In the past several years, a number of papers have attempted to explain the phenomenon of improved oil recovery due to low-salinity waterflooding from the perspective of the thin brine films that wet the surfaces of the pores. Because of the complicated, multiscale nature of low- salinity waterflooding (LSW), the studies on this topic have been largely limited to phenomenological observations and explanations. Improving our understanding of thin liquid film thermodynamics is an important step towards developing our knowledge of LSW and optimizing the effect of brine chemistry on oil recovery.

In this work, we analyze two fundamental thermodynamic problems relating to flat thin liquid films. One problem concerns the expression for the Gibbs energy of the films. Thin liquid films undergo film tension work. There are conflicting results in the literature over whether to include the film tension work in the Legendre transform that relates the Gibbs energy to the internal energy of the films. We show that for the purposes of finding the equilibrium state, the Legendre transform must include the film tension term so that the chemical potentials are equivalent to the partial molar Gibbs energy. The second problem we examine involves the augmented Young equation, which is a condition for mechanical equilibrium involving the contact angle at the boundary between a thin liquid film and the bulk liquid droplet from which the film is formed. Two different expressions for the augmented Young equation have appeared in the literature. We show that under certain assumptions, the two expressions can be made equivalent.

We then briefly discuss how fundamental thermodynamic relations of thin liquid films can help explain the wettability alteration observed in LSW studies. We conclude with a summary of our main results.

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