea sandstone samples to investigate the effect of initial water saturation and aging on the performance outcome of low-salinity waterflooding. Two spontaneous imbibitions were performed using 20 in. length core samples. The objective of these two experiments was to study the effect of initial water saturation distribution across the core on the oil production using X-ray computed tomography.
Results obtained from coreflood and spontaneous imbibition tests suggest injected brine and initial water saturation have predominant influence on the oil recovery performance. As the initial water saturation increased from 23 to 41%, the oil recovery increased from 43.8 to 48.4% of OOIP using low-salinity waterflooding. Comparison between the high-salinity relative permeability and low-salinity water relative permeability showed that the end-point water relative permeability slightly decreased for the cores after using low-salinity brine as injected brine compared to the end-point water relative permeability after flooding with high-salinity brine for Bandera sandstone. The end-point relative permeability to water increased from 0.07 to 0.39 when the reservoir connate water salinity increased from 4,633 to 174,156 ppm for injected brine salinity of 500 and 5,000 ppm, respectively for Buff Berea sandstone. The un-aged core plugs with higher initial water saturation provided oil recovery higher than that with lower initial water saturation by 5% of OOIP for the low-salinity waterflooding.