Wettability alteration of oil-wet carbonate surface induced by self-dispersing silica nanoparticles: Mechanism and monovalent metal ion's effect

Abstract

Various analysis methods including IR, SEM, contact angle determination and spontaneous imbibition experiments were used to investigate the mechanism of wettability alteration of oil-wet carbonate surface by silica nanoparticles (NPs) in the present study. Stable silica nanofluids were first studied through zeta potential measurement. Results show that there is an optimal sonication time interval (5 h) for the preparation of SiO2 nanofluid after the magnetic stirring treatment in this work. The addition of NaCl can decrease the dispersion stability of SiO2 nanofluids. Both IR and SEM results show that the originally water-wet silica NPs are adsorbed on the calcite surface, making the surface water-wet. The role of sodium ions is to compress the electric double layer, giving the nanoparticle (NP) an opportunity to access the palmitic acid molecules. The palmitic acid molecules are replaced by the NPs through competitive adsorption. The longer the contact time, the better the performance of wettability alteration of oil-wet carbonate rock by the nanofluid. The contact angle decreases as the NPs concentration increases. Among the metal ions in the formation water, Na+ ion is closest to the solid surface, making Na+ ion have a great influence on wettability change of oil-wet carbonate surface by silica NP. Na+ ions in the solution can neutralize some negatively charged portion of the carbonate surface, making the adsorption amount of silica NPs on the solid surface increase. Thus, wettability alteration of oil-wet carbonate surface by SiO2 NPs is improved in the presence of Na+ ions. During the spontaneous imbibition experiments, silica NPs are adsorbed on rock surface and gradually form a wedge film in the contact area of the three-phase (oil-water-solid). In this case, structural disjoining pressure is generated due to the presence of electrostatic repulsion and Brownian motion of silica NPs. When the structural disjoining pressure is stronger than the oil droplet adhesion, the contact area of oil droplet on the solid surface gradually decreases, making the oil droplet eventually peel off from the rock surface.