Zeta Potential of the Crude Oil-Brine Interface and Implications for Controlled Salinity Waterflooding

Abstract

Summary It is commonly observed that improved oil recovery (IOR) by controlled salinity waterflooding (CSW) coincides with a change to a more water-wet state. An important parameter controlling the wetting state is the zeta potential of the mineral-brine and oil-brine interfaces, which controls the electrostatic forces acting between. Evidence suggests successful CSW is observed when the injection brine composition is modified so as to increase the electrostatic repulsion acting between the interfaces, leading to detachment of oil from the mineral surfaces and IOR on the core- to field-scale. Measurements of zeta potential at the mineral-brine interface have been reported using the streaming potential method (SPM) at representative reservoir conditions of temperature, salinity and wetting state. It is generally accepted that the mineral-brine zeta potential becomes more negative with a reduction in the injection brine salinity and/or specific ions. However, comparable zeta potential data for the oil-brine interface are scarce. Most report negative values at pHg6, increasing in magnitude with increasing brine dilution, however, these are typically made using commercially available electrophoretic methods which operate far from reservoir conditions, using suspensions of crude-oil droplets in dilute (l0.1M), single-salt species brines and ambient conditions. Here we report novel and systematic SPM measurements of zeta potential of the oil-brine interface in brines of high ionic strength and containing multivalent ions. A strongly oil-wetting, hydrophobic porous substrate was prepared and coated with the crude-oil of interest. The SPM was used to measure the zeta potential of these substrates when saturated with brines of interest to CSW. We find the zeta potential is negative in simple NaCl brines (up to 2 Mol/L) with the magnitude increasing with decreasing brine salinity, consistent with previous literature. The concentration dependence of a given oil depends on properties such as the acid number and base number. Increasing the concentration of specific ions such as Ca2+ causes the zeta potential to become more positive and for some oils can invert the polarity. The sensitivity of the crude-oil to potential determining ions such as Ca2+ varies significantly dependent on the oil properties. Integrating these measurements with CSW corefloods, we find diluting the injection brine yields IOR only when the zeta potential of the oil-brine interface is negative. In samples where the oil-brine zeta potential is positive, no IOR was observed, consistent with the hypothesis that IOR is caused by an increase in the repulsive electrostatic force between the mineral-brine and oil-brine interfaces.