Experimental investigation on mechanical performance and failure modes of shale interbedded with sandstone under triaxial compression

The investigation of the mechanical and failure behavior of shale interbedded with sandstone is of scientific and engineering significance for shale gas development. Triaxial compression tests were conducted to investigate the effects of the confining stress and thickness proportion of sandstone interlayers on the characteristic stresses and macroscopic failure modes of specimens of shale interbedded with sandstone. The characteristics of microflaws within interbedded specimens were also examined from a microscopic perspective based on nuclear magnetic resonance (NMR). According to the test results, the deformation and failure process of shale interbedded with sandstone exhibit obvious stage characteristics. The characteristic stresses are affected by the confining stress and thickness proportion of sandstone interlayers, while the ratios of the different characteristic stresses to the peak stresses remain almost constant. All specimens exhibit a tensile-shear failure mode, and six crack morphology patterns can be summarized at the interface between shale and sandstone interlayers. The microflaws in the failed specimens can be classified into three types, namely, closed flaws, small open flaws, and large open flaws.