## 3 Results and Discussion
### NPs Interaction with Chalk Mineral in Presence of Hydrocarbon
The exchange between Ca\({}^{2+}\) and Mg\({}^{2+}\) may lead to the formation of complex calcium/magnesium minerals with different ratios. The following reaction is for a 1:1 ratio (dolomite):  
\[2\text{CaCO}_{3(\text{s})}+\text{Mg}^{2+}\rightarrow\text{CaMg}(\text{CO}_{3}) _{2}+\text{Ca}^{2+} \tag{4}\]  
Dolomitization has been previously observed by [PERSON] and [PERSON] [57] in the chalk formations of the Ekofisk field. During primary stage by LSW, the ratio of the effluent ion concentration to the injected concentration reached 6.267 and 0.686 for Ca\({}^{2+}\) and Mg\({}^{2+}\) respectively at 16 PV/day. When SK6 was switched to NF prepared in LSW, the ratio of Ca\({}^{2+}\) to injected concentrations fell to 4.26 at the injection rate of 4 PV/day. Increasing the rate to 16 PV/day raised the Ca\({}^{2+}\) concentration slightly to 4.63, which is still below the Ca\({}^{2+}\) concentration during primary stage by LSW. This reduction in Ca\({}^{2+}\) during NF injection (almost 30%) coincided with a comparative increase in levels of Mg\({}^{2+}\) to 0.86 at 4 PV/day and 0.85 at 16 PV/day. These observations may indicate a reduction in calcite dissolution and the formation of calcium/magnesium minerals. The increased amount of Mg\({}^{2+}\) was not significant enough to indicate reduced magnesium/calcium exchange. It is therefore possible that the reduction in Ca\({}^{2+}\) was caused by reduced calcite dissolution during NF injection. Increasing the rate to 16 PV/day, the Ca\({}^{2+}\) concentration stabilized at around 4.6. This also represents a 25% reduction of Ca\({}^{2+}\) production compared with the 4 PV/day flow rate during primary stage. With primary stage in the SK7 case, the effluent ion concentration profiles were close to the injected concentration. When the SK7 was switched to NF prepared in LSW, the Ca\({}^{2+}\) and Mg\({}^{2+}\) ions were initially high. Two main possibilities exist for the increase of Ca\({}^{2+}\) and Mg\({}^{2+}\): (1) production of trapped SSW from the first stage  
Figure 7: Effluent pH profiles for SK5-SSW (**a**), SK6-LSW (**b**) and SK7-mixed (**c**). and/or (2) dissolution of possible calcium sulfate formed during the first stage. The latter may sound more realistic because of a rapid reduction in Ca\({}^{2+}\) and Mg\({}^{2+}\) concentrations. Thereafter, the Ca\({}^{2+}\) concentration was around 1.5 at 4 PV/day. The Ca\({}^{2+}\) concentration during this stage is almost three times lower than at the same stage in SK6. When increasing the injection rate to 16 PV/day, the Ca\({}^{2+}\) concentration fluctuates between 2.9 and 1.16. This concentration at 4 PV/day is almost three times lower than during the same stage in SK6. The differential pressure drop (dP) data recorded for SK5-7 is shown in Figure 9.  
At 4 PV/day with SSW injection in SK5, the pressure drop stabilized at about 1.79 bar. When the injection rate was increased to 16 PV/day, the dP increased and stabilized at around 3.70 bar. After the injection fluid was switched to NF prepared in SSW, the pressure rose steadily from 0.68 to 2.17 bar. [PERSON], et al. [58] stated a general rule of thumb that, if the particle size of the suspended solids exceeds one-third of the pore diameter, the particles will cause plugging behavior. The average pore size of the SK chalk used in this study is around 200 nm [59]. The size of the NPs in SSW was shown earlier to be 88.1 nm at 80 \({}^{\circ}\)C. It is possible that some of the smaller pore throats are blocked by the NPs. For SK6 however, the recorded dP for NF (in LSW) was 0.439 bar, which is almost three times lower than the dP recorded for LSW injection in the first stage. The measured particle size of the NPs in LSW is about 38 nm, which is significantly lower than the average pore throat of the chalk used. The recorded dP at 16 PV/day was slightly higher for the NF compared with LSW alone. However, the difference (\(\approx\)0.04 bar) is within the uncertainty range of the measured dP (\(\pm\)0.1 bar). The resistance to flow was therefore lower at the low flow rate.