Analysis of the Microscopic Evolution of Rock Damage Based on Real-Time Nuclear Magnetic Resonance

Under the action of a load, internal pores and cracks expand, and irreversible plastic deformations occur. Compared with conventional rock mechanics tests, nuclear magnetic resonance (NMR) can characterize the size and distribution of pores at the microscopic scale. In this study, a series of low-confining-stress triaxial compression tests were performed on different types of sandstone samples using real-time T2-weighted NMR spectra and imaging. It was found that the area of macropores in sandstone significantly increased only during the initial loading stage, but played an opposite role in the damage evolution process. This phenomenon is contrary to our expectations and provides a new basis for understanding the evolution of damage in rocks. Furthermore, during the linear deformation stage, the number of pores and mesopores increased, whereas the number of macropores decreased. A damage model based on the NMR results is proposed. The value of D_n sharply increases during the initial stage due to the expansion of pores, then decreases, and finally begins to increase again before the failure stage and until the sample fractures owing to the development of macroscopic cracks. In conclusion, the structure of micropores has a significant influence on the failure mode of sandstone rocks in low-confining-pressure triaxial compression tests.