Electro-Kinetic Induced Wettability Alteration for Enhanced Oil Recovery in Carbonates

Abstract

SmartWater and alkali/surfactant assisted chemical flooding processes are gaining some attention for enhanced oil recovery in carbonate reservoirs. The wettability alteration toward water-wet conditions caused by electro-kinetic induced favorable surface charge alteration is one main mechanism responsible for oil recovery in these methods. In this experimental investigation, the impact of electro-kinetic induced wettability alteration in carbonates was studied by measuring zeta potentials at calcite/brine and crude oil/brine interfaces using phase analysis light scattering technique. Two types of alkalis (NaOH and Na2CO3) were used at a dilute concentration of 0.1 wt%. Four different low salinity water recipes representing SmartWater (10-times reduced ionic strength high salinity brine), NaCl, Na2SO4, and MgSO4 brines were chosen at a fixed salinity. The results showed that alkali can increase the magnitude of zeta potentials at both interfaces towards more negative for Na2SO4 brine. Only a slight increase in negative zeta potentials was observed with NaCl brine. Na2CO3 effects on the zeta potential of calcite is found to be more pronounced when compared to NaOH due to the availability of carbonate ions as potential determining ions. In contrast, NaOH showed an improved effect on the zeta potentials of crude oil as NaOH is a stronger alkali to favorably result in the dissociation of the acids from crude oil. These results indicate that Na2SO4 brine is better suited with alkali whereas NaCl brine and SmartWater can synergistically combine with anionic surfactant to electrokinetically induce wettability alteration in carbonates. This work for the first time evaluated electro- kinetic induced wettability alteration effects in chemical flooding processes by measuring zeta potentials at calcite/brine and oil/brine interfaces. The new knowledge gained from this electro-kinetic study will provide useful practical guidelines on how to select right water chemistries for waterflooding and chemical enhanced oil recovery (EOR) processes in carbonate reservoirs.