Mechanistic study of the effects of dynamic fluid/fluid and fluid/rock interactions during immiscible displacement of oil in porous media by low salinity water: Direct numerical simulation

Abstract

Low salinity waterflooding (LSWF) is a process in which by lowering the ionic strength and/or manipulation of the composition of the injection water, the long term equilibrium in oil/brine/rock system is disturbed to reach a new state of equilibrium through which the oil production will be enhanced due to fluid/fluid and/or rock/fluid interactions. In spite of recent advances in the simulation of the LSWF at core scale and beyond, there are very few works that have modelled and simulated this process at the pore scale specially using direct numerical simulation (DNS). As a result the effects of wettability alteration and/or Interfacial Tension (IFT) change on the distribution of the phases at pore scale, as well as dynamics and mechanisms of the oil displacements in porous media are neither thoroughly investigated nor well understood. With this objective, the present study takes into account modelling both rock/fluid (dynamic contact angle) and fluid/fluid (dynamic oil/water IFT as well as diffusion between high salinity and low salinity water) interactions. Additionally, the proposed model is capable to effectively capture the time scale pertinent in the wettability alteration by LSWF via reaction rate and near solid surface diffusion coefficients. The model is developed and incorporated into OpenFOAM and both the Navier-Stokes equation for oil/water two-phase flow and the advection-diffusion equation for ion transport are solved. The developed model is validated against one dimensional (1D) sinusoidal micromodel experiments reported in the literature, and then applied to two dimensional (2D) heterogeneous micromodel to investigate the sole effect of wettability alteration as well as its coupled effect with IFT variations. Different degrees of contact angle alteration, different trends of IFT variations reported in the literature, as well as different injection scenarios (secondary or tertiary) are considered. 1D sinusoidal micromodel simulations visualize a dominant dynamic recovery mechanism which boosts the low salinity effect (LSE) via coalescence of the detached or partially detached residual oil blobs which forms a large oil ganglion towards the downstream and can effectively produce the residual oil saturation at shorter period of time compared to the case if such a coalescence didn't happen. The sensitivity analysis in 2D heterogeneous micromodel shows that the performance of the LSWF is a function of the degree of wettability alteration as well as IFT variations. Although the IFT decreasing trend with salinity can enhance the oil production through synergetic effect, the IFT increasing trend has a detrimental effect on the oil production and surpasses the effectiveness of wettability alteration interactions. The results confirm the importance of fluid/fluid interactions such as IFT variation on the LSE which has been previously ignored in numerical simulation LSWF altogether. The presented simulations shed the lights to the reasons behinds some of the discrepancies reported in the literature.