Micromechanical Contrast of Ordos Basin Sandstone-Mudstone Interbedded Layered Rocks

Layered rocks are heterogeneous media composed of multiscale minerals with contrasting geomechanical properties. It remains unclear how the mechanical responses of constituent minerals among different adjacent layers vary in a microscale. Based on the grid nanoindentation tests assisted by high-resolution scanning electron microscopy and energy-dispersive spectroscopy techniques, we quantified the mechanical responses of various minerals in an interbedded sandstone-mudstone core sample and provided visual mineralogies and indentation patterns of different adjacent layers. Results show that the Young's modulus, hardness, and elasticity index of quartz and dolomite decrease, whereas their fracture toughness and plastic work ratio increase from the sandstone layer to the mudstone layer. Quartz displays elastic-dominated deformations in the sandstone layer whereas transforms into medium-plastic deformations in the mudstone layer. The mean values of Young's modulus, hardness, and elasticity index of quartz in the sandstone layer are 1.28, 2.00, and 1.56 times higher than that in the mudstone layer, respectively. Dolomite and phyllosilicate minerals show prominently plastic-dominated deformations in each layer. When an indenter encounters multiple minerals simultaneously, the mechanical responses are determined by the softer phases and irregular indent impressions are generated. Shear and radical cracks are prone to occur in the elastic-dominated minerals, while chipping damage is induced in the plastic-dominated minerals. This work provides new insights and fundamental understandings in geomechanical properties contrasts in multilayered rocks, and can facilitate the geomechanical modeling and upscaling schemes in multilayered formations. Multilayered formations often consist of lithological layers with significant variations in geomechanical properties. In Ordos Basin, China, the multilayered formations not only contain a large amount of unconventional oil and gas resources, but also provide a potentially safe and economic trap for CO2 geological storage. The geomechanical contrast between layers generally controls the rock heterogeneity and fracture containment in layered rocks. However, the mechanical response of an individual layer such as the pure sandstone on layered rocks is difficult to quantify on core samples since the thinly interbedded layer is less prone to extract for the standard mechanical tests. Moreover, it is infeasible to interpret the geomechanical heterogeneity in microscale by well logging results restricted by its resolution. Hence, we used nanoindentation tests combined with scanning electron microscopy and energy-dispersive spectroscopy techniques to explore the mechanical properties and deformation behaviors of the microsized minerals among adjacent layers including sandstone, mudstone, and their interfaces. This study is expected to provide direct visual insights into the mechanical responses and contrasts among various minerals in adjacent layers, and will possibly facilitate the rock-physics modeling and upscaling, which can help to guide for the hydraulic fracturing designing, waste water injection, and CO2 injection for carbon capture and storage, etc. Geomechanical responses of individual minerals and multiple minerals were quantified in different layers of an interbedded core sampleElasticity index of quartz and dolomite decreases whereas fracture toughness and plastic work ratio increase from sandstone to mudstoneMechanical responses of multiple minerals are determined by the softer phases, resulting in irregular indent impressions in each layer

Automated summary

This study quantified the mechanical responses of different minerals in layered rocks using nanoindentation tests and high-resolution scanning electron microscopy. The results showed that the mechanical properties of quartz and dolomite varied between different layers, with quartz displaying elastic-dominated deformations in the sandstone layer and medium-plastic deformations in the mudstone layer.