Experimental study on brittle characteristics and a brittleness prediction model for Longmaxi shale in the Sichuan Basin

The assessment of shale reservoir brittleness plays an important role in evaluating reservoir fracturability, designing fracture stages, and optimizing fracture stimulation outcomes. The brittleness indices (B1∼B14) of the shale with different bedding orientations from Longmaxi were calculated via the summarized methods under triaxial compression tests, and the anisotropic brittleness characteristics of the shale with different bedding angles were explored. Finally, utilizing the U-shaped trend of the brittleness index B3, a predictive model for shale brittleness as a function of bedding orientation (θ) was established via nonlinear fitting. The model requires only the brittleness indices at bedding orientations of 0° (B0) and 90° (B90) as inputs. This model was validated on the basis of many available experimental data in the literature. The experimental results reveal that the brittleness indices of the shale specimens calculated via the summarized methods present obvious anisotropy. However, the brittleness index B3 shows an overall trend of decreasing first and then increasing with increasing bedding orientation, which is similar to the variation in the peak strength and elastic modulus of shale under triaxial compression. On the basis of the experimental results of B3, a method for predicting the brittle index of shale as a function of bedding orientation was established through a nonlinear fitting methodology. The proposed method is validated by experimental data from the published literature, which shows that the predictive results are in accordance with the experimental results to some degree. This method can provide a theoretical reference for evaluating the brittleness characteristics of deep shale reservoirs.