Observations of breakage for transversely isotropic shale using acoustic emission and X-ray computed tomography: Effect of bedding orientation, pre-existing weaknesses, and pore water

Abstract

Shale is typically an anisotropic material for which the mechanical characteristics can vary significantly along different orientations with regard to its bedding planes. In underground formations, the strength of shale shows strong dependence on the pre-existing weaknesses and the water content. This study is concerned with the mechanical behavior of Marcellus shale specimens under three-point bending where loading is applied in different directions with respect to its bedding plane orientation under both dry and saturated conditions. Acoustic Emission (AE) is recorded in order to investigate the nucleation of microfracturing via AE locations and Moment Tensor Analysis (MTA). Complimentary to the AE detection and analysis, X-ray Computed Tomography (CT) is utilized to image internal damage sustained due to loading. Experimental results show lower nominal tensile strength when bending loads are applied parallel to bedding in dry samples. Further comparisons between tests under different conditions point to the strong impact of the pore water and pre-existing weaknesses on the samples. Detailed results from MTA show tensile microfracturing is the dominant crack type. Surprisingly, event locations show a large number of AE events occur in areas of the beam that are nominally subjected to compressive stress, implying that fracturing can be strongly impacted by local stress fluctuations which may result in opening of the interfaces between bedding layers even when the nominal stress in an area is compressive. In several cases, AE associate with visible fractures in the CT scan which are not connected to the main rupture surface through the specimen. Additionally, visualization of the internal fracture paths using image segmentation on the CT results show more complex fracture systems in parallel loading cases, which brings together localized fractures generated by different mechanisms.