Model selection for dynamic interfacial tension of dead crude oil/brine to estimate pressure and temperature effects on the equilibrium tension

Abstract

Interfacial tension (IFT) measurements using pendant drop techniques are typically time-dependent and embed the underlying physics that can be exploited. A physics-based modeling approach of the time-varying IFT is chosen to estimate the equilibrium IFT and to identify which phenomena are leading the dynamic. This approach is both consistent and reproducible, hence advantageous for routine research and industrial applications because typical approaches are especially challenging when the techniques rely on long-time measurements or detailed characterization of phase composition and transport properties. The approach is applied to the Brazilian pre-salt dead crude oil/brine system when the equilibrium between the phases and between bulk and interface concentrations are not assured initially. Selected models based on a different physical phenomenon, mainly focused on diffusion and sorption, and with a particular time scale behavior are tested on measured data. The model which better fits the data in its full-time range should point out which phenomenon is dominant. The results indicate that the model with t time scale seems preferable for this oil/brine system, which is associated with a diffusion-controlled process in multiple regimes, for instance, bulk-to-interface diffusion of naturally present interface chemicals and their rearrangement at the interface. The effect of pressure (14.7 psi–4000 psi) and temperature (22 °C–60 °C) variations on the equilibrium IFT of the Brazilian pre-salt reservoir dead crude oil/brine at various conditions is characterized and discussed using the consistent physics-based estimate for the equilibrium IFT. The results show that the equilibrium IFT does not change significantly with pressure and increases with temperature, further supporting the identified presence of naturally present interface chemicals in the transient IFT measurements and the brine ionic valence influence. Furthermore, the wettability of carbonate rock samples with different mineralogical compositions to the dead crude oil/formation water is assessed using adhesion tension which embeds both IFT and contact angle contributions. Pressure changes do not affect wettability considerably, while the effects of temperature increase show that every sample tended to be wetter to its respective fluid affinity, following the IFT temperature behavior.