Underground CO2 storage represents the most viable Negative Emission Technology capable of significantly reducing atmospheric carbon dioxide levels. Ideal reservoirs for CO2 sequestration are thick highly-porous and highly-permeable formations ensuring a high storage capacity at industrially meaningful injection rates (MtCO2/year) [1]. Selecting a suitable CO2 storage site requires thorough analysis of the geology, structure and hydrology of each reservoir. One of the critical concerns during this assessment is the potential for induced seismicity, particularly due to the presence of faults in the vicinity of the reservoir. To evaluate the mechanical stability of fault systems caused by changes in the hydro- mechanical state of a reservoir, extensive numerical modelling is employed. In this study, we assess the suitability of a potential site for CO2 storage with a focus on mechanical stability of a fault system. To achieve this, we have developed a geomechanical simulator that incorporates hydro-mechanical coupling and modelling fractures and faults discretely. The hydraulic component accounts for single-phase fluid flow in the bulk, as well as laminar flow along rock discontinuities. Meanwhile, the mechanical aspect incorporates poroelasticity in the bulk and considers contact and friction along fractures and faults. The numerical system of equations is discretized using the Finite Element Method and solved in a fully-implicit manner. To handle nonlinearities in the hydro-mechanical behavior of interfaces, we utilize the Newton-Raphson method. Additionally, we employ the block conjugate gradient method [2] to solve the linearized system of equations. This approach allows us to use separate solver for each subproblem while maintaining a fully-implicit solution strategy. Furthermore, the MPI code parallelization enables simulations of large numerical models with a high-resolution level of geological features [3]. By modelling the injection of CO2 into a potential 3D reservoir adjacent to a fault, we predict the evolution of pressure and stress within the reservoir and the caprock. Moreover, we assess the potential impact on the mechanical state of the fault. This geomechanical analysis contributes valuable insights to the process of site selection for safe and efficient CO2 storage. [1] Kelemen P, Benson SM, Pilorgé H, Psarras P and Wilcox J (2019) An Overview of the Status and Challenges of CO2 Storage in Minerals and Geological Formations. Frontiers in Climate 1:9. [2] Prevost J (1997) Partitioned Solution Procedure for Simultaneous Integration of Coupled- Field Problems. Communications in Numerical Methods in Engineering 13:4 [3] Richart N, and Molinari J-F (2015) Implementation of a Parallel Finite-Element Library: Test Case on a Non-Local Continuum Damage Model. Finite Elements in Analysis and Design 100: 41–46.