Effect of Brine Salinity on Interfacial Tension in Arab-D Carbonate Reservoir, Saudi Arabia

Abstract

Abstract Carbonate reservoirs contain more than 50% of the worlds's hydrocarbons reserves. In Saudi Arabia, most of the current hydrocarbon production is from Arab-D carbonate reservoir (Upper Jurassic limestone). Oil production started 50 years ago. So, the various fields had been subjected to prolonged pressure support by waterfloods (since 1970) using high salinity brine (Arabian Gulf water). Oil recovery using waterflooding is about a third of the oil original in place (OOIP); while two-thirds still remain as a target for additional recovery. Low salinity waterflooding was introduced as an enhanced oil recovery technique from at least 65 years. Interfacial tension (IFT) between oil and brine during waterflooding has a significant effect on oil recovery and production strategy. So, this study was carried out to investigate the effect of brine dilution, temperature, and pressure on IFT of dead and recombined oil in Arab-D carbonate reservoir as the first phase in evaluating the potential of low salinity flooding technique to improve oil recovery. In this study, Arab-D reservoir brine with total dissolved solids (TDS) of 214,943 ppm was mixed with distilled water in two proportions resulting in a first solution with TDS = 107,906 ppm and a second with TDS = 52,346 ppm. For reservoir brine and each diluted brine, crude oil/water IFT was measured at reservoir conditions. Test results showed a decrease in IFT values for both dead and recombined oil as the volume percent of brine in the mixture decreased. IFT decreased with increasing temperature at constant pressure and increased with pressure at constant temperature. Such results show that low salinity flooding may be a good technique to improve oil recovery in Arab-D carbonate reservoir. This will require an intensive experimental coreflooding tests due to complexity of the Arab-D reservoir. Introduction Carbonate reservoirs make up about 20 % of the world's sedimentary rocks and contain 40 % of the world's oil. In Saudi Arabia, most of the current hydrocarbon production is from Arab-D carbonate reservoir (Upper Jurassic limestone). Various fields had been subjected to prolonged pressure support by waterfloods (since 1970) using high salinity brine (Arabian Gulf water). Oil recovery using waterflooding is about a third of the oil original in place (OOIP); while two-thirds still remain as a target for additional recovery. Hydrocarbon recovery depends mainly on the overall efficiency with which oil is displaced by some other fluid. When the fluids are brine and oil, this displacement is characterized by viscous and capillary forces, and by the original saturation and saturation history. It has been recognized that surface forces play an active part in the oil production and in determining the amount of unrecoverable oil. The magnitudes of these forces are governed by the value of interfacial tensions (Abrams, A., 1975). Low salinity waterflooding was introduced as an enhanced oil recovery technique 65 years ago. This technique received less attention than other enhanced recovery methods. The recovery mechanisms of low salinity waterflood are complex and not well defined especially for carbonate reservoirs. There are some investigators maintained that low salinity behaves in a fashion similar to alkaline flooding. Like alkaline flooding, low salinity reduces the interfacial tension between the reservoir oil and brine (Tang and Morrow, 1997, 1999). However, studies conducted to evaluate the effect brine salinity on interfacial and contact angle are very limited (Vijapurapu and Rao, 2003; Buckley and Fan, 2005).