Experimental investigation of Sinian shale rock under triaxial stress monitored by ultrasonic transmission and acoustic emission

Abstract

To investigate the acoustic and mechanical properties of shale gas reservoir rocks, fracture experiments under triaxial compression were conducted using Sinian shale samples collected from the Sichuan Basin and cored along an angle of ∼15° with respect to bedding orientation. Ultrasonic transmission (UT) and Acoustic emission (AE) were used to monitor the response of anisotropic wave velocity and fracture behavior to elevated stress. X-ray CT (computed tomography) scan was used to explore the heterogeneous structure and geometric property of the fracture zone that ultimately developed in the sample. The evolution of microstructure such as closure of pores, development of microcracks and formation of macro fractures at elevated stress is likely responsible for the observed responses of elastic wave velocity and AE activity. During the hydrostatic compression stage, the closure of cracks and pores leads to a little bit of increase of velocity and a small number of AE events. As the mean effective stress further increases, there is no significant change in the microstructure of shale sample derived from the non-response of velocity to elevated stress. During the dynamic fracture stage, the generation of macro-fracture along the bedding plane accounts for the decrease of velocity as well as the rapid increase of AE activity. Our results indicate that the Sinian shale of the Sichuan Basin is characterized by weak anisotropy and strong heterogeneity in core scales. The intrinsic anisotropy keeps almost unchanged to the elevated stress, while significant increase in velocity anisotropy is induced by shear fracture. Moreover, the fracture experiments show that the shale sample performs a purely brittle fracture behavior, which is beneficial for the hydraulic fracturing stimulation of shale gas reservoirs. Both AE hypocenters and X-ray CT scan image show that the deformation and fracture process correlates strongly with the bedding structure in the sample.