Quantitative characterization of water absorption pore structure evolution in sandstone based on nitrogen adsorption and mercury intrusion

To investigate the mechanism of water’s effect on the pore structure of rocks, this study relies on the sandstone formation in Wanfu Coal Mine, Shandong, China. Scanning electron microscopy (SEM), nitrogen adsorption-desorption, and high-pressure mercury intrusion combined with pore structure testing experiments were conducted on sandstone samples subjected to different immersion times. The aim was to analyze the variations in the pore structure of micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm). The results indicate that the pore structure of the sandstone consists mainly of narrow and wedge-shaped structures. The pores have the ability to connect and form fissures. During the water absorption process, the porosity of the sandstone initially decreases and then increases. The proportion of micropores decreases initially and then increases, while the proportion of mesopores and macropores increases initially and then decreases. According to the quantitative assessment of pore structure using generalized fractal dimension and spectral width, it is found that there is a positive correlation between pore structure and pore quality. That is, as the difference in fractal dimension and spectral width increases, the pore structure becomes more complex. There is a negative correlation between pore structure and porosity, that is, larger porosity corresponds to smaller differences in fractal dimension and spectral width.