Characterization of meso-scale mechanical properties of Longmaxi shale using grid microindentation experiments

Mechanical properties, such as the hardness H, Young's modulus E, creep modulus C, and fracture toughness K_c, are essential parameters in the design of hydraulic fracturing systems for prospective shale gas formations. In this study, a practical methodology is presented for obtaining these properties through microindentation experiments combined with quantitative observations of the mineralogical phases using X-ray diffraction (XRD), scanning electron microscopy (SEM) with backscattered electron (BSE) imaging, and energy-dispersive X-ray spectroscopy (EDS) analyses. We apply this method in the case of three types of Longmaxi shales with different mineralogies (i.e. carbonate-, clay-, and quartz-rich, respectively), which allows us to determine the characteristic indentation depth, h_c = 8–10 μm, beyond which the mechanical response of the carbonate-rich shale is homogeneous and independent of its complex heterogeneous microstructure. Moreover, exploiting the results of a large number of indentation tests, we demonstrate that the indentation modulus M of the shale increases as a power-law of hardness H, and its creep modulus C increases linearly with H. We also compute the fracture toughness K_c from the indentation data by assuming a perfectly plastic behavior of the sample. Our results are in good agreement with independent measurements of K_c determined by microscratch tests. Finally, further tests on quartz- and clay-rich samples of the Longmaxi shale suggest further variations in the samples’ mechanical properties depending on their burial conditions and the mechanical properties of their dominant mineral phases.