MICROSCOPIC MECHANISMS OF DISPLACEMENT ENHANCEMENT IN POROUS MEDIA BY NANOPARTICLE-SURFACTANT SYNERGY

Multiphase displacement in porous media is of great scientific significance and practical relevance to applications such as soil remediation, geological carbon sequestration, and hydrocarbon recovery. Although nanoparticle suspensions and surfactant systems have been widely used for wettability alteration and displacement enhancement, the applicability and mechanical origin of their synergistic effects remain under debate. Based on microfluidic visualization experiments and microscopic characterization, combined with theoretical analysis and topological statistics, this study systematically investigates the mechanisms of wettability alteration and displacement enhancement by the nanoparticle-surfactant composite system. The results show that nanoparticles substantially alter surface wettability via adsorption on solid surfaces; however, under displacement interfacial conditions, adsorption is confined to the aqueous phase side, and nanoparticle effect becomes significant only when the original contact angle meets the critical condition for corner flow. Surfactants reduce the contact angle through chemical modification, but the degree of variation is limited. The synergistic system leverages the pre-spreading of wetting films induced by the surfactant to enable nanoparticles to exert their structural effect, thereby maximizing wettability alteration. During displacement in porous media, the nanoparticle-surfactant composite system exhibits a pronounced displacement enhancement compared with either component alone. The aqueous film flow in regions occupied by the displaced phase is strengthened, detachment and mobilization of residual droplets is promoted, and the sweep area is enlarged. This work elucidates the mechanical basis of the synergistic effects for displacement enhancement by nanoparticles and surfactants, providing a new perspective for wettability manipulation under porous flow conditions.