## 3 Results and Discussion
### Dynamic Adsorption  
This section deals with the dynamic adsorption of NPs in chalk and its effect on fluid/rock interactions. Three salinities: DIW, LSW and SSW were used in the dynamic tests as outlined in Section 2.3. The adsorption/transport behavior of the used Silica NPs (DP9711) in sandstones has been addressed previously by some researchers [17; 45; 49]. They generally reported low adsorption of NPs on sandstone minerals. To best our knowledge, the adsorption behavior of Silica NPs in chalk and its effect on fluid rock interactions has not been addressed previously. The first test (SK1), was done with a core saturated with DIW. After several PVs pre-flush with DIW, a 1.5 PV slug containing LiCl tracer (0.1 M) and NPs (1 g/L) was injected into the core, followed by post-flush with DIW. The effluent fluid was analyzed for the concentrations of NP, Ca and Li (tracer). The results are presented in Figure 4. As shown, a difference of about 1 PV between the peak concentration of the tracer and the peak concentration of NPs. It was also observed that the tracer peak concentration declined faster than that for the NPs. The delay of NP decline may indicate interaction between the NP and core surface. After about 11 PV of injection (from the start), the NP concentration showed a linear increase, while the Li declined to a level close to zero concentration. The increased of NPs' concentration after 11 PV, may indicate the NPs' desorption. Now the question is, why did the NP concentration decline below 0.01 g/L before the start of the desorption process (at \(\approx\)11 PV)? The adsorption/desorption process may be related to the pH of post flowing fluid in contact with calcite surface and adsorbed NPs. The effluent pH profile for core SK1 is shown in Figure 4b. It is shown that the pH after nanofluid slug increased steadily for approximately 1 PV, and then steeply increased to a pH of about 10, before it started to decline to reach  
Figure 4: (**a**) NP and tracer concentration profile for SK1. (**b**) Effluent pH profile for SK1 (**c**) Effluent Ca concentration for SK1 (with NPs) and SK2 (without NPs). a pH of about 8.6. The highest pH coincided with the peak concentration of NP in the effluent and the desorption of NP from 11 PV (Figure 4a) coincided with the steady increase of the pH from about 8 to about 9. Equation (1) may explain the associated increase of pH with calcite dissolution. \[\mathrm{CaCO_{3}(calcite)+H_{2}O\to Ca^{2+}+HCO_{3}^{-}+OH^{-}} \tag{1}\]  
Therefore, as calcite dissolves, the pH increases. Increase of the pH increases the dissolution of NPs (SiO\({}_{2}\)) according to the following equations:  
\[\mathrm{SiO_{2}(s)+2H_{2}O\leftrightarrow H_{4}SiO_{4}} \tag{2}\]  
\[\mathrm{H_{4}SiO_{4}\leftrightarrow H_{3}SiO_{4}^{-}+H_{3}O^{+}} \tag{3}\]  
Equation (2) shows the dissolution of SiO\({}_{2}\). [PERSON] and [PERSON] [PERSON] and [PERSON] (1992) stated that SiO\({}_{2}\) solubility increases at neutral to slightly alkaline pH ranges according to the above equation, producing silicic acid, which is a weak acid. Silicic acid further dissociates and reduces the pH (Equation (3)). The progression of this equation may have reduced the effluent pH, as shown in Figure 4b. The reduction of the pH to 8, may have resulted in more adsorption of NP on the calcite surface, hence reduced the produced NPs in the effluent (Figure 4a). As the pH started to increase, desorption of NPs increased. The increase of the pH from about 8 till about 9 at the termination point of the experiment (PV \(\approx\) 18) may be caused by calcite dissolution by Equation (1). These observations may be summarized as: maximum NP concentration in the effluent occurred at the highest pH (\(\approx\)10) of this experiment. As the pH declined, less concentration of NPs were detected in the effluent until NPs reached minimum at 11 PV (pH \(\approx\) 8), after which, NP started to rise again and to reach 0.03 g/L (at the time of the experimental termination). This counts for about 50% higher than the peak concentration of 0.02 g/L.