Effect of Water Salinity and pH on the Wettability of a Model Substrate

While the wettability of oil reservoirs has been the focus of many studies, little is known as to whether, and to what extent, the wettability evolves during oil recovery by waterflooding. To this end, a model silicate substrate, namely glass, was treated by oil drainage of the surrounding salt solution and aging (representing the initial state), followed by oil displacement by a second salt solution (flooded state). The two states were analyzed by scanning electron microscopy of the oil components attached to the substrate and measurement of their influence on macroscopic contact angles. Initial-state wettability took the form of an incomplete asphaltenic film interrupted by nanoscale channels and pockets of trapped salt solution. The film was observed to remain fluidic and, on flooding, could retract and detach to leave a more incomplete coverage, usually of oil nanodroplets. The influence of pH of the initial and flooding solutions on these two states was generally opposite; high pH, at which oil-substrate repulsion is prevalent, tended to reduce film coverage in the initial state but aid its retention by the substrate on flooding. Contact angles on flooded substrates depended on this residual adhering nanoscale oil and on the ability of bulk oil to adhere by reconnecting to it. Again, the pH dependence of these two factors was opposite. The results suggested a possible supplementary mechanism for enhanced recovery by low salinity flooding.