A three-dimensional model is presented and used to reproduce the laboratory hydraulic fracturing test performed on a thick-walled hollow cylinder limestone sample. This work aims to investigate the implications of the fluid flow on the behaviour of the micro structure of the rock sample, including the material strength, its elastic constants and the initialisation and propagation of fractures. The replication of the laboratory test conditions has been performed based on the coupled Discrete Element Method and Computational Fluid Dynamics scheme. The numerical results are in good agreement with the experimental data, both qualitatively and quantitatively. The developed model closely validates the overall behaviour of the laboratory sample, providing a realistic overview of the cracking propagation towards total collapse as well as complying with Lame's theory for thick walled cylinders. This research aims to provide some insight into designing an accurate DEM model of a fracturing rock that can be used to predict its geo-mechanical behaviour during Enhanced Oil Recovery (EOR) applications.

This work presents a series of solid-fluid-coupled simulations of a natural fractured limestone sample. The aim of this paper is to investigate the effects of the fluid injection on its mechanical behaviour on the particlescale. A detailed study of the influence of the fluid flow on the microstructure of the virtual model, including its internal stress state, the fracture initialization and propagation, and also the interactions between the existing fracturing networks and the new hydraulically induced fractures, has been performed. The results show that the change of the angle of the natural fracture alters the internal stress pattern of the model, concluding that for fractures between 15°and 45°, the stress regime below the fracture is always higher than the one related with the fractures between 45°and 90°. Furthermore, the propagation of cracks has also been affected by the fracture angle, where for fractures below 45°, the cracks tend to propagate downwards and travelling mostly as a group of cracks. In contrast, for fractures above 45°, easier upwards movement of the cracks and the formation of clusters that stray from the main volume of cracks are observed. The overall fracture growth is in agreement with what conventional theory generally states, where a hydraulic fracture is extended along the direction of the maximum compressive principal stress. However, the magnitude of this observation was restricted by the sample dimensions due to increased computational time for larger samples. Finally, the relationship between the important cracking events (abrupt increases of microcracks) and the energy release within the model confirms the postulate that bond breakage causes further movement of particles and therefore increases the internal kinetic energy.