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

TLDR

The mechanical and creep properties of shale strongly influence artificial hydraulic fracturing, wellbore stability, and the evaluation of reservoir performance in shale gas exploration. The study aimed to characterize shale’s mechanical and creep properties at the microscale using nanoindentation and to determine how indentation load and loading strain rate affect these properties, thereby standardizing testing parameters and enhancing understanding of microscale deformation mechanisms. Nanoindentation tests were performed at varying loads (2–400 mN) and loading strain rates (0.005–0.1 s⁻¹) to assess their influence on Young’s modulus, hardness, indentation creep, and creep strain‑rate sensitivity. The results showed that Young’s modulus and hardness decreased sharply with increasing load up to 200 mN and then plateaued, while both increased slightly with higher loading strain rates; indentation creep and creep strain‑rate sensitivity rose with load but fell with strain rate, indicating that higher loads or lower strain rates amplify creep deformation, and the observed m values (0.040–0.124) point to dislocation power‑law creep as the dominant mechanism.

Abstract

The mechanical and creep properties of shale strongly influence artificial hydraulic fracturing, wellbore stability, and the evaluation of reservoir performance in shale gas exploration. This study characterized these mechanical and creep properties at the microscale through nanoindentation tests and evaluated their dependence on the indentation test parameters, specifically, the indentation load and the loading strain rate. The mechanical parameters (the Young's modulus and hardness) of shale were strongly influenced by the magnitude of an indentation load (2-400 mN). Both parameters decreased sharply as the load increased from 2 to 200 mN; they then remained relatively stable at loads of 200-400 mN, suggesting that large indentation loads (200-400 mN) can be used to detect the mechanical responses of bulk shale. In contrast, both parameters increased slightly as the loading strain rate increased from 0.005 to 0.1 s-1. The indentation creep (CIT), related to creep behavior, and the creep strain rate sensitivity (m), related to the creep mechanism of shale, both increased with increasing the indentation load, whereas they decreased with increasing the loading strain rate. This demonstrates that increasing the load or decreasing the loading strain rate can increase creep deformation in shale during nanoindentation creep testing. The values of m varied from 0.040 to 0.124 under different loading conditions, suggesting that dislocation power-law creep may be the main mechanism controlling creep in shale. This study standardizes the testing parameters for the characterization of the mechanical properties of shale by nanoindentation testing and also advances our understanding of the deformation mechanisms of shale at the microscale.