## 3 Results and Discussion
### Dynamic Adsorption
The dissolution of calcite can be inferred from the effluent calcium concentration profiles. Since the injected fluid did not contain any calcium, the effluent calcium observed may be attributed to the calcite dissolution by Equation (1). A second test (SK2) was done without NPs to provide a baseline for comparing the calcite dissolution in SK1. The pH of the slug without NPs in SK2 was adjusted to be at the same pH level as that of nanofluid in SK1. The calcium concentration in the effluent from test SK1 (with NP) and SK2 (without NP) is shown in Figure 4c. Calcite dissolution could be detrimental to chalk matrix integrity, causing severe subsidence and this is a major concern for the operators of chalk reservoirs. It is interesting to observe that calcium concentration during tests of SK1 and SK2, Figure 4c, there is no significant difference between the Ca in the two cases during DIW pre-flush. However, after the slug injection, the difference in the trend of Ca for the two experiments started to increase. At 10-12 PV in the case of test fluid with NP the calcium concentration was about 80% less than that for the fluid without NP. The general mechanism may, then, be deduced as follow: the dissolution of calcite increases the pH, which in turn reduces NP adsorption on the calcite surface. [PERSON], [PERSON], [PERSON] and [PERSON] (2007) have made similar observation where increasing the pH from around 7.5 to 10 reduced the adsorption of silica NPs on the calcite surface by about 33.33%. As mentioned earlier, effluent calcium was reduced by about 80% for brine with NPs. As the pH increases, the layer closer to the calcite surface becomes less positive, so that NP adsorption decreases. As observed, more NPs were produced when the pH reached about 10, with peak concentration at pH \(\approx\) 10. SiO\({}_{2}\) dissolves in the alkaline range of the pH (Equation (2)). Dissociation of silicic acid (Equation (3)) increases the negative ions and thereby reduces the adsorption process. In addition, the dissociation of (weak) silicic acid slightly increases the acidity of the solution, which may again increase the adsorption of NPs. The greater the NPs' adsorption, the faster the pH decline, as observed in Figure 4a. As the desorption process proceeded, the pH steadily increased until it reached about 9, which was the point where the experiment was terminated. It is estimated that the adsorbed on the chalk about 0.46 mg per gram of chalk. This was obtained from the integrated area under the curve (Figure 4a) and the injected NPs. This process was investigated further in saline environments by injecting SSW as base fluid in SK3 and LSW as base fluid in SK4. In SK3 test, both the pre-flush and post-flush was performed with SSW. The nanofluid slug with tracer was also prepared in SSW. The results for the test SK3 are shown in Figure 5.  
It can be seen in Figure 5a that unlike in test SK1, the NP concentration profile for test SK3 follows the tracer profile closely. In addition, the NP production stops about 0.25 PV before the tracer. That is unlike SK1, no NP was detected in the effluent after the unreacted tracer has passed through the chalk core. This together with high adsorption of the NPs observed in SSW during static adsorption experiments may indicate strong irreversible adsorption of the NPs on the chalk surface. Integrating the area under the curve in Figure 5a and comparing it to the known amount of NPs injected into the core showed that about 86% of NPs were adsorbed on the chalk surface. Further, the effluent pH and ion concentration profiles for test SK3 in SSW are shown in Figure 5b,c respectively. At elevated salinity conditions in SK3, the chalk surface and the contacting fluid is at equilibrium [43] and therefore ionic activity due to fluid/rock interaction was expected to be low. For ease of comparison, the effluent concentration (Co) was normalized with respect to the injected concentration (Ci).