## 3 Reservoir-Scale Simulations  
The main objectives of a reservoir engineering study for CO\({}_{2}\) sequestration include determining the overall reservoir capacity, the reservoir injectivity, the project cost, and the contributions of the various trapping mechanisms to the ultimate fate of the CO\({}_{2}\), together with assessing the risks involved, such as reservoir stability and caprock integrity [79; 80]. Due to the limited data available from the field, many studies involve reservoir simulations. Reservoir simulations also enable the testing of '_what_-if' scenarios to trial potential reservoir engineering ideas, such as different injection regimes and well-placings [80]. Although two-dimensional (2D) models are practical and helpful, they cannot be fully representative of all the complexities of a geological formation, such as all of the stress and strain distributions resulting from the various interactions between different processes [81].  
Potential leakage mechanisms in caprocks and interlayers include capillary pressure breakthrough, the diffusion of dissolved CO\({}_{2}\), and migration along faults and fractures, which could occur during the upwards migration of the plume. Once leakage commences either, or both, positive and/or negative feedback processes can also eventually be initiated, which can enhance or curtail the leak. Therefore, it is crucial to understand the short- and long-term impact of CO\({}_{2}\)-brine-rock interactions during and after CO\({}_{2}\) injection [82]. Once the CO\({}_{2}\) invades the caprock, geochemical CO\({}_{2}\)-rock interaction may result in a change in porosity and permeability through mineral precipitation and dissolution. In addition, any change in the density and viscosity of the fluids, together with the change in the fluid pressure, will have an impact on the fluid flow and thus on the transportation rate of chemical species. Moreover, the overpressure introduced by the injection of scCO\({}_{2}\) results in a change in loads or internal pressure, which leads to a change in the geo-mechanical and hydrological properties of the seal. These include, for example, stresses and deformation of the rock and capillary forces, respectively. The variation in the applied load will generate an expansion in the porous media of the seal and reservoir, which may lead to the stress state moving close to shear or tensile failure [82]. This may therefore compromise the seal integrity by opening up new flow paths via the sealing area. As a result, understanding the inter-relation between the various fluid flow-chemical-mechanical coupling phenomena plays a vital role in geological formation storage in terms of predicting capacity and ensuring safety [83]. A numerical simulation is an important tool for investigating the aforementioned complex issues regarding the operation of a geological carbon storage site. The majority of the models related to carbon storage consider two-phase flow problems (liquid and gas) conducted at a continuum scale to overcome computational constraints. Simulations involve the use of three-dimensional governing equations, including those for the permeability-saturation relationship, capillary pressure-saturation and residual saturation [84].