Experimental Investigation on Mechanical and Acoustic Parameters of Different Depth Shale Under The Effect of Confining Pressure

Abstract

Investigations on the effects of confining pressure and burial depth on mechanical and acoustic parameters are significant for studying the deep shale deformation, failure rule, and hydraulic fracturing. Shale specimens at six different depth levels ranging from 1535 to 1635 m have been collected from a shale gas well in southwest China. RTR-1000 rapid rock triaxial testing system has been applied to carry out the mechanical and acoustic experiments with 19 shale specimens. The experimental results revealed that the peak deviatoric stress, peak axial strain, peak volumetric strain, residual stress, and axial wave velocity are directly proportional to confining pressure and burial depth, whereas elastic modulus is not affected much by both confining pressure and burial depth and the Poisson ratio decreases with the increase of confining pressure. In the burial depth range 1535–1635 m, the variation of Poisson’s ratio is not obvious with the depth changes, and their relationship has high randomicity. The Wave velocity Anisotropy of tested shale is greatly affected by the degree of stratification development rather than the corresponding crustal stress. The coefficient of variation and the degree of stratification development are inversely proportional to burial depth. The axial wave velocity of shale is closely related to the loading process. In the compaction stage, wave velocity increases rapidly with the increase of strain. And in the elastic yielding stage, wave velocity increases slowly along with the strain build-up. This study will help to deepen the understanding of the physical and mechanical properties of Wuxi shale and guide the shale gas exploitation in China.