Study on failure models and fractal characteristics of shale under seepage-stress coupling

The existence of bedding in shale plays an important role in the physical properties and destruction processes of shale. In order to study the failure mechanism of shale with different dip angles under the coupling of seepage and stress, the study uses RFPA2D-Flow software to advance the seepage-stress coupling numerical simulation for seven groups of different bedding direction shale. Research shows that (1) the compressive strength and elastic modulus of shale are significantly affected by the bedding directions. The compressive strength can be observed with the apparent anisotropy of the shale compressive strength with the change of the bedding angle alpha; the elastic modulus increase with the increase in alpha as a whole. (2) The ultimate failure mode of shale under different bedding angles can be divided into V type (0 degrees), inverted V type (15 degrees), multi-line type (30 degrees), oblique type I (45 degrees, 90 degrees), and oblique N type (60 degrees, 75 degrees); the failure of shale in each group direction is mainly tensile failure with a small amount of shear failure. It can be found that the spatial distribution of acoustic emission (AE) reflected the macroscopic failure mode of shale. (3) The fractal dimension can well reflect the failure mode of the sample. From the trend of the dip-fractal dimension curve, the fractal dimension of the multi-line type reach to maximum, which is 1.41 699, and the D value of the oblique type I is the smallest, between 1.28 191 and 1.286 181. And the values of the inverted V type, V type and oblique N type, between 1.286 181 and 1.41 699. Therefore, the larger the value is, the more complex the shale failure mode is.