Geomechanical Properties of Thinly Interbedded Rocks Based on Micro- and Macro-Scale Measurements

Clarifying the geomechanical properties and inherent heterogeneity of layered rocks is essential for predicting fracture morphology and designing hydraulic fracturing pumping schemes. Grid nanoindentation combined with high-resolution scanning electron microscopy and energy-dispersive spectroscopy (SEM–EDS) techniques were used to investigate mechanical differences between the sandstone layer and the mudstone layer for the downhole thinly interbedded core rocks (layered core rocks). Subsequently, the indent impressions and mechanical responses of individual minerals and multiple minerals were quantified. On this basis, the bulk mechanical properties and failure modes of the layered core rocks are investigated under confining pressures. Results indicate that quartz exhibits the highest hardness and Young’s modulus with the smallest indent impression, whereas kaolinite exhibits the opposite in each layer. When the indenter covers multiple minerals simultaneously, the mechanical responses of which are determined by the softer phases. Moreover, minerals exhibit diverse deformations and cracking patterns in different layers. Quartz shows elastic-dominated deformation in the sandstone layer, whereas medium-plastic deformation in the mudstone layer. Since the unstable sheet structures, kaolinite exhibits plastic-dominated deformation in each layer. Shear cracks and radical cracks are prone to occur in elastic-dominated minerals, while chipping damage is induced in plastic-dominated minerals. In addition, the failures of the layered core rocks tend to create along the mudstone layer since the lower mechanical properties. With the increasing of confining pressure, the compressive strength of layered core rocks gradually grows and the failure changes from tensile splitting to tensile–shear mixed failure mode and shear failure mode. The key findings of this paper can provide reliable input data for multiscale geomechanical modeling in understanding proppant embedment mechanisms and designing hydraulic fracturing treatments in coal measure strata.