Understanding the impact of water salinity and ionic compositions on rock-fluids interactions and subsequently wettability plays a major role to determine the performance of different recovery processes in carbonate formations. Various studies have shown that surface electric charge manipulation is the main driving mechanism behind wettability alteration observed in controlled ionic composition waterflooding (CICW) processes. Therefore, investigation of electrokinetic interactions at both brine/calcite and brine/crude-oil interfaces is important to optimize the injection water compositions used for chemical enhanced oil recovery (EOR) in carbonates.

In this investigation, the electrokinetic interactions of different surfactants at calcite/brine/crude-oil interfaces are studied using Surface Complexation Modeling (SCM) approach. First, the three low salinity water recipes of NaCl brine, Na2SO4 brine, SmartWater, and a high salinity water (HSW) are analyzed as baseline for zeta-potential comparison. The salinities of low salinity water recipes are fixed at the same salinity as 10-times diluted high salinity water. Then, four different surfactants are added at 0.1 wt% concentration to the brine recipes, where the first two surfactants are anionic, the third one is amphoteric, and the fourth one is non-ionic surfactant. The SCM results are compared with experimental zeta potential measurements for calcite/brine and crude oil/brine suspensions in different aqueous solutions containing surfactants.

The SCM results reasonably matched the experimental zeta potential data trends obtained with different surfactants at brine/calcite and crude-oil/brine interfaces. Both the anionic surfactants altered the zeta-potential values of brine/calcite and crude-oil/brine interfaces towards more negative in all brine recipes. This impact is found to be more pronounced for SmartWater and HSW. For amphoteric surfactant that includes both anionic and cationic charges, the opposite trend is observed. The zeta potentials became less negative at calcite/brine and oil/brine interfaces thereby making it unattractive for chemical EOR in carbonates. The negative surface charge of NaCl, and Na2SO4 brines decreased when non-ionic surfactant is added to the aqueous solution. However, much favorable effect is observed with HSW in conjunction with non-ionic surfactant, wherein the zeta-potential magnitudes became increasingly negative at the two interfaces. In SCM framework, the trend is accurately captured by reducing the surface equilibrium reaction constants for divalent cations (Ca+2, and Mg+2) to result in less adsorbed concentrations of divalent cations at the interfaces. This assumption can be rationalized by the optimal phase behavior of non-ionic surfactant observed in HSW to further explain such high effectiveness from electrokinetics perspective.

The novelty of this work is that it captures the electrokinetic interactions of different surfactant chemicals at calcite/brine/crude-oil interfaces besides validating the SCM results with experimental zeta potential data. These modeling results will provide new insights on defining optimal water compositions to synergize with surfactants and further improve oil recovery in carbonate reservoirs.

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