Experimental Investigation of the Influence of Bedding Planes and Differential Stress on Microcrack Propagation in Shale Using X-Ray CT Scan

In past couple of decades, scanning electron microscope and digital image correlation has been extensively used to investigate the microcrack propagation in rocks. Only recently, X-ray computed tomography (CT) scans were used for more detailed understanding of the physico-mechanical behavior of rock specimens. In current study, the deformational behavior of bedded Marcellus shale was studied through the changes in the geometry of microcracks. Quasi-static triaxial tests at three successive levels of differential stress were conducted on cylindrical-shaped shale specimens at a confining pressure of 6.89 MPa. Mineralogical analysis indicated the homogenous composition of specimen, however, triaxial tests resulted varying modulus of stiffness at similar confining pressure. The X-ray CT scanned images of specimens prior to the triaxial stress showed the significant density of pre-existing microcracks. During triaxial tests, the successive levels of differential stress influenced the geometry of pre-existing microcracks. The differential stress equivalent to 55% and 65% of the differential strength significantly closed the pre-existing microcracks. However, differential stress equivalent to 70% of the differential strength increased the density of microcracks. The orientation of the bedding plane only influenced the direction of microcrack propagation. The perpendicular-bedded specimen experienced significant microcrack propagation in the axial direction, while the parallel-bedded specimen experienced significant increase in the aperture of microcracks.