## 3 Reservoir-Scale Simulations
### Geochemical Processes  
In addition to geomechanical considerations, geochemical numerical modelling is also a vital aspect to be considered as a part of investigating potential geological carbon storage sites in order to examine the site sustainability and to obtain the limitations in terms of the injection rate and sealing integrity [111]. The injection of CO\({}_{2}\) into the subsurface as a part of geological carbon storage would result in geochemical changes within and around the storage formation. In the clay-rich caprocks that typically lay on top of proposed storage sites, the chemical interactions with CO\({}_{2}\) can include the dissolution and re-precipitation of carbonates, the dissolution of plagioclase feldspar and clay minerals (illite, chlorite, and kaolinite), and the precipitation of smectite [112]. However, due to the complex network of chemical reactions that can develop, prediction is difficult. In calcite-rich rocks, dissolution in preferable flow pathways can create positive feedback, leading to the rapid development of 'wormhole structures' [83]. Geochemical changes can lead to changes in the geomechanical properties of rock, such as its elastic behaviour, which has consequences for the stress and strain distributions and for fracture initiation and propagation [81]. For this activity to be carried out safely, and not damage the ecosystem, then sufficient isolation needs to be provided to prevent CO\({}_{2}\) from escaping into the atmosphere. Therefore, monitoring the geochemical response is important. Accordingly, the IPPC special report on carbon capture and storage recommended that a geochemical reaction assessment, for both reservoir and seal rock, needs to be carried out. Therefore, several studies have monitored the impact of changes to geochemistry on the porosity and permeability of the reservoir and the seal, while ignoring geomechanical effects. For example, [PERSON] et al. [113] performed a simulation study to evaluate the influence of mineral precipitation and dissolution on alterations in the porosity and permeability of a reservoir. The results indicated that although a porosity increase occurred near the injection well area, caused by mineral dissolution, conversely, a porosity reduction was caused by mineral precipitation in areas far away from the well. In addition, permeability followed a similar trend to that of porosity. However, the change in porosity and permeability was minor [113]. The presence of CO\({}_{2}\) in the pore water alters the chemical equilibria occurring in the rock. For simulations of the Shenhua carbon sequestration project, [PERSON] et al. [114] found that the mudstone caprock'self-healed' due to mineral precipitation in the lower part of the caprock exceeding dissolution. It was found that the K-feldspar content was the key mineral controlling the integrity of the caprock, with the lack of K-feldspar enhancingthe self-sealing of the lower part of the caprock. They also found that the precipitation of carbonate minerals consumed CO\({}_{2}\) in the caprock, resulting in a decrease in the CO\({}_{2}\) concentration and rock permeability, thereby enhancing the seal's efficiency. However, the greater precipitation of Ca-smectite and the dissolution of albite in the lower part of the caprock was the opposite to what was happening to these minerals in the top of the caprock. Additionally, [PERSON] et al. [115] carried out geochemical modelling to evaluate the change in the caprock permeability due to the evolution of the mineralogy resulting from CO\({}_{2}\) injection, with the outcomes showing that the dissolution of feldspar and albite minerals played a major role in increasing the permeability of the seal. On the other hand, some caproks do not contain minerals within their pore throats that are sensitive to CO\({}_{2}\) injection (such as carbonate), and therefore no precipitation and dissolution will occur. For example, a study performed by [PERSON] et al.