Phase Behavior and Interfacial Properties of Salt-Tolerant Polymers: Insights from Molecular Dynamics Simulations

Mobilization of the capillary trapped oil beyond the conventional recovery methods is essential to enhance the oil extraction process and meet the global energy demands. Partially hydrolyzed polyacrylamide (HPAM) is a water-soluble polymer widely used to control the mobility ratio between the injected and displaced fluids to improve the sweep efficiency. However, its viscosifying capacity is substantially affected by the harsh reservoir conditions (high temperature and high salinity conditions). Using equilibrium molecular dynamics simulations and well-tempered metadynamics, we have investigated the effect of sidechain sulfonation on the polyacrylamide (PAM) phase behavior and interfacial properties. The sulfonated polymer exhibits weak interactions with the brine's cations; it interacts almost equally with Na+ and Ca2+, while it displays an even weaker interaction with Mg2+ ions. Contrarily, the pristine HPAM shows stronger and specific interactions with Ca2+ and Mg2+ ions. Furthermore, HPAM encapsulates the metal cations, which induce a significant conformational contraction, leading to viscosity reduction. Combining our results and the reported works of the literature, the salt tolerance and the thermal stability of this kind of polymer originate from the sulfonated sidechain-ion weak interactions and its slower hydrolysis compared to the acrylamide group. Finally, the interfacial properties of the sulfonated polymer at the oil-brine interface are examined. These findings can help to enrich our understanding of the polymer salt tolerance and thermal stability in order to design polymers with better performance.