Molecular simulation of double layer expansion mechanism during low-salinity waterflooding

Abstract

Low-salinity waterflooding is an improved oil recovery technique whose mechanisms are still elusive. Double Layer Expansion (DLE) is one of the main proposed mechanisms which have been settled in the petroleum literature. However, recent molecular simulation studies have questioned its validity. In this study, we discuss the current understanding of DLE mechanism and design molecular simulations to investigate it at the molecular level. We used kaolinite and montmorillonite substrates in contact with water having varying concentrations of monovalent and divalent ions. Our results confirm that the location of the adsorption planes is independent of the ionic strength. However, the electric potential developed over these surfaces and how it decays depends on both the ionic strength and ion type. A shrinkage is observed in the double layer for the case of low salinity, supported by both film thickness estimations and interaction energy analysis. This shrinkage, which contradicts the prevailing assumption, is consistent with molecular simulation literature, and casts some doubts on the efficiency of double layer expansion as a mechanism for improved oil recovery observed during low-salinity waterflooding.