## 3 Results and Discussion
### Nanofluids Stabilized by MPTMS  
In this part of the study, surface modification of the porous medium was performed with nanofluids that were stabilized by addition of MPTMS. As stated previously, to avoid confusion, NP concentration is stated in g/L units and MPTMS concentration is stated in g/100 mL units. The effluent concentration profiles of NP during the post flush (with DIW) for the cores treated with nanofluids at different concentrations (Experiment: 4-8) are shown in Figure 7a. As shown, the nanofluids with 1 g/L and 2.5 g/L NP show similar profile of NP production. After DIW flush of about 1.5 PV for 2.5 g/L and 1 PV for 1 g/L the NP production in the effluent stopped. To ensure that this difference does not arise due to dissimilarities in the core, experiments 4 and 5 were repeated. The effluent concentration profiles shown in Figure 7a for the repeated cases are close to the initial experiments 4 and 5. For the nanofluid at 4 g/L concentration, the behaviour is completely different. The percentage of NP adsorbed in the core (Experiment 4-8) as the post flush progresses is shown in Figure 7b. In these experiments, the cores were vacuum saturated with the nanofluid. Therefore, it may be assumed that the spatial distribution of the NP in the core was uniform. Gradually decreasing retention for the case of 1 g/L and 2.5 g/L possibly suggests desorption of particles in the core or that the retained particles were forced out during the post flush. However, for 4 g/L it can be inferred from the almost flat nature of the curve that possibly only the NP near the outlet of the core were produced and substantial channelling of the fluid may be caused by blockage of some pore throats. This was confirmed by the monitoring the corresponding pressure drop during the experiments (Figure 8). Figure 8 shows the recorded pressure drop (dP) during the post flush with DIW after application of nanofluid (Experiments 4-6). The recorded pressure drop during initial DIW injection was taken as a base line for the pressure drop. It is interesting to see that the pressure drop peaked at about 0.25 PV indicating entry resistance. The lowest pressure drop peak occurred for 1 g/L followed by the base fluid (DIW), then 2.5 g/L. In the case of 4 g/L, the pressure drop increased to above 0.3 bar. For cores saturated with NP concentration of 1 g/L, more fluctuations in dP was observed, followed by two  
Figure 6: SEM image of (**a**) berea sandstone sample; (**b**) berea sandstone treated with nanofluid; (**c**) magnified look at the adsorbed/adhered silica. peaks between 1-2 PV, while the others (DIW and 2.5 g/L) dP declined smother. It may be concluded that 1 g/L flowed through the core with occasions of resistance to the flow. From mass balance, in the case of 1 g/L, 69.47% (0.01389 g) and for 2.5 g/L, 85.44% (0.044 g) of NP were adsorbed/ retained in the core, i.e., the loss of the NP in the core for the case of 2.5 g/L exceeded 3 times higher compared to 1 g/L, yet the dP curves eventually became almost equal to the initial DIW injection, this may indicate that the retained/adsorbed NP did not hinder the flow. However, the surface modification of he surfaced by this nanofluid does not lead improvement in water injectivity. In contrast, for the 4 g/L, where the pressure drop increased for more than 2 PV before it was stabilized at dP > 0.3 bar, this may indicate possible aggregation of NPs that restricted/blocked some of the pore throats. Figure 9 shows the amount of MPTMS adsorbed in the core. The concentration of MPTMS was measured by UV/VIS. The amount of MPTMS retained in the core was calculated from the mass balance. It may be observed that a high amount of the stabilizing fluid in adsorbed in the core. High adsorption of NP stabilizing fluid (MPTMS) in the porous media suggest that this method may not be suitable for subsurface application.