## 1 Introduction
### Shale Interactions (Multi-Mineral)
[58] did not detail the chemical processes that were taking place. The content of quartz and the small amount of kaolinite showed very little change in this experiment. To date, only one laboratory-based geochemical experiment has been conducted on the exposure of the Sleipner field caprock, Nordland shale, to CO\({}_{2}\). This focused only on dissolved CO\({}_{2}\) interactions, since the sample was submerged in porewater prior to CO\({}_{2}\) injection. [PERSON] et al. [59] designed an investigation to evaluate the impact of CO\({}_{2}\) on the sealing capacity of Nordland shale. Several 'batch' experiments were set up, running for up to 5 years. Each 100 mL container carried 8 g of caprock and 80 g of synthetic porewater. The porewater was based upon samples collected from the host reservoir rock, namely, the Utsira Sand. The samples were kept at 30 \({}^{\circ}\)C and injected with CO\({}_{2}\) to a pressure of 8 MPa (approximate reservoir conditions). A control sample, consisting of a reactor under nitrogen, was also included for reference purposes. Reactant fluids were regularly sampledfor the first 14 months and again at 5 years. Levels of pH, carbonate/bicarbonate, and various other cations/anions were tested. The caprock samples were characterised using XRD analysis on completion of the experiment, along with an untreated sample. [PERSON] et al. [59] considered that the CO\({}_{2}\) had reacted with the pore water relatively quickly. Most of these reactions would have occurred before any reaction with the rock samples. Although pH levels initially decreased over time, a slight increase was seen in the final measurement after 5 years. [PERSON] et al. [59] suggested that this may have been due to an increase in pH buffering. A general increase in bicarbonate concentration was observed, which was initially rapid up to 3200 mg L\({}^{-1}\) after a few weeks, and which then continued more sedately to 3500 mg L\({}^{-1}\) after 5 years. This response came directly from CO\({}_{2}\) dissolution, as well as the dissolution of caprock sample carbonates caused by the lowering of the pH level. The slowing of the bicarbonate concentration increase after the initial burst was said to be due to a decelerating rate of carbonate mineral dissolution. A further cause was the slow reaction of aluminosilicate minerals, which 'consume' acidity and generate bicarbonate ions as a by-product. Dissolution of caprock sample carbonates was also indicated by the swift increase in the calcium ion concentration and those of other minor elements in the pore water samples. It was not clear whether an equilibrium was reached or whether total consumption of the carbonate phase in the sample had occurred. XRD analysis showed that the calcium content had dropped from 2.4% to <0.5%, which is consistent with the pore water results. It was noted that the calcium content was present mostly as shell debris. No other significate mineralogical changes were detected. This included downsonite, which was otherwise predicted to form in models developed by [PERSON] et al. [60]. [PERSON] et al. [59] also noted that reaction rates may have been accelerated by the continuous mixing of the solution, but this would also have prevented the formation of reaction fronts, which may occur in intact caprock. The lack of reactivity seen in the nitrogen control experiments may indicate equilibrium between caprock and synthetic porewater. This verified the accuracy of the chosen constituents of the water and indicated that caprock and host rock porewater were broadly similar.