Abstract: The shale of Longmaxi formation in Sichuan Basin contains a large amount of shale gas for development. In order to study the mechanical properties of the shale, a uniaxial compression test is first performed on a shale sample; and then the test is simulated based on the particle flow code(PFC) numerical simulations (discrete element method). Finally, the numerical simulations of triaxial compression are performed to study the brittle failure characteristics of rock under different stress conditions. The research results show that the rock of the area is a typical brittle rock with high strength. It is found that the Longmaxi shale has a high content of brittle minerals; and it is the high compression strengths and high brittleness rock. The Longmaxi formation shale has similar failure characteristics under different stress conditions，and the peak strength of shale is linear with the confining pressure, which is consistent with the linear law of Mohr Coulomb. The distribution of microcracks does not have obvious directionality and regionality before the damage stress is reached; the microcrack distribution may be related to the heterogeneity of the rock at this stage; but the expansion of the microcrack will be affected by the stress after the damage stress is reached. The cracking stress, damage stress and peak stress of the shale are linearly related to the confining pressure; but the peak stress is more affected by the confining pressure. The reason is that the microcracks are independent at the crack stable growth stage; but it is necessary to consider the influence between microcracks, and the macroscopic crack surface are not smooth at the crack unsteady growth stages.

Key words: shale, brittle failure, discrete element method, crack initiation stress, damage stress