Bridging nanoindentation and triaxial creep tests on a shale

Shale is known to exhibit significant creep across spatiotemporal scales. In this work, we performed indentation and triaxial creep tests on organic-rich Woodford shale and showed that the measured creep responses at the nanometer and millimeter scales are statistically correlated in the sense that the indentation creep tests can be used to predict the triaxial creep response. The link between the two extreme scales is facilitated by an elasto-viscoplastic constitutive model based on critical state theory. Even though this constitutive model contains several material parameters, only the elastic modulus and viscosity played a major role in statistically correlating the creep responses at the two scales. Given that shale is commonly represented as a transversely isotropic material, we conducted indentation and triaxial creep tests in both bed-normal (BN) and bed-parallel (BP) directions on the rock samples. For the shale that was tested, the creep responses at the nanometer and millimeter scales were statistically correlated in the BN direction where the rock is weaker. However, the creep deformations at the two scales in the BP direction, where the rock is stronger, were significantly smaller than in the BN direction, and so, they were not as strongly correlated. Nevertheless, the results of this study suggest that shale exhibits strongly anisotropic creep properties.