An Investigation on the Pore Structure Characterization of Sandstone Using a Scanning Electron Microscope and an Online Nuclear Magnetic Resonance System

The micropore structure of porous media (such as natural rocks and man-made materials) is very complex and has strong micro heterogeneity, and pore structure is a critical parameter to estimate the rock quality. However, the pore structure characterization of rocks under load is not studied well. In this paper, sandstone specimens were preloaded to six different stress levels, and then the pore structure of rock was characterized by SEM and NMR, respectively. The results show the following: (1) The damage in sandstone increases with predefined stress, and the rate significantly increases over 0.8 uniaxial compressive strength (UCS). (2) There is a critical value in the process of rock damage (0.8 UCS), and when it is less than this critical value, the microstructure in the rock is mainly composed of pores and micro-cracks, and the length is generally less than 5 μm; when it exceeds the critical value, there are obvious cracks or even groups of cracks inside the rock. (3) The changes in porosity can be divided into three stages, showing a “√” shape tendency. (4) The pore structure can be visually presented using NMR and SEM, and the distribution mode of the pores changes from separated points to concentrated patches to finally interconnected networks of pores with an increase in the predefined stress. Overall, NMR provides a new method for characterizing rock damage and studying rock microstructure.