Smart waterflooding (SWF), an emerging EOR technique, was first investigated in sandstone reservoirs. However in recent years, the effect of smart water on carbonate reservoirs has gained wide interest. Despite the increasing number of research and publications in this area, the mechanism enhancing the recovery is still not fully understood. Wettability alteration and reduction in interfacial tension (IFT) have been identified as the two most dominant mechanisms. Divalent ions such as sulfate (SO42-) are found by many investigators to show promising results during SWF. However, limited research has been carried out to investigate the effect of trivalent ions. The objective of this study was to investigate the potential of phosphate (PO43-) spiked brines as a viable EOR technique in tight carbonate reservoirs.
Seawater with different dilution factors and varying concentration of phosphate ions were investigated against non-phosphate brines. Impact of these brines on wettability alteration was qualitatively monitored by tracking the rate of contact angle change on several aged-carbonate core samples subjected to varying aging periods. Various designed phosphate brines were tested at elevated temperature of 90 °C, and IFT was also measured at both ambient and elevated temperature (90 °C) to complement our findings.
To identify the impact of phosphate ions on wettability alteration due to brine/crude oil/rock interactions, carbonate core samples were prepared having initial contact angle ranging from 130° to 160°. The rate of contact angle change observed ranged from 9° to 36° as a function of varying composition of smart brines. A slight decrease in IFT was observed when lower salinity brines were used without phosphate. Phosphate spiked brines show a near surfactant reduction in IFT, compared to the low salinity brines. Smart brines with higher concentration of phosphate achieved better results than those with lower concentration.