Experimental Evidence on Nanoparticle Adsorption and Wettability Alteration for Enhancing Oil Recovery from Tight Rocks

In this study, we investigate the effects of highly surface-modified silica nanoparticles (NPs) on rock–fluid interactions and imbibition oil recovery from tight rocks. The protocol consists of the following steps: (1) measuring the fluid properties of NP solutions, including interfacial tension (IFT), particle size distribution, and zeta potential; (2) evaluating the efficiency of NPs on wettability change and enhanced oil recovery by conducting systematic contact angle and imbibition tests; (3) visualizing NP adsorption on rock surfaces before and after NP treatment through SEM/EDS analysis; and (4) evaluating the change of adhesion forces between a silica tip and the rock surface with and without NPs using atomic force microscopy analysis. Our results indicate that adding NPs reduces oil–water IFT values from 15 to less than 4 mN/m, and alters the initial natural-wet rocks toward more water-wet. The IFT reduction and wettability alteration can explain the improved oil recovery by NP solutions. Compared to the base cases with less than 10% oil recovery factor, the oil recovery factor from the oil-saturated samples soaked in NP solutions is enhanced, ranging from 15.2 to 26.1%. Increasing the concentration of NPs beyond the critical micelle concentration results in more oil recovery because a higher NP concentration ensures a sufficient supply for the extensive pore surfaces in tight rocks. Increasing salinity results in less oil recovery due to weaker osmosis and less wettability alteration and an enhanced cation bridging effect. Based on the interaction energy calculation for the rock–water–NPs system using the DLVO theory, NP adsorption primarily occurs on carbonate or hydrocarbon residue surfaces. The adsorption of NPs on the rock surface is proportional to NP concentrations, and the adhesion force between the rock surface and the silica tip increases after NP adsorption. This can be explained by higher adhesion energy between the rock surface and NPs obtained from the interaction energy profiles. By increasing salinity, the energy barrier for NPs to adsorb on the rock surface reduces, enhancing NP adsorption.