## 3 Results and Discussion
### NPs Interaction with Chalk Mineral in Presence of Hydrocarbon  
Nanofluid are prepared in brines and some studies have investigated the combined role of salinity and NPs on the wettability change process [51] and NP adsorption on mineral surfaces [52]. [PERSON] and [PERSON] [51] stated that nanofluid flooding is sensitive to water salinity especially in the presence of divalent ion (Ca\({}^{2+}\) and Mg\({}^{2+}\)). The effect of injection brine salinity on the recovery process has been well documented in the literature [36; 39; 53; 54; 55; 56]. As discussed in the previous section, the adsorption of NPs on the mineral surface alters the rock surface hence fluid/rock interactions. The experiments were divided into two stages with brine alone and NPs dispersed in the selected brine. The injection was performed at lower flowrates that are closer to real field cases and to give the injected fluid sufficient residence time in the core for the interaction. [PERSON] and [PERSON] [43] and [PERSON] and [PERSON] [41] previously studied the effect of low salinity composition on primary and secondary recovery in SK chalk by systematically diluting the SSW. They found that LSW at a 1:10 SSW dilution was the optimum for the investigated brines for EOR. Therefore, LSW 1:10 dilution of SSW is used here. In the SK5 case, both primary and secondary stage the injection of the fluid was SSW. For the secondary stage, however, silica NPs (DP 1 g/L) was mixed with SSW. In SK6, primary stage was performed with LSW followed by injection of NF (DP 1 g/L) prepared in LSW. The third scenario was for SK7, where primary stage SSW was used, followed by NF prepared in LSW (DP 1 g/L). Those three scenarios represent the various possible combinations. The effluent pH profiles were recorded for the SK5, SK6 and SK7 and are shown in Figure 7. For SK5 (all SSW), the effluent pH during primary and secondary stages were lower than the pH of the injected SSW. This observation is similar to that previously made by [PERSON] and [PERSON] [43]. In the case of SK6 (LSW and LSW with NPs) however, an interesting behavior was observed. The effluent pH was higher than the injection pH and continued rising until it stabilized at about 7.8. Increasing the flow rate to 16 PV/day led to a slight increase in pH to around 7.95, after which it stabilized at around 7.91. The increase in pH may be explained by calcite dissolution, in accordance with the Equation (1). The pH in the case of SK7 (SSW and LSW with NPs) shows that the pH remained below the injected pH in primary stage. However, the pH increased slightly to about 7.56 when the injection was switched to LSW with NF. When the NF injection rate was increased to 16 PV/day, the pH of SK6 and SK7 stabilized at about 8.06 and 7.75, respectively. As expected, the pH was higher for SK6 case than in the case of SK7. The concentrations of Ca\({}^{2+}\) and Mg\({}^{2+}\) in the effluents of SK5-7 are plotted in Figure 8. For ease of comparison, effluent concentration (Co) was normalized with respect to the injected concentration (Ci). Figure 8a shows that the Ca\({}^{2+}\) effluent in SK5 was lower than the injected concentration during primary stage and even more so during secondary stage. This may indicate slight calcite dissolution with high injection salinity (SSW). This observation was also supported by the low pH recorded for this SK5 in Figure 7a. The Mg\({}^{2+}\) concentration in effluent was close to the injected concentration. Where the SK6 (all LSW) is concerned, however, the Ca\({}^{2+}\) ion concentration during primary stage by LSW was consistently higher, indicating calcite dissolution similar to the observation made during dynamic adsorption experiments in SK4. This observation is supported by the high pH recorded during this stage in Figure 7b. Along with the excess Ca\({}^{2+}\), there was a deficiency of Mg\({}^{2+}\) in the effluent. It is well established that calcite has a tendency to accommodate Mg\({}^{2+}\) in its structure [50].