Modeling the Viscoelastic Behavior of Quartz and Clay Minerals in Shale by Nanoindentation Creep Tests

Abstract

The viscoelastic behavior of minerals in shales is important in predicting the macroscale creep behavior of heterogeneous bulk shale. In this study, in situ indentation measurements of two major constitutive minerals (i.e., quartz and clay) in Longmaxi Formation shale from the Sichuan Basin, South China, were conducted using a nanoindentation technique and high-resolution optical microscope. Firstly, quartz and clay minerals were identified under an optical microscope based on their morphology, surface features, reflection characteristics, particle shapes, and indentation responses. Three viscoelastic models (i.e., three-element Voigt, Burger’s, and two-dashpot Kelvin models) were then used to fit the creep data for both minerals. Finally, the effects of peak load on the viscoelastic behavior of quartz and clay minerals were investigated. Our results show that the sizes of the residual imprints on clay minerals were larger than that of quartz for a specific peak load. Moreover, the initial creep rates and depths in clay minerals were higher than those in quartz. However, the creep rates of quartz and clay minerals displayed similar trends, which were independent of peak load. In addition, all three viscoelastic models produced good fits to the experimental data. However, due to the poor fit in the initial holding stage of the three-element Voigt model and instability of the two-dashpot Kelvin model, Burger’s model is best in obtaining the regression parameters. The regression results indicate that the viscoelastic parameters obtained by these models are associated with peak load, and that a relatively small peak load is more reliable for the determination of viscoelastic parameters. Furthermore, the regression values for the viscoelastic parameters of clay minerals were lower than those of quartz and the bulk shale, suggesting the former facilitates the viscoelastic deformation of shale. Our study provides a better understanding of the nanoscale viscoelastic properties of shale, which can be used to predict the time-dependent deformation of shale.

Using Nanoindentation to Characterize the Mechanical and Creep Properties of Shale: Load and Loading Strain Rate Effects

“…The creep curve with the “pop-in” phenomenon and/or data with a relatively large thermal drift must be discarded. 33 , 57 …”

“… Using a heterogeneous and anisotropic sample (i.e., shale) during the test might have induced complex creep behavior. The creep curve with the “pop-in” phenomenon and/or data with a relatively large thermal drift must be discarded. , …”

“…The creep curve with the "pop-in" phenomenon and/or data with a relatively large thermal drift must be discarded. 33,57 Shale is a type of fine-grained sedimentary rock composed of a variety of minerals (clay, quartz, feldspar, pyrite, and carbonate) and organic matter. Each component has its own creep behavior.…”

“…Wick reported that nanoindentation gave results that were comparable with those of conventional uniaxial compression testing for creep behavior; thus, they believed that nanoindentation measurements were reliable in determining the creep behavior of shales . Recent research has focused on the effects of shale composition − and temperature on its nanoindentation creep behavior, creep models for shale, and the relationship between the creep parameters obtained from nanoindentation and triaxial creep tests. ,, Nanoindentation testing has proved useful for both the characterization of mechanical properties and evaluating creep behavior.…”