Experimental investigation on dynamic and static rock mechanical behavior, failure modes, and sequences of frequent interbedded sand and shale reservoirs

High variability in diagenetic strength and rock mechanical properties in interbedded sand and shale necessitates a sustained interest in the study of the dynamic and static rock mechanical behavior and failure modes. We have analyzed the rock mechanical behavior, rupture characteristics, and sequences of the clastic reservoirs of the Xu 5 member, Western Sichuan Depression, China. The results indicate that the Xu 5 shale can produce longer plastic creep behavior than tight sandstone at the same load rate. This causes greater stresses to build up inside the shale than in adjacent sandstone formations. The average internal friction angle of sandstone is 43 degrees, whereas the average internal friction angle of shale is 33 degrees. The failure modes of the Xu 5 member sandstone are mainly brittle-tensile failure and brittle X-type shear failure. We observed that the tensile rupture is dominant, accounting for approximately 75.2%. Shale failure forms mainly include plastic-tensile failure and X-type shear failure, in which shear failure accounts for approximately 67.9%. The sequence of failures of similar clastic reservoirs is generally tensile failure or tensile-shear failure to extension failure. We found that the Xu 5 shale has high plasticity, and the stress conditions required for its failure are higher and more complicated. In addition, our test results indicate that, for the same lithology, the tensile failure is the initial rupture rather than shear failure.

Automated summary

Studying the rock mechanical behavior and failure modes of interbedded sand and shale is important for understanding the mechanical properties and fracture mechanisms of rocks. The study found that at the same load rate, shale can exhibit longer plastic creep behavior compared to sandstone, resulting in higher stress accumulation inside the shale. Additionally, there are differences in the internal friction angle and failure modes between sandstone and shale.