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.
Highlights
The marine Longmaxi shale in South China is characterized by extremely low porosity and permeability, and has been exploited for shale gas by multistage hydraulic fracturing and horizontal drilling techniques [1]
The size of the residual imprints observed on clay minerals was larger than that of quartz for a specific peak load, which is consistent with the higher strength and stiffness of quartz
The creep rate trends were similar
Summary
The marine Longmaxi shale in South China is characterized by extremely low porosity and permeability, and has been exploited for shale gas by multistage hydraulic fracturing and horizontal drilling techniques [1]. Statistical nanoindentation methods have commonly been used to determine the creep behavior of shale composites and their constituents [4, 8, 15, 21, 22] This method requires a large amount of indentation data to be acquired, and the relevant parameters for each phase are extracted with a Gaussian distribution model [27,28,29]. The Gaussian distribution model can determine the creep behavior of a single mineral phase in a shale It requires clear identification of the phase composition of the material, and care is needed during the indentation not to indent contacts between two or more phase, which will affect the test results. The results improved our understanding of the intrinsic creep behavior and mechanisms of major constituents in shale at nanoscale, which further facilitates an accurate prediction of time-dependent behavior of shale reservoir during gas production
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