## 3 Reservoir-Scale Simulations
### Geomechanical Processes  
The injection of large volumes of scCO\({}_{2}\) into the subsurface perturbs the pressure, followed by changes in the state of the stresses. As result, expansion of the subsurface porous media occurs and brings the stress state close to failure, which potentially leads to the upwards migration of scCO\({}_{2}\) towards the atmosphere. The two main failure mechanisms that might arise are tensile fracturing and the shear-slip reactivation of cohesionless pre-existing fractures ([PERSON] et al., 2018). Tensile fracturing develops when the minimum effective stress becomes negative and its absolute magnitude exceeds the tensile strength of the rock matrix. Shear-slip reactivation occurs according to the Mohr-Coulomb criterion. Hence, monitoring the geomechanical response to scCO\({}_{2}\) injection is vital for ensuring the seal integrity. For example, [PERSON] et al. [PERSON] et al. (2019) found that a sharp front of reduction in effective stress migrated upward through the Morrow shale caprock with CO\({}_{2}\) injection. Therefore, it is necessary to solve the hydromechanical coupling equations numerically. This is because ignoring the geo-mechanical effects would lead to misleading results, and there are several instances in the literature supporting this. For example, the build-up of pore pressure during CO\({}_{2}\) injection increases the volumetric strain, and thus porosity, leading to increased permeability. This increased permeability results in negative feedback, wherein the pore pressure increase is slowed, and thus the effective stress changes are different compared to cases in which these mechanical effects on flow are neglected ([PERSON] et al., 2018; [PERSON] et al., 2019). Furthermore, incorrect predictions of fracture pressure occur when simplified analytical solutions are used ([PERSON] et al., 2018; [PERSON] et al., 2019). Purely hydraulic simulations fail to predict the correct response of the fluid pressure in the seal (since they predict an instant rise, whereas it actually decreases at the beginning of the injection due to dilatancy ([PERSON] et al., 2018)) and purely hydraulic codes do not simulate the effect of the initial stress state, which has a great effect on the seal failure behaviour ([PERSON] et al., 2018; [PERSON] et al., 2019). The importance of including geomechanics in assessments of seal integrity has been demonstrated by several studies. In terms of playing a critical role in CO\({}_{2}\) injection sites, this was confirmed both by investigations ([PERSON] et al., 2019) of the Mountaineer project in West Virginia, and also by the modelling and monitoring of the In Salah storage project locatedin Algeria [100; 101; 102]. These studies concluded that an increase in pore pressure, which was induced by CO\({}_{2}\) injection, increases the potential for fault reactivation and for surface uplift, the magnitude of which is controlled by poro-elastic expansion arising from the pressure change within the adjacent caprock. Currently, geomechanical problems are gaining more attention from geological carbon stakeholders, especially following the publication of a paper by [PERSON] and [PERSON] [103] which concluded that the injection of a large volume of CO\({}_{2}\) would induce minor-to-moderate earthquakes, which would potentially threaten the seal integrity. Furthermore, geomechanical numerical modelling is a vital aspect of the investigation of potential geological carbon storage in order to examine the site's sustainability and to calculate the limitations in terms of the injection rate and sealing integrity. Several studies have been conducted to evaluate only the impact of geo-mechanical changes on seal integrity. [PERSON] et al. [104], [PERSON] et al. [105], and [PERSON] et al. [106] have all conducted studies to demonstrate the behaviour of natural fractures within the caprock during scCO\({}_{2}\) injection.