Tuning chitosan retention and performance in enhanced oil recovery via hydrophobic side chain modification

Adsorption of injected chemicals during enhanced oil recovery (EOR) applications plays a crucial role in effectiveness and economic feasibility of oil production. Hydrophobically modified chitosan (HM-chitosan) polymers, green-based polymers with interfacial activity capable of reducing interfacial tension and shifting wettability toward a more water-wet state, have shown great potential to replace fossil-based upstream chemicals. This study investigates the adsorption of two HM-chitosan polymers, grafted with alkyl chains of various lengths (i.e., –COC₅H₁₁ and –COC₆H₁₃) on two types of reservoir rock: sandstone (Berea sandstone) and carbonate (Indiana limestone and Austin chalk). Static adsorption experiments were conducted in both deionized (DI) water and seawater (SW) with results compared to those obtained for the pristine chitosan. The results reveal that salinity and pH have a significant impact on polymer adsorption, and that even slight variations in side-chain structure can lead to notable differences in performance across rock types and brine conditions. To better understand the molecular-level mechanisms, multiscale simulations were conducted. Semi-empirical GFN2-xTB simulations of chitosan oligomers revealed that surface charge and hydrophobic side chain modifications significantly influence the polymer conformation. Complementary DFT calculations elucidated key intermolecular interactions driving adsorption onto silica and calcite surfaces. Brine composition was also found to modulate polymer aggregation and adsorption efficiency. These results demonstrate the potential of hydrophobic side chain engineering to reduce polymer retention and improve chitosan performance in enhanced oil recovery (EOR) applications.