Elucidating Wettability Alteration on Clay Surface Contacting Mixed Electrolyte Solution: Implications to Low-Salinity Waterflooding
Authors:   The University of Texas at Austin, Silveira de Araujo Isa , Heidari Zoya ,   The University of Texas at Austin
Published: November 2024

Many mechanisms have been proposed in the literature to explain wettability alteration at low-salinity waterflooding. Examples of these mechanisms include electrical double layer (EDL) expansion and multicomponent ion exchange. However, no consensus has been reached on which one is the key mechanism in low-salinity enhanced oil recovery. Moreover, these mechanisms are poorly understood. Parameters such as salinity, electrolyte type, and the presence of clay minerals are often associated with the degree to which the injection of low-salinity water increases oil production. Therefore, an investigation of the geochemistry of the clay-fluid interface is crucial to understand the role of petrophysical properties such as wettability on oil production. We use molecular dynamics (MD) to (i) investigate, at an atomistic scale, wettability alteration mechanisms such as EDL expansion and multicomponent ion exchange, (ii) quantify the impacts of different types of electrolyte and its mixture at varying ionic strengths on interfacial properties between oil/brine/clay system, and (iii) investigate wettability alteration by means of water adsorption quantification. In this work, we investigate interfacial interactions of systems composed of oil/brine/clay. Clay is represented by illite, and brine is composed of water molecules and different electrolyte types, such as NaCl, CaCl, BaCl, and their mixtures at varied concentrations. Oil is represented in this system by heptane molecules. Initially, the modeled oil/brine/clay interface is composed of brine containing only one type of electrolyte. These systems will be studied at a range of chlorine concentration from 0.3 to 1.0 mol/dm3. Subsequently, we investigate the interfacial properties of the oil/water/clay where brine is composed of mixtures of electrolytes. MD simulations were performed at 330 K, and number density profiles of water, hydrocarbon, and ions inside the illite nanopores were computed. From the density profile of water, hydrocarbon, and ions perpendicular to the clay surface, the structure of the fluid was analyzed. As salinity increases, sodium cations tend to leach out potassium cations from the illite surface into the solution. The position of the adsorption planes of the ions was identified at different salinity levels. These adsorption planes do not change with salinity, indicating that EDL expansion might not be the dominant wettability alteration mechanism in low-salinity waterflooding. When mixtures of salts were analyzed, it was found that sodium ions are always fully hydrated by water molecules. However, divalent cations such as calcium are present in the stern layer and can form bridges between clay and hydrocarbon. Regarding the distribution of hydrocarbon in the nanopores, it is found that these organic molecules form aggregates inside the nanopore at all simulated ionic strengths. Despite being widely known as an efficient method for achieving enhanced oil recovery, the underpinning mechanism for wettability alteration at low-salinity waterflooding is still not fully understood. The outcomes of this work improve our understanding of the most effective mechanism in wettability alteration. Moreover, the molecular-scale simulations of clay, water, and hydrocarbon interactions occurring during enhanced oil recovery elucidate the role of different types of ions and the impact of salinity on this process.