川南龙马溪组页岩不同应力条件下脆性破坏特征室内实验与数值模拟研究

Laboratory experiment and numerical simulation on brittle failure characteristics of Longmaxi formation shale in Southern Sichuan under different stress conditions

四川盆地龙马溪组页岩具有较好的生烃潜力。为研究龙马溪组页岩力学特性,首先对完整页岩试样开展了单轴压缩试验,基于颗粒流离散单元法反演单轴压缩岩石动态破坏过程,通过模拟常规三轴压缩试验,研究不同应力条件下页岩脆性破坏特征。研究表明,该龙马溪组页岩脆性矿物含量高,强度大,脆性特征明显;不同应力条件下龙马溪组页岩发生脆性破坏时具有相似的破坏特征,且峰值强度符合Mohr-Coulomb线性规律。加载到岩石损伤应力以前,微裂纹的分布无明显方向性和区域性,该时微裂纹的分布应与岩石的非均质性有关;损伤应力后微裂纹的萌生、聚集和贯通将受应力影响。页岩的特征应力(起裂应力、损伤应力、峰值强度)与围压表现出强相关性,但围压对峰值应力的影响程度比对损伤应力和起裂应力的影响程度更大,因裂纹稳定扩展阶段,微裂纹独立产生无相互影响,峰值应力还需考虑微裂纹间的相互作用及形成的宏观破裂面粗糙性的影响。

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.