Surface charge change in carbonates during low-salinity imbibition

Area of Science:

• Petroleum Engineering
• Geochemistry
• Physical Chemistry

Background:

• Low-salinity water injection is recognized for improving oil recovery in laboratory settings, but the underlying mechanisms remain debated.
• Existing research often emphasizes brine composition, overlooking the crucial role of rock-fluid interactions in carbonate reservoirs.

Purpose of the Study:

• To investigate the influence of diverse carbonate rock materials on surface charge, wettability alteration, and spontaneous imbibition.
• To elucidate the impact of specific ions (calcium, magnesium, sulfate) on rock surface charge and wettability.
• To develop a numerical model that links surface charge changes to capillary pressure and oil recovery.

Main Methods:

• Zeta potential measurements were conducted to analyze the surface charge responses of different carbonate rock materials to various brine salinities (Formation water, Seawater, Diluted Seawater).
• Spontaneous imbibition tests were performed to evaluate oil recovery and wettability alteration under different brine conditions.
• A numerical model was developed, incorporating the distinction between computed surface charge and experimentally determined zeta potential, and linking surface charge changes to capillary pressure alterations.

Main Results:

• Each carbonate rock material exhibited unique surface charge responses to different brines, with varying sensitivity to calcium, magnesium, and sulfate ions.
• A decrease in the system's surface charge compared to the initial carbonate-formation water system correlated with increased oil recovery and a shift towards water-wet conditions.
• The numerical model successfully reproduced the experimentally observed ion effects on zeta potential and history-matched spontaneous imbibition tests by linking surface charge to capillary pressure.

Conclusions:

• Carbonate rock material plays a critical role in wettability alteration and oil recovery during low-salinity water injection.
• Accurate modeling requires distinguishing between computed surface charge and zeta potential, considering the shear plane location.
• While laboratory-scale wettability alteration is observed over weeks, field-scale molecular diffusion-driven alteration is estimated to take centuries.