Wettability Alteration by Low-salinity Water: pH-Dependent Response on Organic Acid Monolayer Structures

Abstract

Low salinity water flooding (LSWF) has emerged as a sustainable enhanced oil recovery technique, in which wettability alteration is the dominant mechanism. Wettability alteration is greatly affected by the initial adsorption structures of oil components in the presence of initially saturated water, but the controlling mechanisms remain unresolved. This study investigated the effects of the initial adsorption structures of organic acid monolayers on wettability alteration. Ca-bridged stearic acid (SA) monolayers were formed on muscovite in the presence of initially saturated water to mimic the initial surface condition of reservoir rock mineral. Atomic force microscopy (AFM) was used to evaluate the monolayer structure. Subsequently, contact angles of NaCl brine droplets of varying salinity and pH were measured in the n-decane. AFM confirmed that high Ca²⁺ and SA concentrations form mostly continuous monolayers, whereas lower concentrations yield patchy or islands of SA monolayer. The contact angle measurements show that the patchy adsorption promotes a pronounced LSW effect by many pathways, where LSW can easily access the water film under the SA monolayer. pH dominated the stability of the SA monolayers, as protonation of carboxylate at pH 2 causes desorption of SA. These findings highlight that LSW-induced wettability alteration is driven by the coupled effects of pH and the interfacial pathways to the initially saturated water under the layer of organic acids adsorbed to the mineral surface, suggesting a fundamental microscale mechanism for LSWF.