The recovery of residual oil from mature and non-producing reservoirs remains a major challenge owing to unfavourable mobility ratios and persistent rock fluid interactions. Although polymer flooding is a developed chemical enhanced oil recovery (cEOR) technique, the long-term performance of both synthetic and natural polymers is often limited by thermal degradation, salinity sensitivity, and shear-induced instability under reservoir conditions. These limitations highlight the development of polymer systems with modified molecular structures capable of enhancing mobility control and interfacial interactions. In this study, a chemically modified guar gum-based biopolymer was developed to overcome the limitations of conventional polymer flooding. The polymer structure was modified through a chemo-selective approach involving graft copolymerization, esterification, gel modification, crosslinking, and nanocomposite functionalization to achieve improved rheological stability and interfacial performance. Structural modification was confirmed by spectroscopic characterization, and the polymer system was systematically evaluated under reservoir-simulated conditions. Rheological investigations showed a significant enhancement in dynamic viscosity compared to native guar gum, along with stable pseudoplastic behaviour over a temperature range of 30-70 °C, showing improved resistance to gravitational flow and enhanced sweep efficiency. Interfacial studies showed a significant reduction in oil–water interfacial tension to 27 dyne/cm, accompanied by a decrease in surface tension, facilitating improved oil mobilization. Wettability alteration experiments on oil-aged carbonate rocks showed a clear transition from oil-wet to water-wet conditions. These findings were confirmed by flotation and qualitative wettability tests, including flotation and two-phase separation experiments, where polymer-treated carbonate powders migrated to the aqueous phase. The modified polymer also exhibited enhanced emulsifying activity with the formation of finer and more stable oil water droplets. The displacement efficiency of the developed polymer was further validated through oil reservoir simulating bioreactor (ORSB) flooding experiments conducted at 70 °C. Polymer flooding resulted in an additional oil recovery of more than 10% original oil in place (OOIP). The enhanced recovery performance results from the synergistic effects of viscosity enhancement, viscoelastic flow behaviour, interfacial tension reduction, wettability alteration, and stable emulsion formation. Overall, this study demonstrates that rational chemo-selective modification of natural polymers provides a sustainable and cost-effective pathway for developing high-performance EOR agents. The developed biopolymer system shows strong potential for deployment in mature and non-producing reservoirs under harsh reservoir conditions.

“Next-Generation Biopolymer Systems for Sustainable and Energy-Efficient Oil Recovery”