Wettability alteration is the principal low-salinity-effect (LSE) in many oil-brine-rock (OBR) systems. Our recent experimental results have demonstrated that wettability alteration by low salinity is slow. It is expected that the electrical behavior of oil/brine and rock/brine interfaces and the water film geometry control both the transient hydrodynamic pressure, and the time-scale of ionic transport in the film, thus the kinetics and degree of wettability alteration. In this paper, the electro-diffusion process induced by the imposed ionic strength gradient is simulated by solving Poisson-Nernst-Planck equations in a water film bound between two charged surfaces, using a finite element-based computational fluid dynamics method. Both the non-equilibrium electric-double-layer (EDL) pressure and the time-scale of diffusion under different plausible electrical boundary conditions (BCs) are determined. The numerical results show that electro-diffusion in the thin film is non-Fickian, strongly dependent on the electrical BCs, and significantly (10–20 times) slower than Fickian diffusion. Among various BCs, those which lead to the strengthening of the electrostatic force, or electric field (such as constant charge BC), are the most favorable in terms of observing LSE. Moreover, it is found that the contribution of the osmotic pressure in the vicinity of the pore (bulk) fluid is negligible and that Maxwell stress is the dominant source of EDL force build-up. This force can then trigger wettability alteration. Furthermore, while both film length and thickness influence the electrical interaction of interfaces, the film thickness affects mainly the EDL force rather than the rate of ionic transport. On the contrary, the film length has a significant effect on the time-scale of diffusion. The effect of the ionic strength gradient on the time-scale of diffusion and LSE is relatively minor. This study provides novel insights into the role of the electrical behavior of OBR interfaces and film phenomena in the rate of ionic transport and establishment of low salinity in the film. Thin film modeling is a means to develop predictive capability for LSWF, screen OBR candidates, and to determine favorable conditions to observe LSE. Moreover, the slow kinetics of LSE necessitates accounting for the time-effect in the experimental evaluation of LSWF.
|Colloids and Surfaces A: Physicochemical and Engineering Aspects
|Early online date
|1 Apr 2021
|Published - 5 Jul 2021