This dissertation investigates the implications of the fluid flow on the behaviour of the particle-scale structure of a porous hard rock, based on the Discrete Element Method (DEM). This project is driven by the need to contribute towards a better understanding of the mechanical behaviour of porous rock formations under intense injection conditions and the influence of natural pre-existing rock damage to the hydraulic fracturing mechanism. The proposed numerical scheme incorporates different methods for computing both the solid and co-existing fluid phases. The solid phase (rock sample) has been characterized as a collection of discrete interacting particles, bound by spring-like contacts according to the DEM. Meanwhile, the fluid phase has been modelled by discretising the Navier-Stokes equations for porous media, utilising the fluid coupling algorithm embedded in the Particle Flow Code (PFC3D) software by Itasca. The outcome of this dissertation suggests that the DEM approach is an advanced computational method that can reproduce accurate rock models, adequately describe the inter-particle dynamics and thus contribute towards direct numerical and experimental comparisons, and interpret the geo-mechanical behaviour of the rock materials. Furthermore, this study identifies the importance of shear cracking in the hydraulic fracturing models, whereas conventional theory relates hydraulic fracturing with tensile cracking. Finally, this study focuses on the influences of various parameters, such as the external stress regime, fluid viscosity and pre-existing fractures, on the mechanical behaviour of the rock material in the particle-scale and the hydraulic fracturing process as a whole. This work is in an early stage and it aims to simulate hydraulic fracturing experiments with the use of a 3D modelling and the DEM approach, and to investigate the micromechanical response of the rock. Further research may include areas such as the 3D modelling of pre-cracked rocks using a larger variety of fracture angles.