Chemical properties and interfacial behavior of fluids during enhanced oil recovery under direct current electric fields

Carbonate reservoirs are characterized by low oil recovery and complex residual oil distribution due to their strong heterogeneity. Electro-assisted enhanced oil recovery technology has emerged as a promising, efficient, and environmentally friendly approach for improving oil displacement efficiency. However, the oil-water-rock interfacial characteristics remain a critical challenge in optimizing the performance of electro-assisted oil recovery. In this study, long-core displacement experiments integrated with nuclear magnetic resonance fluid distribution characterization were conducted, along with interfacial tension, wettability, Zeta potential, and molecular functional group analyses, to investigate the mechanisms by which the timing of electric field application influences oil-water-rock interfacial behavior. The results indicate that applying an electric field after the water-free oil production period is the most effective strategy, leading to a 16.90 % increase in oil recovery compared to conventional water flooding. Early-stage electric field application suppresses the formation of preferential water flow channels, effectively delaying water breakthrough and enhancin displacement efficiency. Furthermore, the introduction of an electric field significantly reduces injection pressure and enhances oil mobilization in various pore structures. Specifically, in the water-electric coupled flooding mode, the peak pressure decreased from 11.47 MPa (water flooding) to 5.74 MPa, while post-water-flood electric field application improved low-permeability oil mobilization to 40.98 % and increased macropore oil displacement efficiency from 45.30 % to 49.89 %. The enhanced oil recovery performance is primarily attributed to electric field-induced interfacial tension reduction and wettability alteration, which enhance displacement efficiency, expand sweep efficiency, and improve oil film detachment. Additionally, molecular-scale electrochemical processes facilitate hydrocarbon restructuring, indicated by an increase in asphaltene content to 10.77 % in the produced oil and a 30 % reduction in the COOH/COO^- ratio, which stabilizes interfacial activity, increases rock surface hydrophilicity, and improves crude oil mobility. This study integrates macroscopic oil displacement experiments with microscopic interfacial mechanism analysis to elucidate how electric fields modulate oil-water-rock interactions and influence fluid distribution in porous media. By examining the chemical reactions and compositional changes induced by electric fields, this research provides new insights and theoretical guidance for the electro-assisted development of carbonate reservoirs.