A Modified Derjaguin-Landau-Verwey-Overbeek (DVLO) Model Accounting for Steric Effects at High Ionic Strength: Implications for Low Salinity Waterflooding

Abstract

Summary Traditionally, the Poisson-Boltzmann equation is solved to describe the electrical potential in the diffuse layer of an electrolyte adjacent to a charged interface and the electrostatic contribution to the total interaction potential between the interface and the ionic species. A major assumption of the Poisson-Boltzmann equation is that ions act as point charges, which allows for an infinite ion and charge density near to the charged interface, and thus predicts that the zeta potential falls to zero at high ionic strength (typically >0.1M). However, experimental measurements have reported small but measurable zeta potential values at high ionic strength, showing the zeta potential does not tend to zero as predicted. Electrostatic forces acting between electrically charged mineral-brine and oil-brine interfaces in the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory have been used to explain observed increases in oil recovery during low salinity waterflooding. Injection of a low salinity brine is expected to create a more negative zeta potential at the mineral-brine interface. If the oil-brine interface is negatively charged, this increases the electrostatic repulsive force per unit area which, if larger than the disjoining pressure, should lead to improved oil recovery. However, the calculation of the electrostatic forces are normally based on the traditional Poisson-Boltzmann model which underestimates the electrostatic contribution at high salinity. Here, a modified Poisson-Boltzmann equation ( Borukhov et al., (1997) , Physical Review Letters, 79(3), 435), which accounts for steric effects in the diffuse layer at high salinity but recovers the original Poisson-Boltzmann equation at low salinity, has been combined with a triple layer model which accounts for charge in the Stern layer ( Revil et al., (1999) , Journal of Geophysical Research, 104). This combined model has been used to match experimental zeta potential measurements made on natural, intact sandstones across the ionic strength range 10–5 – 5M, including small and constant zeta potentials observed at ionic strength >0.4M. The effect of this modified Poisson-Boltzmann model on the total interaction potential and DLVO theory has further been investigated. Our relatively simple modification shows that the electrostatic forces at high salinity are larger than previously thought and should not be neglected when calculating total interaction forces. Previous models using classic DLVO theory for understanding low salinity waterflooding may be inaccurate as they incorrectly estimate the changes in the electrostatic forces that occur during injection of low salinity brines.