Implication of Oil/Brine/Rock Surface Interactions in Modeling Modified Salinity Waterflooding in Carbonate and Sandstone Reservoirs
Authors: Sanaei Alireza , Sepehrnoori Kamy 
Published: October 2018

Abstract
It is widely accepted that oil recovery during waterflooding can be improved by modifying the composition of the injected brine. A typical approach is diluting the formation water to a specific lower salinity. However, various recent experimental studies have shown the adverse effect of water dilution on oil recovery which depends on the rock composition and oil properties, especially in carbonates. In this study we investigated the effect of water chemistry on wettability and oil recovery by considering the complex interplay interaction of rock, brine, and oil system.
We used a coupled in-house compositional simulator and geochemical (IPhreeqc) framework for this study. Using this simulator we were able to capture true physics of the modified salinity waterflooding process. We modeled the wettability alterations as a function of zeta-potential between the oil-brine and brine-rock system. We calculated the surface charge at oil-brine and rock-brine interfaces as a function of surface complexation, ion exchange, oil acid and base numbers, and rock composition. Moreover, using DLVO theory, we calculated disjoining pressure and contact angle in a brine/oil/rock system and compared with recently published experimental data. For sandstones we assumed that multi-ion exchange and double layer expansion are the main mechanisms of modified salinity waterflooding. For carbonates, surface-charge change is the considered mechanism for wettability alteration. In order to validate our simulation approach, the results of our simulations were compared with experiments selected from recently published corefloods.
The results of this study indicated that DLVO theory can be used to qualitatively analyze the effect of water chemistry on wettability alteration in an oil/brine/rock system. By changing the water composition and zeta potentials we observed the trend of changing toward less attractive forces and a more water-wet surface. We observed that the divalent cations contribute more to wettability alteration as compared to monovalent cations. Moreover, the results of contact angle and comparison with the experimental published data indicated that although the calculated and measured values are not the same, but the change in the contact angle as the system changes is in a good agreement with experiment data. Our zeta potential calculations based on surface complexation model reproduced the experimental data of oil/brine, brine/calcite, and brine/sandstone zeta potential measurements. Our results of coreflood history-matching indicated that for sandstones, diluting the formation brine results in incremental oil recovery due to double layer expansion and multi-ion exchange. In carbonates, the change in surface charge and consequently contact angle as a result of changes in water chemistry is the underlying mechanism of low salinity waterflooding in carbonates. We believe this is the first study that a comprehensive compositional reactive transport simulator is used to assess modified salinity waterflooding in both sandstones and carbonates as a function of contact angle and zeta potential.