Influence of carbonate impurities on smartwater effect: Evaluation of wettability alteration process by geochemical simulation

Low salinity water flooding (LSWF) has been considered as a promising technique for enhanced oil recovery (EOR). The wettability alteration towards a more water-wet state is recognised as the main mechanism for the positive LSWF effect. Electrokinetic interactions occurring at crude oil/brine and rock/brine interfaces affect the wettability alteration. Most of the studies reported in the existing knowledge considered synthetic calcite to understand the electrokinetics of natural carbonate with brines. The mineral impurities present in natural carbonate could influence the electrokinetics and subsequently wettability alteration. In this study, the surface complexation model, thermodynamic equilibrium model, and extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were combined to evaluate the effect of impurities (dolomite and anhydrite) in natural carbonate on wettability alteration. The surface complexation modelling parameters were first estimated by fitting the calculated zeta potential to that of measured value in various brines and then validated in smartwater. The thermodynamic equilibrium of impurities is largely insensitive to crude oil/brine interface properties. However, the calculated carbonate/brine surface or zeta potential changed from positive to negative with anhydrite equilibrium in the brines containing Ca2+ or Mg2+ ions. The attractive or repulsive forces between crude oil and carbonate were estimated from disjoining pressure using extended DLVO theory and compared with measured adhesion forces. The computed disjoining pressure was used as an indicator to evaluate wettability alteration. It is found that brine with SO42- ions can result in highest water-wet condition, followed by smartwater (brine with Ca2+, Mg2+, and SO42-) and brine with either Ca2+ ions or Mg2+ ions. The de-ionic (DI) water is unfavourable for wettability alteration. The effect of impurities equilibrium and the crude oil surface site densities on the disjoining pressure were also discussed. (C) 2021 Elsevier B.V. All rights reserved.