## 3 Results and Discussion
### Untmodified Silica Nanofluids  
Figure 3a compares effluent NP concentration profiles for cores treated with 1 PV of unmodified silica NP dispersed in DIW. It may be observed that for both 1 and 2.5 g/L, the majority of the NP seized to be produce at about 0.5 and 1 PV, respectively. Long tail in the effluent concentration profile was observed for 2.5 g/L. The percentage of NP adsorbed in the core (calculated from mass balance) as the post flush progressed is plotted in Figure 3b. The estimated adsorbed NP was higher for 2.5 g/L nanofluid (88.82%) as compared to that for 1 g/L (85.82%) nanofluid. The recorded pressure drop during these experiments is shown in Figure 4.  
Figure 4 shows that after treatment with 1 and 2.5 g/L silica NP, pressure drop profiles were lower than that for the initial DIW injection in unmodified berea. It may be observed that the entry resistance post application of NP was lower than the initial DIW injection as indicated by pressure peak of about 0.12 and about 0.09 bar, for initial DIW injection and post flush respectively (treatment with 2.5 g/L of nanofluid). It may be concluded from Figure 4a,b that using 1 g/L concentration leads to greater improvement in water injectivity, as there is a greater difference between the pressure drop for initial DIW injection and post flush after treatment with NP. Fines refer to solid mineral particles of the sandstone minerals that lose their coherence due to fluid/mineral interaction and become mobilized with the flowing fluids. In this study, the injection has been performed with DIW wherein the salinity is almost zero hence lower than the Critical Salt Concentration (CSC), which may induce interaction with minerals and produces fines that migrate and increase resistance to flow which may ultimately lead to formation damage [29]. [PERSON] and [PERSON] [8] studied the applicability of NP for mitigating fine migration in engineered porous media (glass beads). In their work, they studied the application of different metal oxide NP to mitigate fines formation. They found that treating the porous medium with NP caused reduction in concentration of fines particles in the effluents as compared to untreated porous media. For example, they observed that treating the porous medium with silica dispersed in DIW led to approximately 20% reduction in effluent fines concentration as compared to the reference case. They observed that porous media that has been treated with NPs acts as a strong adsorbent  
Figure 4: Pressure drop profiles for silica dispersed in deionized water (DIW) at (**a**) 1 g/L and (**b**) 2.5 g/L concentration. of fine particles [9]. [PERSON] et al. [30] made a similar observation wherein they observed that for a sand pack treated with NP, the pressure drop across the sand pack was 10% lower than of sand pack without NP, showing an improvement in water injectivity. To verify the effects of surface modification by silica NP and the associated improvement in water injectivity, experiment 3 was performed. In experiment 3, the core sample was initially injected with DIW and the stabilized dP was recorded at increasing injection rates. Thereafter the core was unloaded from the core holder, vacuum dried and treated with 1 g/L silica nanofluid prepared in DIW and flushed again with DIW. Stabilized dP was recorded at increasing injection rates. The results are shown in Figure 5. It can be observed that saturating the porous medium with the NP improves the water injectivity as indicated by lower dP (\(\approx\)8%) during DIW flooding post treatment with NP. To quantify the remedial effect due to the application of NP, the ratio of the original pressure drop during the initial DIW injection (dPi) to the pressure drop post treatment with nanofluid (dPnp) was estimated and plotted as shown in Figure 5. As shown, the nanofluid treatment was effective at lower injection rates i.e., in reducing pressure drop. This is perhaps due to the increased hydrodynamic forces at higher injection rates.