Simulation of the hydraulic fracturing process of fractured rocks by the discrete element method

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 particle-scale. 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 15A degrees and 45A degrees, the stress regime below the fracture is always higher than the one related with the fractures between 45A degrees and 90A degrees. Furthermore, the propagation of cracks has also been affected by the fracture angle, where for fractures below 45A degrees, the cracks tend to propagate downwards and travelling mostly as a group of cracks. In contrast, for fractures above 45A degrees, 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.