Nanoparticles (NPs) have been suggested for subsurface projects, including enhanced oil recovery (EOR), carbon capture and storage (CCS), and hydrogen storage, due to their small size, high surface energy, controlled surface properties, and significant effects on the underground formation properties. Silica is an excellent NP that efficiently modifies the surface properties of subsurface formations for incremental oil recovery and to derisk carbon and hydrogen leaks. However, the retention of injected NPs in a narrow area near the injection inlet can limit a project’s feasibility. In the present study, hydrophobic (hybrid) NPs were produced via silanization of silica NPs, and the mobility behavior of bare (hydrophilic) and hybrid (hydrophobic) silica NPs in limestone cores was probed via scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques using nanofluid core flooding methods. Experimentally, nanofluids with different NP concentrations and hydrophilicity were injected at a constant rate (0.1 mL/min) into limestone cores at reservoir temperature (50°C) and various salinities (0 to 5 wt% NaCl). Subsequently, the existence of silica in various parts of the core was measured. Moreover, NPs’ stability and size growth were examined via zetasizer and dynamic light scattering (DLS). Findings revealed that, regardless of the hydrophilicity, the initial NP size is key for NP transport in limestone, as was confirmed by differential pressure measurements. Further, despite the remarkable resistance of hybrid NPs to the increased salinity, bare NPs showed higher mobility at all salinities (0–5 wt% NaCl). Low salinity (≤0.5 wt% NaCl) compared to hybrid NPs is mainly due to the smaller initial particle size. Moreover, increased salinity (e.g., 5 wt.% NaCl) significantly decreased the mobility of bare NPs, while the low mobility ratio of hybrid NPs was almost constant over all salinities (0–5 wt.% NaCl). Furthermore, the addition of a slight amount of electrolyte (≥0.5 wt.% NaCl) significantly decreased the zeta potential of bare NPs (from −25 to −9 mV) to 0 mV when electrolyte concentration increased to 5 wt% NaCl at pH = 6.25, while the zeta potential of hybrid NPs showed higher resistance to the change in salinity even at the highest level (5 wt% NaCl). These results of NP transport agree well with the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. Apparently, hybrid NPs are unsuitable for subsurface applications due to their larger initial size, resulting from agglomeration during salinization steps, compared to bare NPs.
