Nanoparticle to Surfactant Transition Behavior and EOR Performance of an Alkali/Styrene-Acrylate Copolymer Emulsion Binary Flooding System

During surfactant flooding, the significant adsorption loss on rock surfaces often results in a lower-than-expected oil displacement efficiency when the surfactant reaches the target reservoir area. This paper proposes a novel binary composite flooding system composed of a styrene-acrylate copolymer emulsion and alkali. As the acrylate in the styrene-acrylate copolymer emulsion hydrolyzes, the composite system gradually transitions from a water-insoluble polymer nanoparticle dispersion to a polymeric surfactant. By use of the weak adsorption of polymer nanoparticles on rock surfaces, more surfactants can reach the target area. This work first optimized the preparation conditions of the styrene-acrylate copolymer emulsion and measured its static adsorption on quartz sand. Then, the solution properties of the styrene-acrylate copolymer emulsion + alkali binary composite system were studied. Finally, the injectivity and oil displacement performance of the binary composite system in different states (0, 8, and 24 h of hydrolysis) were investigated. Based on the experimental results, the recommended synthesis conditions for the styrene-acrylate copolymer emulsion are acrylate as methyl acrylate (MA), MA to styrene mass ratio of 8:2, initiator dosage of 0.2% of the total monomer mass, reaction temperature of 75 °C, total monomer concentration of 10%, emulsifier dosage of 1.2% of the aqueous phase mass, and reaction time of 4 h. Adsorption experiments showed that the adsorption of the styrene-acrylate copolymer emulsion is lower than that of commonly used surfactants. Injectivity experiments demonstrated that the binary system in different states exhibits good injectivity in cores with gas permeability ≥ 100 mD. When the core permeability was 600 mD, oil displacement experiments indicated that the 24 h hydrolyzed binary system increased the oil recovery by 23.41%, higher than the binary system in other states.