The effect of natural fractures on the failure mechanism of granite: Implication for enhanced geothermal systems

Hot dry rock is considered to be an important renewable energy source typically existing with extensive natural fractures. Studying the deformation and failure pattern of the granite containing natural fractures is of great engineering significance for the in-depth understanding of the hydraulic fracture intersecting with natural fractures. In this paper, three different types of natural fractures were obtained according to the cementing degree of the filling mineral. Afterward, the fracture toughness of the natural fractures was evaluated according to the nanoindentation-based energy approach. Then, the mechanical experimental tests including uniaxial compression tests, triaxial compression tests, and Brazilian disc tests were conducted on granite samples containing pre-existing natural fractures monitored by real-time acoustic emission (AE) monitoring technology. The results indicated that granite with weakly sealed natural fractures presented the lowest mechanical strength, followed by granite with partially sealed natural fractures and strongly sealed natural fractures. For the granite with the partially sealed or weakly sealed natural fracture, the characteristic of the stress curve showed a multimodal pattern with constant stress drop and rise due to slippage of the natural fracture, in which a substantial amount of the AE counts were recorded accordingly. The rise angle and average frequency analysis demonstrated that a larger proportion of shear cracks would be generated with the existence of natural fractures. Further, the partially sealed natural fractures were activated and connected with other newly formed cracks, resulting in more complex fracture networks than the other samples, which leads to a higher fracture volume ratio of granite samples. The strongly sealed natural fractures are difficult to get stimulated under the deformation of the samples, resulting in rather simple fracture networks similar to that of intact granite. The results suggested fracturing granite with partially sealed natural fractures could possibly form complex interconnected fracture networks, potentially favoring the development of the enhanced geothermal system. The findings are expected to provide theoretical guidance for the stimulation of the hot dry rock reservoir.