Effect of divalent cations in formation water on wettability alteration during low salinity water flooding in sandstone reservoirs: Oil recovery analyses, surface reactivity tests, contact angle, and spontaneous imbibition experiments

Abstract

This paper demonstrates the importance of Ca2+ in formation water on low salinity oil recovery, and the importance of Mg2+ in formation water with Ca2+ and alone. A series of Berea sandstone cores were flooded with formation water (FW) water as a secondary flood, then low salinity (LS) water was injected for the tertiary stage. The temperature of the experiments was 90 °C (reservoir temperature). Surface reactivity tests were run where water saturated cores were flooded with the same FW and LS water that was used in oil recovery experiments. While injecting brines, samples of the effluent were analyzed for pH, Mg2+, and Ca2+. We also measured contact angles between sandstone cores and crude oil in the presence of LS water and FW after some flooding stages. Oil recovery tests with double Ca2+ and Mg2+ (especially Ca2+) concentration in FW revealed a lower LS water EOR effect, indicating that the cores are apparently more water-wet. The LS water effect was much larger when the concentration of Ca2+ and Mg2+ in the FW was decreased by half. Additionally, the oil recovery due to LS water is greater for cores with Mg2+ abundance in the FW than for those with Ca2+. The LS water effect is greater as the Mg2+ exists. Surface reactivity tests, contact angle measurements, and spontaneous imbibition results were consistent with the oil recovery tests. Generally, contact angle measurements and spontaneous imbibition predict similar wettability trends because both tests depend on the same physics concepts. In this study, we claim that the smaller Mg2+ radius causes Mg2+ to be more tightly enveloped in water molecules than Ca2+, which might mean that Ca2+can actually get closer to the oil and mineral surfaces than Mg2+and have a bigger effect.The tests conducted in this study to investigate the role of divalent cations on LS water response should be routine practice as a supplement to conventional core flooding experiments. Such tests will give the operator clear guidance as to proper LS water design to maximize oil recovery for a given FW and core.