Experimental Investigation of Mechanical Characteristics in Tight Sandstones by Combining Nanoindentation and AFM

ABSTRACT Acquiring the micromechanical property of phases in tight sandstones by conventional mechanical testing is a big challenge because of the heterogeneity and complexity of mineralogical composition in sandstone systems. In this study, nanoindentation and AFM (atomic force microscope) technologies were employed to figure out the mechanical characteristic of tight sandstones. Nanoindentation results show that the average Young modulus of quartz is 72.78 GPa in tight sandstones. We found that nanoindentation needs more methods like SEM and EDS to separate the modulus of phases in tight sandstone. In contrast, AFM can easily recognize the elastic modulus of various phases with high accuracy and distinction. AFM results show that the Young modulus of quartz is 7.4 GPa and the Young modulus of minerals is 2.28 GPa. Moreover, the modulus value in nanoindentation is higher than that in AFM. The difference in modulus results of quartz by AFM and nanoindentation is due to the selection in probe in AFM. Overall, it shows that AFM is more suitable for achieving the goal of identifying the modulus of various phases in tight sandstones. The knowledge obtained from this paper is promising in the application of nanoindentation and AFM in tight sandstones. It will also benefit the study of micromechanical characteristics of various phases in tight sandstones. INTRODUCTION Acquiring the mechanical property is beneficial for several engineering aspects. Such as designing hydraulic fracturing schemes, enhancing oil recovery, and keeping wellhole stability(Li et al., 2019; Shen et al., 2022; Q. Wang et al., 2022). Rock consisting of several minerals is widely accepted as a homogenous and complex material. In the conventional mechanical tests, the obtained results represent the overall mechanical characterization of rock, which is regarded as the identical mechanical property of minerals (Brotóns et al., 2013). Uniaxial and triaxial pressure tests are typically applied to characterize the mechanical property of rock(Palchik, 2011; J. Wang et al., 2021). However, from the micro-scale, there are different pore structures and components of the minerals in the rock. The result is that the micromechanical characterizations between several minerals differ a lot(Liu et al., 2021; Wu et al., 2020). Therefore, acquiring the micromechanical property of phases in rocks by conventional mechanical testing is a big challenge because of the heterogeneity and complexity of mineralogical composition in rock systems(T. Wang et al., 2022). That is to say, by obtaining the micro-mechanical properties of phases of minerals in the rock, we can have an explicit knowledge of the mechanical property of rock.