Stress-Induced Damage Evolution in Unconventional Shale Monitored by Acoustic Emission

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Abstract This paper experimentally investigates the damage evolution (or the deformation process) induced by externally applied stress on unconventional shale samples. The focus is to understand the physics behind the complex failure process occurring at the micro-scale, and thus provide useful information on the macro-fracture mechanisms of unconventional shales. We perform the compression tests along with the acoustic emission (AE) measurement on cylindrical rock samples selected from Berea sandstone (BS), Eagle Ford (EF) and Marcellus formation. Tests are conducted under the same stress condition: uniaxial unconfined compression test and triaxial compression test with a confining pressure of 10 MPa. AEs are continuously collected to characterize the progressive damage process. The axial stress, axial and radial displacement are also recorded during experiments. Shale samples are drilled perpendicular or parallel to the beddings. The X-ray diffraction (XRD) analysis is used to measure the mineralogy and clay content. Prior to geomechanical testing, the high-resolution micro-CT imaging system is used for quality assessment of rock samples and characterization of microstructure and fracture presence. All samples studied exhibit distinct characteristics of AE counts. In Berea sandstone, AE counts initially increase slowly with the axial strain, and then surge dramatically after reaching the yield stress point. This is consistent with the observed stress-strain behavior: a linear elastic part followed by the inelastic deformation till the rock fails. Compared to Berea sandstone, shale samples show a different characteristics of AE counts. AEs resulting from EF vertical sample monotonically increase until the sample fails, while three stages of AE activity are observed in the horizontal sample: initial increase followed by a stage of quiet period (no AE events), and increase again. Marcellus samples behave just the opposite: three stages of AE events are observed in the vertical sample, while monotonically increase in the horizontal sample. AEs are also observed to be affected by the test condition: different from the triaxial compression test, Marcellus vertical and horizontal samples have a similar trend in AE counts during the uniaxial compression test. Both initially increase in AE activity with the axial stress, and after a certain stress, a surge in AE activity is observed. The observed discrepancy of AEs can be due to the complexity of shale mineralogical compositions, pore geometry and thin layer structure. More detailed laboratory experiments are necessary to understand the physics behind the complex failure process and AEs occurring at the micro-scale in unconventional shales.