Zeta Potential in Intact Carbonate Samples: Impact of Brine Composition, Temperature and Wetting State with Application to Controlled Salinity Waterflooding

Summary Laboratory experiments and field trials have shown that oil recovery from carbonate reservoirs can be increased by modifying the injected brine composition in a process termed controlled salinity water-flooding (CSW). However, the mineral- to pore-scale processes responsible for improved recovery during CSW remain ambiguous and there is no method to predict the optimum CSW composition for a given crude oil/brine/rock (COBR) system. The zeta potential is a measure of the electrical potential at mineral-brine and oil-brine interfaces and controls the electrostatic forces acting between these interfaces. Measured values of zeta potential at the mineral-brine interface in carbonate rocks remain scarce, particularly at reservoir conditions of temperature, salinity and wetting state. Moreover, there are no measured zeta potential data for the oil-brine interface at these conditions. Here, we report zeta potential measured using the streaming potential method and intact samples of outcrop and reservoir carbonates. Measured values were obtained in clean, water-wet samples and in the same samples after aging in two different crude oils. Measurements were conducted at laboratory temperature, or at elevated temperatures >70°C. In some samples, CSW was then conducted to determine whether changing the injection brine composition increased recovery. The measured zeta potential in clean samples saturated with synthetic formation brine was consistently positive across all the samples tested, while zeta potential in the same (clean) samples saturated with low salinity brine (dilute seawater) was consistently negative, irrespective of temperature. Consequently, changing the brine composition during CSW from formation brine to low salinity brine in these samples is expected to invert the polarity of the zeta potential. After aging in crude oil and formation brine, the zeta potential consistently became more positive in samples aged in one crude oil, and less positive in samples aged in the other crude oil. This result can be interpreted in terms of the zeta potential at the oil-brine interface: the crude oil yielding a more positive sample zeta potential after aging has a positive zeta potential at its interface with the brine; the crude oil yielding a less positive sample zeta potential after aging has a negative zeta potential at its interface. Injecting low salinity brine yielded improved recovery with the ‘negative’ oil, but no response with the ‘positive’ oil, consistent with the hypothesis that improved recovery follows from an increase in the repulsive electrostatic force acting between mineral-brine and oil-brine interfaces.