A mechanistic investigation of low salinity water flooding coupled with ion tuning for enhanced oil recovery

Abstract

Oil recovery by low salinity water flooding (LSWF) from carbonate reservoirs has gained tremendous attention in recent years due to its cost-effectiveness and environment-friendly nature. The mechanisms of low salinity water flooding for enhanced oil recovery are very complex and depend on the mineralogy of the formation rock, properties of injection brine and reservoir fluids. The present work aimed at the optimization of salinity and concentration of potential determining ions (PDIs) in injection water for enhanced oil recovery from carbonate reservoirs. Initially, we conducted a series of experiments on the dilution effect of seawater (SW) with the help of rock/fluid and fluid/fluid interactions via interfacial tension (IFT), zeta potential and contact angle measurements. This offered an optimum salinity (20dSW) with an 11% increase in recovery of the original oil in place (OOIP) over the SW injection in secondary flooding mode. Then, the ion tuning was done on the optimum salinity (20dSW) by manipulating the PDIs (Ca²⁺, SO₄ ^2- and Mg²⁺) while keeping ionic strength constant. The properties of ion tuned brine were optimized by zeta potential and contact angle measurements. The core flooding experiments performed with the injection of designed ion tuned water obtained by dilution and ion tuning of SW showed more than 20% OOIP as incremental recovery over the SW injection. Effluent analysis after the flooding confirms that the main mechanisms for enhanced oil recovery include calcite dissolution and wettability alteration due to interplay of PDIs.

Fig. 1. Proposed methodology of experiment.

Fig. 2. (a) IFT effect on dilution of SW (b) pendant drop image of lowest IFT at 90 °C.

Fig. 3. (a) Effect of salinity on zeta potential. (b) Effect of pH on zeta potential at rock/brine interface with salinity. (c) Effect of pH on zeta potential at oil/brine interface with salinity at 25 °C.

Fig. 4. Effect of salinity on contact angle at 90 °C.

Fig. 5. Oil recovery diagram for injection of SW, 20dSW and 40dSW in secondary mode at 90 °C.

Fig. 6. Effluent analysis of (a) SW (b) 20dSW (c) 40dSW.

Fig. 7. Schematic mechanisms prevailing at 20dSW oil recovery (a) injection of low salinity water (b) calcite dissolution and interplay of PDIs taking place.

Fig. 8. Ion tuning effect of PDIs on IFT at optimum dilution.

Fig. 9. Variation of zeta potential (a) rock/brine interface (b) oil/brine interface of ion tuned brine at optimum dilution of 20dSW at 25 °C.

Fig. 10. Time variant change in contact angle at 90 °C for (a) Ca²⁺ ion tuning (b) Mg²⁺ ion tuning and (c) SO₄²⁻ ion tuning.

Fig. 11. Oil recovery diagram for injection of 20dSW, 20dSW × 3Mg²⁺ ion and 20dSW × 2Ca²⁺ ion tuned brine in secondary mode at 90 °C (ion tuning effect coupled with dilution).

Fig. 12. Effluent analysis of (a) 20dSW × 3Mg²⁺ (b) 20dSW × 2Ca²⁺.

Fig. 13. Comparison of oil recovery obtained both by dilution and ion tuned water flooding of SW, 20dSW, 20dSW × 3Mg²⁺ and 20dSW × 2Ca²⁺ brines.