## 1 Introduction  
With the rising demand for energy and fast approaching end of the era of easy oil, the petroleum industry today faces unique challenges especially in the Enhanced Oil Recovery (EOR) area. Over the past decade, many of researches have focused on application of nanoparticles (NP) as an EOR method. The small size and high specific surface area of NP offer unique advantages like allowing them to easily pass through pore throats and enhanced interaction in the reservoir even with a small quantity of NP. NPs have displayed the potential to act as surface modifiers that could alter the wettability and reduce the oil/water interfacial tension leading to better mobility of the oil phase [1; 2; 3; 4; 5; 6; 7] and reduce fines migration [8; 9]. Recent laboratory studies have indicated that nanofluids, which are colloidal dispersions of NP in a dispersing medium have the potential to increase oil recovery [2; 7; 10; 11; 12; 13]. Over the past decade, special focus has been directed to silica NPs for EOR due to its hydrophilic nature and ease of functionalization. [PERSON] et al. [14] postulated the presence of silanol groups (Si-OH) on the silica surface that causes its hydrophilicity, wherein silanol groups act as binding sites (H\({}^{+}\) bonds) for water. The protonation and deprotonation of these silanol groups determine the surface charge of silica NP and the extent of the repulsive energy that keep them dispersed in the solution [15]. For two-phase systems like nanofluids, one of the most important issue is their colloidal stability i.e., no or low rate of agglomeration of the NP. Stability of NP is essential for injection application as EOR agents in oil reservoirs. Agglomeration can lead to blockage of micro channels, formation damage, hinder the transport of NP and the displacing fluid in the reservoir. The main strategies utilized to enhance the stability of nanofluids are: (a) electrostatic stabilization [16] (by varying pH of the nanofluids); (b) employing stabilizing fluid/surfactant [17]; (c) surface modification [18; 19] (functionalization) of the NP. Electrostatic stabilization (for example by varying the pH) is expected to fail in the presence of dissolved salts in brines. Electrolytes could destabilize particle dispersions by compressing the electrical double layer. As the electrolyte concentration increases, the energy barrier is lowered to an extent that kinetic energy of particles dictates the kinetics of particle aggregation [15]. For a given surface charge, the aggregation of silica NP occurs because of the presence of electrolytes. [PERSON], [PERSON], [PERSON] and [PERSON] [15] studied the effect of pH, cation type, temperature and electrolyte concentration on the stability of silica dispersions. They found that pH does not have a significant effect on stability in the presence of electrolytes. Although addition of stabilizing fluid can be an effective way to enhance the dispersion of NP, it might cause several problems like foaming and stabilizing fluid adsorption in porous media leading to loss of the intended stabilization. Surface modification of NPs (functionalized NP) is a promising approach towards increasing the stability of NPs. [PERSON] and [PERSON] [18] presented a work on the synthesis of functionalized silica (SiO\({}_{2}\)) NP by grafting silanes directly to the surface of silica NP which showed good stability. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [19] systematically performed surface modification of silica with different silanes and studied the wettability of the modified nanomaterials. However, it is essential to examine the effect these stabilization strategies have on the effectiveness of the nanofluids. For stable nanofluids, which can be utilized as EOR agents, an important factor is the interaction of the NPs with the rock minerals over a wide area of the reservoir. When NPs are introduced into porous medium, different processes may take place such as adsorption, desorption, blocking, transportation and aggregation [4].