Smart water EOR effects in preserved sandstone reservoir cores, comparison between sea water and low salinity brines at 136°C
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Low salinity waterflooding is an emerging EOR technique that injects water at significant lower ions concentration as compared to the formation water. Laboratory experiments and field tests show that it can enhance the oil recovery over conventional higher salinity waterflooding. Until now, the mechanism behind low salinity waterflooding is under consideration for further discussions, but it is generally accepted that low salinity waterflooding improves microscopic sweep efficiency by modifying rock wettability. For low salinity condition, it has been suggested that desorption of polar oil components as result of pH increase makes the rock more water-wet. In this thesis, three coreflood experiments were performed to determine the effect of different water salinities on the oil recovery. Two homogeneous reservoir cores which contain active clays with crude oil which has enough polar organic compounds were used during the experiments. Formation water salinity was 60,461 ppm while the injected brines were modified sea water, (SWm) 30,122 ppm, and modified low salinity brine, (LSm) 1,538 ppm. All experiments were conducted at reservoir temperature, 136°C. Coreflood effluents were sampled regularly to investigate crude oil-brine-rock interactions by measuring pH, density, and different ions concentration of produced water. The oil recovery factor by using SWm injection was 51% of OOIP. Increased oil recovery was observed during LSm injection, by 12% in the secondary mode (51% compared to 63%), and 9% in the tertiary mode after SWm injection (51% compared to 60%). Also the ultimate recovery was reached much faster using LSm in the secondary mode in comparison to the tertiary mode. The pH increase by performing SWm injection was only 0.4 pH unit while LSm injection resulted in 1.5 pH unit. Even though most experiments in the literature are done at temperature below 100°C, this study shows that there is also a possibility to see low salinity EOR effect at high temperature, up to 136°C.
Master's thesis in Petroleum engineering