Experimental Investigation of Low Salinity Waterflooding to Improve Heavy Oil Recovery from the Schrader Bluff Reservoir on Alaska North Slope

Abstract

Abstract Alaska North Slope (ANS) contains vast resources of heavy oil which have not been developed efficiently using conventional waterflooding. Recently, low salinity waterflooding (LSWF) has been considered to enhance oil recovery by reducing residual oil saturation in the Schrader Bluff heavy oil reservoir. In this study, lab experiments have been conducted to investigate the performance of LSWF in heavy oil reservoirs on ANS. Fresh-state core plugs cut from preserved core samples with original oil saturations have been flooded sequentially with high salinity produced water, low salinity water, and softened low salinity water. The cumulative oil production and pressure drop across the core plugs have been recorded by the AFS-300 coreflooding system. The oil recovery factors and residual oil saturation after each flooding have been determined based on material balance. In addition, restored-state core plugs saturated with heavy oil have been employed to conduct unsteady-state displacement experiments to measure the oil-water relative permeabilities using high salinity produced water and low salinity water, respectively. It has been found that the core plugs are very unconsolidated, with porosity and absolute permeability in the range of 33 – 36% and 155 – 330 mD respectively. Produced crude oil sample having a viscosity of 63 cP at ambient conditions was used in the experiments. The total dissolved solids (TDS) of the high salinity produced water and the low salinity water are 28,000 mg/L and 2,940 mg/L, respectively. After softening, the TDS of softened low salinity water has little change, but the concentration of Ca2+ has been reduced significantly. The residual oil saturations are reduced gradually by applying LSWF and softened LSWF successively after high salinity water flooding. On the average, LSWF can improve heavy oil recovery by 6.3% of original oil in place (OOIP) over high salinity water flooding, while the softened LSWF further enhances the oil recovery by 1.3% OOIP. The pressure drops observed in the LSWF and softened LSWF demonstrates more fluctuations than that in the high salinity water flooding, which indicates potential particle migration in LSWF. Furthermore, it was found that regardless of the salinity the calculated water relative permeabilities are much lower than the typical values in conventional rock-fluid systems, implying more complex interactions between the reservoir rock, heavy oil, and injected water. This study provides fundamental lab data for evaluating the technical and economic benefits of LSWF in heavy oil reservoirs on ANS.