A new method for shale brittleness evaluation

High brittleness is the prerequisite for the creation of a hydraulic fracture network, which is crucial for shale reservoir development. The test evaluation methods are restricted in the field by lack of continuous brittleness profiles. Mineralogical evaluation cannot incorporate the external factors like stress state. The most widely used method is mechanical evaluation method based on statistical results of Young’s modulus and Poisson’s ratio. This method cannot explain the brittle tensile failure mechanism of shale and contradiction between high brittleness and barriers of high strength rock. With these methods, brittle evaluation and interval selection errors will happen in the application. Starting from the mechanism of rock failure and considering the effect of rock mechanical properties on brittleness, a new method for rock brittleness evaluation has been established. Firstly, the failure surface features were analyzed and the dominant mechanisms in complex failure were identified with triaxial test and scanning electron microscopy test. Secondly, a stress model for shale reservoirs with thin sand and shale interbedded was set up based on elasticity and the factors affecting the stress distribution and failure modes have been analyzed. Eventually, based on experiments and theoretical recognition, the brittleness index for shale was proposed by integrating Young’s modulus, Poisson’s ratio and fracture toughness. Moreover, a method for acquiring brittleness index from well logs was developed from which brittleness profiles for shale formations in Sichuan basin were obtained. Failure features analysis demonstrates that tensile failure dominates during rock brittle failure. It was observed that compression stress on rock is only relevant to mechanical parameters and independent of space location under both internal and external stresses. Finally, comparison with the results from Rickman’s method and microseismic monitoring demonstrates that the method developed in this work possesses more satisfactory results of brittleness evaluation and interval selection in field application.