Evaluating the Microstructure Evolution Behaviors of Saturated Sandstone Using NMR Testing Under Uniaxial Short-Term and Creep Compression

Understanding the micromechanical mechanism of the rock creep process is of great importance for studying the macroscopic time-dependent behavior of rocks. In this study, the evolution characteristics of the microstructure (cracks and pores) of saturated sandstones under short term and creep uniaxial compression conditions were investigated with the nuclear magnetic resonance (NMR) technique. The samples were first loaded to different stress levels and creep stages and then completely unloaded for NMR testing. Based on the testing results, the macroscopic deformation behavior, moisture migration law, pore size distribution, porosity, and microstructure change of the each sample under the short-term loading or different stages of creep were quantitatively analyzed. After that, by introducing a nonlinear elasto-viscoplastic damage creep model (EVP) by Zhao et al. (18:04017129, 2018), the relationships between the macroscopic irreversible strains and microscopic porosity increments were established. Overall, it was observed that: (1) regardless of the stress level, the magnitudes of the axial and lateral critical strains of samples at the onset of the accelerating creep stage are both relatively constant, and the axial strain is almost comparable to that at the peak stress in the short-term test, while the lateral strain is larger than that of the short-term test. (2) During the mechanical tests, the moisture in the samples migrates from large pores into small pores, and after mechanical tests, the porosities of the samples increase, in which the small pores always account for a larger proportion. (3) Corresponding to the three creep stages, the porosity of the sample increases slightly after the transient stage, increases to a constant value that is largely independent of stress after the steady stage, and increases significantly after the creep failure. In particular, compared to the initial porosity of 6.7%, the average porosities of samples taken to the onset of the tertiary stage and creep failure is 7.49% and 8.71%, increasing by 16.7% and 29.8%, respectively. (4) The porosity growth of sandstone during the brittle creep is mainly driven by the microscopic subcritical crack growth along the grain boundaries.